OCFS2(7)

NAME
OCFS2 - A Shared-Disk Cluster File System for Linux

INTRODUCTION

OCFS2 is a file system. It allows users to store and retrieve data. The data is stored in files that are organized in a hierarchical directory tree. It is a POSIX compliant file system that supports the standard interfaces and the behavioral semantics as spelled out by that specification.

It is also a shared disk cluster file system, one that allows multiple nodes to access the same disk at the same time. This is where the fun begins as allowing a file system to be accessible on multiple nodes opens a can of worms. What if the nodes are of different architectures? What if a node dies while writing to the file system? What data consistency can one expect if processes on two nodes are reading and writing concurrently? What if one node removes a file while it is still being used on another node?

Unlike most shared file systems where the answer is fuzzy, the answer in OCFS2 is very well defined. It behaves on all nodes exactly like a local file system. If a file is removed, the directory entry is removed but the inode is kept as long as it is in use across the cluster. When the last user closes the descriptor, the inode is marked for deletion.

The data consistency model follows the same principle. It works as if the two processes that are running on two different nodes are running on the same node. A read on a node gets the last write irrespective of the IO mode used. The modes can be buffered, direct, asynchronous, splice or memory mapped IOs. It is fully cache coherent.

Take for example the REFLINK feature that allows a user to create multiple write-able snapshots of a file. This feature, like all others, is fully cluster-aware. A file being written to on multiple nodes can be safely reflinked on another. The snapshot created is a point-in-time image of the file that includes both the file data and all its attributes (including extended attributes).

It is a journaling file system. When a node dies, a surviving node transparently replays the journal of the dead node. This ensures that the file system metadata is always consistent. It also defaults to ordered data journaling to ensure the file data is flushed to disk before the journal commit, to remove the small possibility of stale data appearing in files after a crash.

It is architecture and endian neutral. It allows concurrent mounts on nodes with different processors like x86, x86_64, IA64 and PPC64. It handles little and big endian, 32-bit and 64-bit architectures.

It is feature rich. It supports indexed directories, metadata checksums, extended attributes, POSIX ACLs, quotas, REFLINKs, sparse files, unwritten extents and inline-data.

It is fully integrated with the mainline Linux kernel. The file system was merged into Linux kernel 2.6.16 in early 2006.

It is quickly installed. It is available with almost all Linux distributions. The file system is on-disk compatible across all of them.

It is modular. The file system can be configured to operate with other cluster stacks like Pacemaker and CMAN along with its own stack, O2CB.

It is easily configured. The O2CB cluster stack configuration involves editing two files, one for cluster layout and the other for cluster timeouts.

It is very efficient. The file system consumes very little resources. It is used to store virtual machine images in limited memory environments like Xen and KVM.

In summary, OCFS2 is an efficient, easily configured, modular, quickly installed, fully integrated and compatible, feature-rich, architecture and endian neutral, cache coherent, ordered data journaling, POSIX-compliant, shared disk cluster file system.

OVERVIEW

OCFS2 is a general-purpose shared-disk cluster file system for Linux capable of providing both high performance and high availability.

As it provides local file system semantics, it can be used with almost all applications. Cluster-aware applications can make use of cache-coherent parallel I/Os from multiple nodes to scale out applications easily. Other applications can make use of the clustering facilities to fail-over running application in the event of a node failure.

The notable features of the file system are:

Tunable Block size
    The file system supports block sizes of 512, 1K, 2K and 4K bytes. 4KB is almost always recommended. This feature is available in all releases of the file system.

Tunable Cluster size
    A cluster size is also referred to as an allocation unit. The file system supports cluster sizes of 4K, 8K, 16K, 32K, 64K, 128K, 256K, 512K and 1M bytes. For most use cases, 4KB is recommended. However, a larger value is recommended for volumes hosting mostly very large files like database files, virtual machine images, etc. A large cluster size allows the file system to store large files more efficiently. This feature is available in all releases of the file system.

Endian and Architecture neutral
    The file system can be mounted concurrently on nodes having different architectures. Like 32-bit, 64-bit, little-endian (x86, x86_64, ia64) and big-endian (ppc64, s390x). This feature is available in all releases of the file system.

Buffered, Direct, Asynchronous, Splice and Memory Mapped I/O modes
    The file system supports all modes of I/O for maximum flexibility and performance. It also supports cluster-wide shared writeable mmap(2). The support for bufferred, direct and asynchronous I/O is available in all releases. The support for splice I/O was added in Linux kernel 2.6.20 and for shared writeable map(2) in 2.6.23.

Multiple Cluster Stacks
    The file system includes a flexible framework to allow it to function with userspace cluster stacks like Pacemaker (pcmk) and CMAN (cman), its own in-kernel cluster stack o2cb and no cluster stack.

    The support for o2cb cluster stack is available in all releases.

    The support for no cluster stack, or local mount, was added in Linux kernel 2.6.20.

    The support for userspace cluster stack was added in Linux kernel 2.6.26.

Journaling
    The file system supports both ordered (default) and writeback data journaling modes to provide file system consistency in the event of power failure or system crash. It uses JBD2 in Linux kernel 2.6.28 and later. It used JBD in earlier kernels.

Extent-based Allocations
    The file system allocates and tracks space in ranges of clusters. This is unlike block based file systems that have to track each and every block. This feature allows the file system to be very efficient when dealing with both large volumes and large files. This feature is available in all releases of the file system.

Sparse files
    Sparse files are files with holes. With this feature, the file system delays allocating space until a write is issued to a cluster. This feature was added in Linux kernel 2.6.22 and requires enabling on-disk feature sparse.

Unwritten Extents
    An unwritten extent is also referred to as user pre-allocation. It allows an application to request a range of clusters to be allocated, but not initialized, within a file. Pre-allocation allows the file system to optimize the data layout with fewer, larger extents. It also provides a performance boost, delaying initialization until the user writes to the clusters. This feature was added in Linux kernel 2.6.23 and requires enabling on-disk feature unwritten.

Hole Punching
    Hole punching allows an application to remove arbitrary allocated regions within a file. Creating holes, essentially. This is more efficient than zeroing the same extents. This feature is especially useful in virtualized environments as it allows a block discard in a guest file system to be converted to a hole punch in the host file system thus allowing users to reduce disk space usage. This feature was added in Linux kernel 2.6.23 and requires enabling on-disk features sparse and unwritten.

Inline-data
    Inline data is also referred to as data-in-inode as it allows storing small files and directories in the inode block. This not only saves space but also has a positive impact on cold-cache directory and file operations. The data is transparently moved out to an extent when it no longer fits inside the inode block. This feature was added in Linux kernel 2.6.24 and requires enabling on-disk feature inline-data.

REFLINK
    REFLINK is also referred to as fast copy. It allows users to atomically (and instantly) copy regular files. In other words, create multiple writeable snapshots of regular files. It is called REFLINK because it looks and feels more like a (hard) link(2) than a traditional snapshot. Like a link, it is a regular user operation, subject to the security attributes of the inode being reflinked and not to the super user privileges typically required to create a snapshot. Like a link, it operates within a file system. But unlike a link, it links the inodes at the data extent level allowing each reflinked inode to grow independently as and when written to. Up to four billion inodes can share a data extent. This feature was added in Linux kernel 2.6.32 and requires enabling on-disk feature refcount.

Allocation Reservation
    File contiguity plays an important role in file system performance. When a file is fragmented on disk, reading and writing to the file involves many seeks, leading to lower throughput. Contiguous files, on the other hand, minimize seeks, allowing the disks to perform IO at the maximum rate.

    With allocation reservation, the file system reserves a window in the bitmap for all extending files allowing each to grow as contiguously as possible. As this extra space is not actually allocated, it is available for use by other files if the need arises. This feature was added in Linux kernel 2.6.35 and can be tuned using the mount option resv_level.

Indexed Directories
    An indexed directory allows users to perform quick lookups of a file in very large directories. It also results in faster creates and unlinks and thus provides better overall performance. This feature was added in Linux kernel 2.6.30 and requires enabling on-disk feature indexed-dirs.

File Attributes
    This refers to EXT2-style file attributes, such as immutable, modified using chattr(1) and queried using lsattr(1). This feature was added in Linux kernel 2.6.19.

Extended Attributes
    An extended attribute refers to a name:value pair than can be associated with file system objects like regular files, directories, symbolic links, etc. OCFS2 allows associating an unlimited number of attributes per object. The attribute names can be up to 255 bytes in length, terminated by the first NUL character. While it is not required, printable names (ASCII) are recommended. The attribute values can be up to 64 KB of arbitrary binary data. These attributes can be modified and listed using standard Linux utilities setfattr(1) and getfattr(1). This feature was added in Linux kernel 2.6.29 and requires enabling on-disk feature xattr.

Metadata Checksums
    This feature allows the file system to detect silent corruptions in all metadata blocks like inodes and directories. This feature was added in Linux kernel 2.6.29 and requires enabling on-disk feature metaecc.

POSIX ACLs and Security Attributes
    POSIX ACLs allows assigning fine-grained discretionary access rights for files and directories. This security scheme is a lot more flexible than the traditional file access permissions that imposes a strict user-group-other model.

    Security attributes allow the file system to support other security regimes like SELinux, SMACK, AppArmor, etc.

    Both these security extensions were added in Linux kernel 2.6.29 and requires enabling on-disk feature xattr.

User and Group Quotas
    This feature allows setting up usage quotas on user and group basis by using the standard utilities like quota(1), setquota(8), quotacheck(8), and quotaon(8). This feature was added in Linux kernel 2.6.29 and requires enabling on-disk features usrquota and grpquota.

Unix File Locking
    The Unix operating system has historically provided two system calls to lock files. flock(2) or BSD locking and fcntl(2) or POSIX locking. OCFS2 extends both file locks to the cluster. File locks taken on one node interact with those taken on other nodes.

    The support for clustered flock(2) was added in Linux kernel 2.6.26. All flock(2) options are supported, including the kernels ability to cancel a lock request when an appropriate kill signal is received by the user. This feature is supported with all cluster-stacks including o2cb.

    The support for clustered fcntl(2) was added in Linux kernel 2.6.28. But because it requires group communication to make the locks coherent, it is only supported with userspace cluster stacks, pcmk and cman and not with the default cluster stack o2cb.

Comprehensive Tools Support
    The file system has a comprehensive EXT3-style toolset that tries to use similar parameters for ease-of-use. It includes mkfs.ocfs2(8) (format), tunefs.ocfs2(8) (tune), fsck.ocfs2(8) (check), debugfs.ocfs2(8) (debug), etc.

Online Resize
    The file system can be dynamically grown using tunefs.ocfs2(8). This feature was added in Linux kernel 2.6.25.

RECENT CHANGES

The O2CB cluster stack has a global heartbeat mode. It allows users to specify heartbeat regions that are consistent across all nodes. The cluster stack also allows online addition and removal of both nodes and heartbeat regions.

o2cb(8) is the new cluster configuration utility. It is an easy to use utility that allows users to create the cluster configuration on a node that is not part of the cluster. It replaces the older utility o2cb_ctl(8) which has being deprecated.

ocfs2console(8) has been obsoleted.

o2info(8) is a new utility that can be used to provide file system information. It allows non-privileged users to see the enabled file system features, block and cluster sizes, extended file stat, free space fragmentation, etc.

o2hbmonitor(8) is a o2hb heartbeat monitor. It is an extremely light weight utility that logs messages to the system logger once the heartbeat delay exceeds the warn threshold. This utility is useful in identifying volumes encountering I/O delays.

debugfs.ocfs2(8) has some new commands. net_stats shows the o2net message times between various nodes. This is useful in identifying nodes are that slowing down the cluster operations. stat_sysdir allows the user to dump the entire system directory that can be used to debug issues. grpextents dumps the complete free space fragmentation in the cluster group allocator.

mkfs.ocfs2(8) now enables xattr, indexed-dirs, discontig-bg, refcount, extended-slotmap and clusterinfo feature flags by default, in addition to the older defaults, sparse, unwritten and inline-data.

mount.ocfs2(8) allows users to specify the level of cache coherency between nodes. By default the file system operates in full coherency mode that also serializes the direct I/Os. While this mode is technically correct, it limits the I/O thruput in a clustered database. This mount option allows the user to limit the cache coherency to only the buffered I/Os to allow multiple nodes to do concurrent direct writes to the same file. This feature works with Linux kernel 2.6.37 and later.

COMPATIBILITY

The OCFS2 development teams goes to great lengths to maintain compatibility. It attempts to maintain both on-disk and network protocol compatibility across all releases of the file system. It does so even while adding new features that entail on-disk format and network protocol changes. To do this successfully, it follows a few rules:

1. The on-disk format changes are managed by a set of feature flags that can be turned on and off. The file system in kernel detects these features during mount and continues only if it understands all the features. Users encountering this have the option of either disabling that feature or upgrading the file system to a newer release.

2. The latest release of ocfs2-tools is compatible with all versions of the file system. All utilities detect the features enabled on disk and continue only if it understands all the features. Users encountering this have to upgrade the tools to a newer release.

3. The network protocol version is negotiated by the nodes to ensure all nodes understand the active protocol version.

FEATURE FLAGS
    The feature flags are split into three categories, namely, Compat, Incompat and RO Compat.

    Compat, or compatible, is a feature that the file system does not need to fully understand to safely read/write to the volume. An example of this is the backup-super feature that added the capability to backup the super block in multiple locations in the file system. As the backup super blocks are typically not read nor written to by the file system, an older file system can safely mount a volume with this feature enabled.

    Incompat, or incompatible, is a feature that the file system needs to fully understand to read/write to the volume. Most features fall under this category.

    RO Compat, or read-only compatible, is a feature that the file system needs to fully understand to write to the volume. Older software can safely read a volume with this feature enabled. An example of this would be user and group quotas. As quotas are manipulated only when the file system is written to, older software can safely mount such volumes in read-only mode.

    The list of feature flags, the version of the kernel it was added in, the earliest version of the tools that understands it, etc., is as follows:

    Feature Flags | Kernel Version | Tools Version | Category | Hex Value
    backup-super | All | ocfs2-tools 1.2 | Compat | 1
    strict-journal-super | All | All | Compat | 2
    local | Linux 2.6.20 | ocfs2-tools 1.2 | Incompat | 8
    sparse | Linux 2.6.22 | ocfs2-tools 1.4 | Incompat | 10
    inline-data | Linux 2.6.24 | ocfs2-tools 1.4 | Incompat | 40
    extended-slotmap | Linux 2.6.27 | ocfs2-tools 1.6 | Incompat | 100
    xattr | Linux 2.6.29 | ocfs2-tools 1.6 | Incompat | 200
    indexed-dirs | Linux 2.6.30 | ocfs2-tools 1.6 | Incompat | 400
    metaecc | Linux 2.6.29 | ocfs2-tools 1.6 | Incompat | 800
    refcount | Linux 2.6.32 | ocfs2-tools 1.6 | Incompat | 1000
    discontig-bg | Linux 2.6.35 | ocfs2-tools 1.6 | Incompat | 2000
    clusterinfo | Linux 2.6.37 | ocfs2-tools 1.8 | Incompat | 4000
    unwritten | Linux 2.6.23 | ocfs2-tools 1.4 | RO Compat | 1
    grpquota | Linux 2.6.29 | ocfs2-tools 1.6 | RO Compat | 2
    usrquota | Linux 2.6.29 | ocfs2-tools 1.6 | RO Compat | 4

    To query the features enabled on a volume, do:

    $ o2info --fs-features /dev/sdf1
    backup-super strict-journal-super sparse extended-slotmap inline-data xattr
    indexed-dirs refcount discontig-bg clusterinfo unwritten

ENABLING AND DISABLING FEATURES

    The format utility, mkfs.ocfs2(8), allows a user to enable and disable specific features using the fs-features option. The features are provided as a comma separated list. The enabled features are listed as is. The disabled features are prefixed with no. The example below shows the file system being formatted with sparse disabled and inline-data enabled.

    # mkfs.ocfs2 --fs-features=nosparse,inline-data /dev/sda1

    After formatting, the users can toggle features using the tune utility, tunefs.ocfs2(8). This is an offline operation. The volume needs to be umounted across the cluster. The example below shows the sparse feature being enabled and inline-data disabled.

    # tunefs.ocfs2 --fs-features=sparse,noinline-data /dev/sda1

    Care should be taken before enabling and disabling features. Users planning to use a volume with an older version of the file system will be better of not enabling newer features as turning disabling may not succeed.

    An example would be disabling the sparse feature; this requires filling every hole. The operation can only succeed if the file system has enough free space.

DETECTING FEATURE INCOMPATIBILITY

    Say one tries to mount a volume with an incompatible feature. What happens then? How does one detect the problem? How does one know the name of that incompatible feature?

    To begin with, one should look for error messages in dmesg(8). Mount failures that are due to an incompatible feature will always result in an error message like the following:

    ERROR: couldn't mount because of unsupported optional features (200).

    Here the file system is unable to mount the volume due to an unsupported optional feature. That means that that feature is an Incompat feature. By referring to the table above, one can then deduce that the user failed to mount a volume with the xattr feature enabled. (The value in the error message is in hexadecimal.)

    Another example of an error message due to incompatibility is as follows:

    ERROR: couldn't mount RDWR because of unsupported optional features (1).

    Here the file system is unable to mount the volume in the RW mode. That means that that feature is a RO Compat feature. Another look at the table and it becomes apparent that the volume had the unwritten feature enabled.

    In both cases, the user has the option of disabling the feature. In the second case, the user has the choice of mounting the volume in the RO mode.

GETTING STARTED

The OCFS2 software is split into two components, namely, kernel and tools. The kernel component includes the core file system and the cluster stack, and is packaged along with the kernel. The tools component is packaged as ocfs2-tools and needs to be specifically installed. It provides utilities to format, tune, mount, debug and check the file system.

To install ocfs2-tools, refer to the package handling utility in in your distributions.

The next step is selecting a cluster stack. The options include:

A. No cluster stack, or local mount.

B. In-kernel o2cb cluster stack with local or global heartbeat.

C. Userspace cluster stacks pcmk or cman.

The file system allows changing cluster stacks easily using tunefs.ocfs2(8). To list the cluster stacks stamped on the OCFS2 volumes, do:

# mounted.ocfs2 -d
Device     Stack  Cluster     F  UUID                              Label
/dev/sdb1  o2cb   webcluster  G  DCDA2845177F4D59A0F2DCD8DE507CC3  hbvol1
/dev/sdc1  None                  23878C320CF3478095D1318CB5C99EED  localmount
/dev/sdd1  o2cb   webcluster  G  8AB016CD59FC4327A2CDAB69F08518E3  webvol
/dev/sdg1  o2cb   webcluster  G  77D95EF51C0149D2823674FCC162CF8B  logsvol
/dev/sdh1  o2cb   webcluster  G  BBA1DBD0F73F449384CE75197D9B7098  scratch

NON-CLUSTERED OR LOCAL MOUNT

    To format a OCFS2 volume as a non-clustered (local) volume, do:

    # mkfs.ocfs2 -L "mylabel" --fs-features=local /dev/sda1

    To convert an existing clustered volume to a non-clustered volume, do:

    # tunefs.ocfs2 --fs-features=local /dev/sda1

    Non-clustered volumes do not interact with the cluster stack. One can have both clustered and non-clustered volumes mounted at the same time.

    While formatting a non-clustered volume, users should consider the possibility of later converting that volume to a clustered one. If there is a possibility of that, then the user should add enough node-slots using the -N option. Adding node-slots during format creates journals with large extents. If created later, then the journals will be fragmented which is not good for performance.

CLUSTERED MOUNT WITH O2CB CLUSTER STACK

    Only one of the two heartbeat mode can be active at any one time. Changing heartbeat modes is an offline operation.

    Both heartbeat modes require /etc/ocfs2/cluster.conf and /etc/sysconfig/o2cb to be populated as described in ocfs2.cluster.conf(5) and o2cb.sysconfig(5) respectively. The only difference in set up between the two modes is that global requires heartbeat devices to be configured whereas local does not.

    Refer o2cb(7) for more information.

    LOCAL HEARTBEAT
        This is the default heartbeat mode. The user needs to populate the configuration files as described in ocfs2.cluster.conf(5) and o2cb.sysconfig(5). In this mode, the cluster stack heartbeats on all mounted volumes. Thus, one does not have to specify heartbeat devices in cluster.conf.

        Once configured, the o2cb cluster stack can be onlined and offlined as follows:

        # service o2cb online
        Setting cluster stack "o2cb": OK
        Registering O2CB cluster "webcluster": OK
        Setting O2CB cluster timeouts : OK

        # service o2cb offline
        Clean userdlm domains: OK
        Stopping O2CB cluster webcluster: OK
        Unregistering O2CB cluster "webcluster": OK

    GLOBAL HEARTBEAT
        The configuration is similar to local heartbeat. The one additional step in this mode is that it requires heartbeat devices to be also configured.

        These heartbeat devices are OCFS2 formatted volumes with global heartbeat enabled on disk. These volumes can later be mounted and used as clustered file systems.

        The steps to format a volume with global heartbeat enabled is listed in o2cb(7). Also listed there is listing all volumes with the cluster stack stamped on disk.

        In this mode, the heartbeat is started when the cluster is onlined and stopped when the cluster is offlined.

        # service o2cb online
        Setting cluster stack "o2cb": OK
        Registering O2CB cluster "webcluster": OK
        Setting O2CB cluster timeouts : OK
        Starting global heartbeat for cluster "webcluster": OK

        # service o2cb offline
        Clean userdlm domains: OK
        Stopping global heartbeat on cluster "webcluster": OK
        Stopping O2CB cluster webcluster: OK
        Unregistering O2CB cluster "webcluster": OK

        # service o2cb status
        Driver for "configfs": Loaded
        Filesystem "configfs": Mounted
        Stack glue driver: Loaded
        Stack plugin "o2cb": Loaded
        Driver for "ocfs2_dlmfs": Loaded
        Filesystem "ocfs2_dlmfs": Mounted
        Checking O2CB cluster "webcluster": Online
          Heartbeat dead threshold: 31
          Network idle timeout: 30000
          Network keepalive delay: 2000
          Network reconnect delay: 2000
          Heartbeat mode: Global
        Checking O2CB heartbeat: Active
          77D95EF51C0149D2823674FCC162CF8B /dev/sdg1
        Nodes in O2CB cluster: 92 96

CLUSTERED MOUNT WITH USERSPACE CLUSTER STACK

    Configure and online the userspace stack pcmk or cman before using tunefs.ocfs2(8) to update the cluster stack on disk.

    # tunefs.ocfs2 --update-cluster-stack /dev/sdd1
    Updating on-disk cluster information to match the running cluster.
    DANGER: YOU MUST BE ABSOLUTELY SURE THAT NO OTHER NODE IS USING THIS
    FILESYSTEM BEFORE MODIFYING ITS CLUSTER CONFIGURATION.
    Update the on-disk cluster information? y

    Refer to the cluster stack documentation for information on starting and stopping the cluster stack.

FILE SYSTEM UTILITIES

This sections lists the utilities that are used to manage the OCFS2 file systems. This includes tools to format, tune, check, mount, debug the file system. Each utility has a man page that lists its capabilities in detail.

mkfs.ocfs2(8)
    This is the file system format utility. All volumes have to be formatted prior to its use. As this utility overwrites the volume, use it with care. Double check to ensure the volume is not in use on any node in the cluster.

    As a precaution, the utility will abort if the volume is locally mounted. It also detects use across the cluster if used by OCFS2. But these checks are not comprehensive and can be overridden. So use it with care.

    While it is not always required, the cluster should be online.

tunefs.ocfs2(8)
    This is the file system tune utility. It allows users to change certain on-disk parameters like label, uuid, number of node-slots, volume size and the size of the journals. It also allows turning on and off the file system features as listed above.

    This utility requires the cluster to be online.

fsck.ocfs2(8)
    This is the file system check utility. It detects and fixes on-disk errors. All the check codes and their fixes are listed in fsck.ocfs2.checks(8).

    This utility requires the cluster to be online to ensure the volume is not in use on another node and to prevent the volume from being mounted for the duration of the check.

mount.ocfs2(8)
    This is the file system mount utility. It is invoked indirectly by the mount(8) utility.

    This utility detects the cluster status and aborts if the cluster is offline or does not match the cluster stamped on disk.

o2cluster(8)
    This is the file system cluster stack update utility. It allows the users to update the on-disk cluster stack to the one provided.

    This utility only updates the disk if the utility is reasonably assured that the file system is not in use on any node.

o2info(1)
    This is the file system information utility. It provides information like the features enabled on disk, block size, cluster size, free space fragmentation, etc.

    It can be used by both privileged and non-privileged users. Users having read permission on the device can provide the path to the device. Other users can provide the path to a file on a mounted file system.

debugfs.ocfs2(8)
    This is the file system debug utility. It allows users to examine all file system structures including walking directory structures, displaying inodes, backing up files, etc., without mounting the file system.

    This utility requires the user to have read permission on the device.

o2image(8)
    This is the file system image utility. It allows users to copy the file system metadata skeleton, including the inodes, directories, bitmaps, etc. As it excludes data, it shrinks the size of the file system tremendously.

    The image file created can be used in debugging on-disk corruptions.

mounted.ocfs2(8)
    This is the file system detect utility. It detects all OCFS2 volumes in the system and lists its label, uuid and cluster stack.

O2CB CLUSTER STACK UTILITIES

This sections lists the utilities that are used to manage O2CB cluster stack. Each utility has a man page that lists its capabilities in detail.

o2cb(8)
    This is the cluster configuration utility. It allows users to update the cluster configuration by adding and removing nodes and heartbeat regions. This utility is used by the o2cb init script to online and offline the cluster.

    This is a new utility and replaces o2cb_ctl(8) which has been deprecated.

ocfs2_hb_ctl(8)
    This is the cluster heartbeat utility. It allows users to start and stop local heartbeat. This utility is invoked by mount.ocfs2(8) and should not be invoked directly by the user.

o2hbmonitor(8)
    This is the disk heartbeat monitor. It tracks the elapsed time since the last heartbeat and logs warnings once that time exceeds the warn threshold.

FILE SYSTEM NOTES

This section includes some useful notes that may prove helpful to the user.

BALANCED CLUSTER
    A cluster is a computer. This is a fact and not a slogan. What this means is that an errant node in the cluster can affect the behavior of other nodes. If one node is slow, the cluster operations will slow down on all nodes. To prevent that, it is best to have a balanced cluster. This is a cluster that has equally powered and loaded nodes.

    The standard recommendation for such clusters is to have identical hardware and software across all the nodes. However, that is not a hard and fast rule. After all, we have taken the effort to ensure that OCFS2 works in a mixed architecture environment.

    If one uses OCFS2 in a mixed architecture environment, try to ensure that the nodes are equally powered and loaded. The use of a load balancer can assist with the latter. Power refers to the number of processors, speed, amount of memory, I/O throughput, network bandwidth, etc. In reality, having equally powered heterogeneous nodes is not always practical. In that case, make the lower node numbers more powerful than the higher node numbers. The O2CB cluster stack favors lower node numbers in all of its tiebreaking logic.

    This is not to suggest you should add a single core node in a cluster of quad cores. No amount of node number juggling will help you there.

FILE DELETION
    In Linux, rm(1) removes the directory entry. It does not necessarily delete the corresponding inode. But by removing the directory entry, it gives the illusion that the inode has been deleted. This puzzles users when they do not see a corresponding up-tick in the reported free space. The reason is that inode deletion has a few more hurdles to cross.

    First is the hard link count, that indicates the number of directory entries pointing to that inode. As long as an inode has one or more directory entries pointing to it, it cannot be deleted. The file system has to wait for the removal of all those directory entries. In other words, wait for that count to drop to zero.

    The second hurdle is the POSIX semantics allowing files to be unlinked even while they are in-use. In OCFS2, that translates to in-use across the cluster. The file system has to wait for all processes across the cluster to stop using the inode.

    Once these conditions are met, the inode is deleted and the freed space is visible after the next sync.

    Now the amount of space freed depends on the allocation. Only space that is actually allocated to that inode is freed. The example below shows a sparsely allocated file of size 51TB of which only 2.4GB is actually allocated.

    $ ls -lsh largefile
    2.4G -rw-r--r-- 1 mark mark 51T Sep 29 15:04 largefile

    Furthermore, for reflinked files, only private extents are freed. Shared extents are freed when the last inode accessing it, is deleted. The example below shows a 4GB file that shares 3GB with other reflinked files. Deleting it will increase the free space by 1GB. However, if it is the only remaining file accessing the shared extents, the full 4G will be freed. (More information on the shared-du(1) utility is provided below.)

    $ shared-du -m -c --shared-size reflinkedfile
    4000    (3000)  reflinkedfile

    The deletion itself is a multi-step process. Once the hard link count falls to zero, the inode is moved to the orphan_dir system directory where it remains until the last process, across the cluster, stops using the inode. Then the file system frees the extents and adds the freed space count to the truncate_log system file where it remains until the next sync. The freed space is made visible to the user only after that sync.

DIRECTORY LISTING
    ls(1) may be a simple command, but it is not cheap. What is expensive is not the part where it reads the directory listing, but the second part where it reads all the inodes, also referred as an inode stat(2). If the inodes are not in cache, this can entail disk I/O. Now, while a cold cache inode stat(2) is expensive in all file systems, it is especially so in a clustered file system as it needs to take a cluster lock on each inode.

    A hot cache stat(2), on the other hand, has shown to perform on OCFS2 like it does on EXT3.

    In other words, the second ls(1) will be quicker than the first. However, it is not guaranteed. Say you have a million files in a file system and not enough kernel memory to cache all the inodes. In that case, each ls(1) will involve some cold cache stat(2)s.

ALLOCATION RESERVATION
    Allocation reservation allows multiple concurrently extending files to grow as contiguously as possible. One way to demonstrate its functioning is to run a script that extends multiple files in a circular order. The script below does that by writing one hundred 4KB chunks to four files, one after another.

    $ for i in $(seq 0 99);
    > do
    >   for j in $(seq 4);
    >   do
    >     dd if=/dev/zero of=file$j bs=4K count=1 seek=$i;
    >   done;
    > done;

    When run on a system running Linux kernel 2.6.34 or earlier, we end up with files with 100 extents each. That is full fragmentation. As the files are being extended one after another, the on-disk allocations are fully interleaved.

    $ filefrag file1 file2 file3 file4
    file1: 100 extents found
    file2: 100 extents found
    file3: 100 extents found
    file4: 100 extents found

    When run on a system running Linux kernel 2.6.35 or later, we see files with 7 extents each. That is a lot fewer than before. Fewer extents mean more on-disk contiguity and that always leads to better overall performance.

    $ filefrag file1 file2 file3 file4
    file1: 7 extents found
    file2: 7 extents found
    file3: 7 extents found
    file4: 7 extents found

REFLINK OPERATION
    This feature allows a user to create a writeable snapshot of a regular file. In this operation, the file system creates a new inode with the same extent pointers as the original inode. Multiple inodes are thus able to share data extents. This adds a twist in file system administration because none of the existing file system utilities in Linux expect this behavior. du(1), a utility to used to compute file space usage, simply adds the blocks allocated to each inode. As it does not know about shared extents, it over estimates the space used. Say, we have a 5GB file in a volume having 42GB free.

    $ ls -l
    total 5120000
    -rw-r--r--  1 jeff jeff   5242880000 Sep 24 17:15 myfile

    $ du -m myfile*
    5000    myfile

    $ df -h .
    Filesystem            Size  Used Avail Use% Mounted on
    /dev/sdd1             50G   8.2G   42G  17% /ocfs2

    If we were to reflink it 4 times, we would expect the directory listing to report five 5GB files, but the df(1) to report no loss of available space. du(1), on the other hand, would report the disk usage to climb to 25GB.

    $ reflink myfile myfile-ref1
    $ reflink myfile myfile-ref2
    $ reflink myfile myfile-ref3
    $ reflink myfile myfile-ref4

    $ ls -l
    total 25600000
    -rw-r--r--  1 jeff jeff   5242880000 Sep 24 17:15 myfile
    -rw-r--r--  1 jeff jeff   5242880000 Sep 24 17:16 myfile-ref1
    -rw-r--r--  1 jeff jeff   5242880000 Sep 24 17:16 myfile-ref2
    -rw-r--r--  1 jeff jeff   5242880000 Sep 24 17:16 myfile-ref3
    -rw-r--r--  1 jeff jeff   5242880000 Sep 24 17:16 myfile-ref4

    $ df -h .
    Filesystem            Size  Used Avail Use% Mounted on
    /dev/sdd1             50G   8.2G   42G  17% /ocfs2

    $ du -m myfile*
    5000    myfile
    5000    myfile-ref1
    5000    myfile-ref2
    5000    myfile-ref3
    5000    myfile-ref4
    25000 total

    Enter shared-du(1), a shared extent-aware du. This utility reports the shared extents per file in parenthesis and the overall footprint. As expected, it lists the overall footprint at 5GB. One can view the details of the extents using shared-filefrag(1). Both these utilities are available at http://oss.oracle.com/~smushran/reflink-tools/. We are currently in the process of pushing the changes to the upstream maintainers of these utilities.

    $ shared-du -m -c --shared-size myfile*
    5000    (5000)  myfile
    5000    (5000)  myfile-ref1
    5000    (5000)  myfile-ref2
    5000    (5000)  myfile-ref3
    5000    (5000)  myfile-ref4
    25000 total
    5000 footprint

    # shared-filefrag -v myfile
    Filesystem type is: 7461636f
    File size of myfile is 5242880000 (1280000 blocks, blocksize 4096)
    ext logical physical expected length flags
    0         0  2247937            8448
    1      8448  2257921  2256384  30720
    2     39168  2290177  2288640  30720
    3     69888  2322433  2320896  30720
    4    100608  2354689  2353152  30720
    7    192768  2451457  2449920  30720
     . . .
    37  1073408  2032129  2030592  30720 shared
    38  1104128  2064385  2062848  30720 shared
    39  1134848  2096641  2095104  30720 shared
    40  1165568  2128897  2127360  30720 shared
    41  1196288  2161153  2159616  30720 shared
    42  1227008  2193409  2191872  30720 shared
    43  1257728  2225665  2224128  22272 shared,eof
    myfile: 44 extents found

DATA COHERENCY
    One of the challenges in a shared file system is data coherency when multiple nodes are writing to the same set of files. NFS, for example, provides close-to-open data coherency that results in the data being flushed to the server when the file is closed on the client. This leaves open a wide window for stale data being read on another node.

    A simple test to check the data coherency of a shared file system involves concurrently appending the same file. Like running "uname -a >>/dir/file" using a parallel distributed shell like dsh or pconsole. If coherent, the file will contain the results from all nodes.

    # dsh -R ssh -w node32,node33,node34,node35 "uname -a >> /ocfs2/test"
    # cat /ocfs2/test
    Linux node32 2.6.32-10 #1 SMP Fri Sep 17 17:51:41 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux
    Linux node35 2.6.32-10 #1 SMP Fri Sep 17 17:51:41 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux
    Linux node33 2.6.32-10 #1 SMP Fri Sep 17 17:51:41 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux
    Linux node34 2.6.32-10 #1 SMP Fri Sep 17 17:51:41 EDT 2010 x86_64 x86_64 x86_64 GNU/Linux

    OCFS2 is a fully cache coherent cluster file system.

DISCONTIGUOUS BLOCK GROUP
    Most file systems pre-allocate space for inodes during format. OCFS2 dynamically allocates this space when required.

    However, this dynamic allocation has been problematic when the free space is very fragmented, because the file system required the inode and extent allocators to grow in contiguous fixed-size chunks.

    The discontiguous block group feature takes care of this problem by allowing the allocators to grow in smaller, variable-sized chunks.

    This feature was added in Linux kernel 2.6.35 and requires enabling on-disk feature discontig-bg.

BACKUP SUPER BLOCKS
    A file system super block stores critical information that is hard to recreate. In OCFS2, it stores the block size, cluster size, and the locations of the root and system directories, among other things. As this block is close to the start of the disk, it is very susceptible to being overwritten by an errant write. Say, dd if=file of=/dev/sda1.

    Backup super blocks are copies of the super block. These blocks are dispersed in the volume to minimize the chances of being overwritten. On the small chance that the original gets corrupted, the backups are available to scan and fix the corruption.

    mkfs.ocfs2(8) enables this feature by default. Users can disable this by specifying --fs-features=nobackup-super during format.

    o2info(1) can be used to view whether the feature has been enabled on a device.

    # o2info --fs-features /dev/sdb1
    backup-super strict-journal-super sparse extended-slotmap inline-data xattr
    indexed-dirs refcount discontig-bg clusterinfo unwritten

    In OCFS2, the super block is on the third block. The backups are located at the 1G, 4G, 16G, 64G, 256G and 1T byte offsets. The actual number of backup blocks depends on the size of the device. The super block is not backed up on devices smaller than 1GB.

    fsck.ocfs2(8) refers to these six offsets by numbers, 1 to 6. Users can specify any backup with the -r option to recover the volume. The example below uses the second backup. If successful, fsck.ocfs2(8) overwrites the corrupted super block with the backup.

    # fsck.ocfs2 -f -r 2 /dev/sdb1
    fsck.ocfs2 1.8.0
    [RECOVER_BACKUP_SUPERBLOCK] Recover superblock information from backup block#1048576? <n> y
    Checking OCFS2 filesystem in /dev/sdb1:
      Label:              webhome
      UUID:               B3E021A2A12B4D0EB08E9E986CDC7947
      Number of blocks:   13107196
      Block size:         4096
      Number of clusters: 13107196
      Cluster size:       4096
      Number of slots:    8

    /dev/sdb1 was run with -f, check forced.
    Pass 0a: Checking cluster allocation chains
    Pass 0b: Checking inode allocation chains
    Pass 0c: Checking extent block allocation chains
    Pass 1: Checking inodes and blocks.
    Pass 2: Checking directory entries.
    Pass 3: Checking directory connectivity.
    Pass 4a: checking for orphaned inodes
    Pass 4b: Checking inodes link counts.
    All passes succeeded.

SYNTHETIC FILE SYSTEMS
    The OCFS2 development effort included two synthetic file systems, configfs and dlmfs. It also makes use of a third, debugfs.

    configfs
        configfs has since been accepted as a generic kernel component and is also used by netconsole and fs/dlm. OCFS2 tools use it to communicate the list of nodes in the cluster, details of the heartbeat device, cluster timeouts, and so on to the in-kernel node manager. The o2cb init script mounts this file system at /sys/kernel/config.

    dlmfs
        dlmfs exposes the in-kernel o2dlm to the user-space. While it was developed primarily for OCFS2 tools, it has seen usage by others looking to add a cluster locking dimension in their applications. Users interested in doing the same should look at the libo2dlm library provided by ocfs2-tools. The o2cb init script mounts this file system at /dlm.

    debugfs
        OCFS2 uses debugfs to expose its in-kernel information to user space. For example, listing the file system cluster locks, dlm locks, dlm state, o2net state, etc. Users can access the information by mounting the file system at /sys/kernel/debug. To automount, add the following to /etc/fstab: debugfs /sys/kernel/debug debugfs defaults 0 0

DISTRIBUTED LOCK MANAGER
    One of the key technologies in a cluster is the lock manager, which maintains the locking state of all resources across the cluster. An easy implementation of a lock manager involves designating one node to handle everything. In this model, if a node wanted to acquire a lock, it would send the request to the lock manager. However, this model has a weakness: lock manager’s death causes the cluster to seize up.

    A better model is one where all nodes manage a subset of the lock resources. Each node maintains enough information for all the lock resources it is interested in. On event of a node death, the remaining nodes pool in the information to reconstruct the lock state maintained by the dead node. In this scheme, the locking overhead is distributed amongst all the nodes. Hence, the term distributed lock manager.

    O2DLM is a distributed lock manager. It is based on the specification titled "Programming Locking Application" written by Kristin Thomas and is available at the following link. http://opendlm.sourceforge.net/cvsmirror/opendlm/docs/dlmbook_final.pdf

DLM DEBUGGING
    O2DLM has a rich debugging infrastructure that allows it to show the state of the lock manager, all the lock resources, among other things. The figure below shows the dlm state of a nine-node cluster that has just lost three nodes: 12, 32, and 35. It can be ascertained that node 7, the recovery master, is currently recovering node 12 and has received the lock states of the dead node from all other live nodes.

    # cat /sys/kernel/debug/o2dlm/45F81E3B6F2B48CCAAD1AE7945AB2001/dlm_state
    Domain: 45F81E3B6F2B48CCAAD1AE7945AB2001  Key: 0x10748e61
    Thread Pid: 24542  Node: 7  State: JOINED
    Number of Joins: 1  Joining Node: 255
    Domain Map: 7 31 33 34 40 50
    Live Map: 7 31 33 34 40 50
    Lock Resources: 48850 (439879)
    MLEs: 0 (1428625)
      Blocking: 0 (1066000)
      Mastery: 0 (362625)
      Migration: 0 (0)
    Lists: Dirty=Empty  Purge=Empty  PendingASTs=Empty  PendingBASTs=Empty
    Purge Count: 0  Refs: 1
    Dead Node: 12
    Recovery Pid: 24543  Master: 7  State: ACTIVE
    Recovery Map: 12 32 35
    Recovery Node State:
            7 - DONE
            31 - DONE
            33 - DONE
            34 - DONE
            40 - DONE
            50 - DONE

    The figure below shows the state of a dlm lock resource that is mastered (owned) by node 25, with 6 locks in the granted queue and node 26 holding the EX (writelock) lock on that resource.

    # debugfs.ocfs2 -R "dlm_locks M000000000000000022d63c00000000" /dev/sda1
    Lockres: M000000000000000022d63c00000000   Owner: 25    State: 0x0
    Last Used: 0      ASTs Reserved: 0    Inflight: 0    Migration Pending: No
    Refs: 8    Locks: 6    On Lists: None
    Reference Map: 26 27 28 94 95
     Lock-Queue  Node  Level  Conv  Cookie           Refs  AST  BAST  Pending-Action
     Granted     94    NL     -1    94:3169409       2     No   No    None
     Granted     28    NL     -1    28:3213591       2     No   No    None
     Granted     27    NL     -1    27:3216832       2     No   No    None
     Granted     95    NL     -1    95:3178429       2     No   No    None
     Granted     25    NL     -1    25:3513994       2     No   No    None
     Granted     26    EX     -1    26:3512906       2     No   No    None

    The figure below shows a lock from the file system perspective. Specifically, it shows a lock that is in the process of being upconverted from a NL to EX. Locks in this state are are referred to in the file system as busy locks and can be listed using the debugfs.ocfs2 command, "fs_locks -B".

    # debugfs.ocfs2 -R "fs_locks -B" /dev/sda1
    Lockres: M000000000000000000000b9aba12ec  Mode: No Lock
    Flags: Initialized Attached Busy
    RO Holders: 0  EX Holders: 0
    Pending Action: Convert  Pending Unlock Action: None
    Requested Mode: Exclusive  Blocking Mode: No Lock
    PR > Gets: 0  Fails: 0    Waits Total: 0us  Max: 0us  Avg: 0ns
    EX > Gets: 1  Fails: 0    Waits Total: 544us  Max: 544us  Avg: 544185ns
    Disk Refreshes: 1

    With this debugging infrastructure in place, users can debug hang issues as follows:

    * Dump the busy fs locks for all the OCFS2 volumes on the node with hanging processes. If no locks are found, then the problem is not related to O2DLM.

    * Dump the corresponding dlm lock for all the busy fs locks. Note down the owner (master) of all the locks.

    * Dump the dlm locks on the master node for each lock.

    At this stage, one should note that the hanging node is waiting to get an AST from the master. The master, on the other hand, cannot send the AST until the current holder has down converted that lock, which it will do upon receiving a Blocking AST. However, a node can only down convert if all the lock holders have stopped using that lock. After dumping the dlm lock on the master node, identify the current lock holder and dump both the dlm and fs locks on that node.

    The trick here is to see whether the Blocking AST message has been relayed to file system. If not, the problem is in the dlm layer. If it has, then the most common reason would be a lock holder, the count for which is maintained in the fs lock.

    At this stage, printing the list of process helps.

    $ ps -e -o pid,stat,comm,wchan=WIDE-WCHAN-COLUMN

    Make a note of all D state processes. At least one of them is responsible for the hang on the first node.

    The challenge then is to figure out why those processes are hanging. Failing that, at least get enough information (like alt-sysrq t output) for the kernel developers to review. What to do next depends on where the process is hanging. If it is waiting for the I/O to complete, the problem could be anywhere in the I/O subsystem, from the block device layer through the drivers to the disk array. If the hang concerns a user lock (flock(2)), the problem could be in the user’s application. A possible solution could be to kill the holder. If the hang is due to tight or fragmented memory, free up some memory by killing non-essential processes.

    The thing to note is that the symptom for the problem was on one node but the cause is on another. The issue can only be resolved on the node holding the lock. Sometimes, the best solution will be to reset that node. Once killed, the O2DLM recovery process will clear all locks owned by the dead node and let the cluster continue to operate. As harsh as that sounds, at times it is the only solution. The good news is that, by following the trail, you now have enough information to file a bug and get the real issue resolved.

NFS EXPORTING
    OCFS2 volumes can be exported as NFS volumes. This support is limited to NFS version 3, which translates to Linux kernel version 2.4 or later.

    If the version of the Linux kernel on the system exporting the volume is older than 2.6.30, then the NFS clients must mount the volumes using the nordirplus mount option. This disables the READDIRPLUS RPC call to workaround a bug in NFSD, detailed in the following link:

    http://oss.oracle.com/pipermail/ocfs2-announce/2008-June/000025.html

    Users running NFS version 2 can export the volume after having disabled subtree checking (mount option no_subtree_check). Be warned, disabling the check has security implications (documented in the exports(5) man page) that users must evaluate on their own.

FILE SYSTEM LIMITS
    OCFS2 has no intrinsic limit on the total number of files and directories in the file system. In general, it is only limited by the size of the device. But there is one limit imposed by the current filesystem. It can address at most four billion clusters. A file system with 1MB cluster size can go up to 4PB, while a file system with a 4KB cluster size can address up to 16TB.

SYSTEM OBJECTS
    The OCFS2 file system stores its internal meta-data, including bitmaps, journals, etc., as system files. These are grouped in a system directory. These files and directories are not accessible via the file system interface but can be viewed using the debugfs.ocfs2(8) tool.

    To list the system directory (referred to as double-slash), do:

    # debugfs.ocfs2 -R "ls -l //" /dev/sde1
            66     drwxr-xr-x  10  0  0         3896 19-Jul-2011 13:36 .
            66     drwxr-xr-x  10  0  0         3896 19-Jul-2011 13:36 ..
            67     -rw-r--r--   1  0  0            0 19-Jul-2011 13:36 bad_blocks
            68     -rw-r--r--   1  0  0      1179648 19-Jul-2011 13:36 global_inode_alloc
            69     -rw-r--r--   1  0  0         4096 19-Jul-2011 14:35 slot_map
            70     -rw-r--r--   1  0  0      1048576 19-Jul-2011 13:36 heartbeat
            71     -rw-r--r--   1  0  0  53686960128 19-Jul-2011 13:36 global_bitmap
            72     drwxr-xr-x   2  0  0         3896 25-Jul-2011 15:05 orphan_dir:0000
            73     drwxr-xr-x   2  0  0         3896 19-Jul-2011 13:36 orphan_dir:0001
            74     -rw-r--r--   1  0  0      8388608 19-Jul-2011 13:36 extent_alloc:0000
            75     -rw-r--r--   1  0  0      8388608 19-Jul-2011 13:36 extent_alloc:0001
            76     -rw-r--r--   1  0  0    121634816 19-Jul-2011 13:36 inode_alloc:0000
            77     -rw-r--r--   1  0  0            0 19-Jul-2011 13:36 inode_alloc:0001
            77     -rw-r--r--   1  0  0    268435456 19-Jul-2011 13:36 journal:0000
            79     -rw-r--r--   1  0  0    268435456 19-Jul-2011 13:37 journal:0001
            80     -rw-r--r--   1  0  0            0 19-Jul-2011 13:36 local_alloc:0000
            81     -rw-r--r--   1  0  0            0 19-Jul-2011 13:36 local_alloc:0001
            82     -rw-r--r--   1  0  0            0 19-Jul-2011 13:36 truncate_log:0000
            83     -rw-r--r--   1  0  0            0 19-Jul-2011 13:36 truncate_log:0001

    The file names that end with numbers are slot specific and are referred to as node-local system files. The set of node-local files used by a node can be determined from the slot map. To list the slot map, do:

    # debugfs.ocfs2 -R "slotmap" /dev/sde1
        Slot#    Node#
            0       32
            1       35
            2       40
            3       31
            4       34
            5       33

    For more information, refer to the OCFS2 support guides available in the Documentation section at http://oss.oracle.com/projects/ocfs2.

HEARTBEAT, QUORUM, AND FENCING
    Heartbeat is an essential component in any cluster. It is charged with accurately designating nodes as dead or alive. A mistake here could lead to a cluster hang or a corruption.

    o2hb is the disk heartbeat component of o2cb. It periodically updates a timestamp on disk, indicating to others that this node is alive. It also reads all the timestamps to identify other live nodes. Other cluster components, like o2dlm and o2net, use the o2hb service to get node up and down events.

    The quorum is the group of nodes in a cluster that is allowed to operate on the shared storage. When there is a failure in the cluster, nodes may be split into groups that can communicate in their groups and with the shared storage but not between groups. o2quo determines which group is allowed to continue and initiates fencing of the other group(s).

    Fencing is the act of forcefully removing a node from a cluster. A node with OCFS2 mounted will fence itself when it realizes that it does not have quorum in a degraded cluster. It does this so that other nodes won’t be stuck trying to access its resources.

    o2cb uses a machine reset to fence. This is the quickest route for the node to rejoin the cluster.

PROCESSES

    [o2net]
        One per node. It is a work-queue thread started when the cluster is brought on-line and stopped when it is off-lined. It handles network communication for all mounts. It gets the list of active nodes from O2HB and sets up a TCP/IP communication channel with each live node. It sends regular keep-alive packets to detect any interruption on the channels.

    [user_dlm]
        One per node. It is a work-queue thread started when dlmfs is loaded and stopped when it is unloaded (dlmfs is a synthetic file system that allows user space processes to access the in-kernel dlm).

    [ocfs2_wq]
        One per node. It is a work-queue thread started when the OCFS2 module is loaded and stopped when it is unloaded. It is assigned background file system tasks that may take cluster locks like flushing the truncate log, orphan directory recovery and local alloc recovery. For example, orphan directory recovery runs in the background so that it does not affect recovery time.

    [o2hb-14C29A7392]
        One per heartbeat device. It is a kernel thread started when the heartbeat region is populated in configfs and stopped when it is removed. It writes every two seconds to a block in the heartbeat region, indicating that this node is alive. It also reads the region to maintain a map of live nodes. It notifies subscribers like o2net and o2dlm of any changes in the live node map.

    [ocfs2dc]
        One per mount. It is a kernel thread started when a volume is mounted and stopped when it is unmounted. It downgrades locks in response to blocking ASTs (BASTs) requested by other nodes.

    [jbd2/sdf1-97]
        One per mount. It is part of JBD2, which OCFS2 uses for journaling.

    [ocfs2cmt]
        One per mount. It is a kernel thread started when a volume is mounted and stopped when it is unmounted. It works with kjournald2.

    [ocfs2rec]
        It is started whenever a node has to be recovered. This thread performs file system recovery by replaying the journal of the dead node. It is scheduled to run after dlm recovery has completed.

    [dlm_thread]
        One per dlm domain. It is a kernel thread started when a dlm domain is created and stopped when it is destroyed. This thread sends ASTs and blocking ASTs in response to lock level convert requests. It also frees unused lock resources.

    [dlm_reco_thread]
        One per dlm domain. It is a kernel thread that handles dlm recovery when another node dies. If this node is the dlm recovery master, it re-masters every lock resource owned by the dead node.

    [dlm_wq]
        One per dlm domain. It is a work-queue thread that o2dlm uses to queue blocking tasks.

FUTURE WORK
    File system development is a never ending cycle. Faster and larger disks, faster and more number of processors, larger caches, etc. keep changing the sweet spot for performance forcing developers to rethink long held beliefs. Add to that new use cases, which forces developers to be innovative in providing solutions that melds seamlessly with existing semantics.

    We are currently looking to add features like transparent compression, transparent encryption, delayed allocation, multi-device support, etc. as well as work on improving performance on newer generation machines.

    If you are interested in contributing, email the development team at ocfs2-devel@oss.oracle.com.

ACKNOWLEDGEMENTS

The principal developers of the OCFS2 file system, its tools and the O2CB cluster stack, are Joel Becker, Zach Brown, Mark Fasheh, Jan Kara, Kurt Hackel, Tao Ma, Sunil Mushran, Tiger Yang and Tristan Ye.

Other developers who have contributed to the file system via bug fixes, testing, etc. are Wim Coekaerts, Srinivas Eeda, Coly Li, Jeff Mahoney, Marcos Matsunaga, Goldwyn Rodrigues, Manish Singh and Wengang Wang.

The members of the Linux Cluster community including Andrew Beekhof, Lars Marowsky-Bree, Fabio Massimo Di Nitto and David Teigland.

The members of the Linux File system community including Christoph Hellwig and Chris Mason.

The corporations that have contributed resources for this project including Oracle, SUSE Labs, EMC, Emulex, HP, IBM, Intel and Network Appliance.

SEE ALSO
debugfs.ocfs2(8) fsck.ocfs2(8) fsck.ocfs2.checks(8) mkfs.ocfs2(8) mount.ocfs2(8) mounted.ocfs2(8) o2cluster(8) o2image(8) o2info(1) o2cb(7) o2cb(8) o2cb.sysconfig(5) o2hbmonitor(8) ocfs2.cluster.conf(5) tunefs.ocfs2(8)

AUTHOR
Oracle Corporation

COPYRIGHT
Copyright © 2004, 2012 Oracle. All rights reserved.