# Additional Notes

Here are miscellaneous notes about topics that may not be covered in enough detail in the usage section.

## `--chunker-params`

The chunker parameters influence how input files are cut into pieces (chunks) which are then considered for deduplication. They also have a big impact on resource usage (RAM and disk space) as the amount of resources needed is (also) determined by the total number of chunks in the repository (see Indexes / Caches memory usage for details).

`--chunker-params=buzhash,10,23,16,4095` results in a fine-grained deduplication| and creates a large number of chunks and thus uses a lot of resources to manage them. This is good for relatively small data volumes and if the machine has a good amount of free RAM and disk space.

`--chunker-params=buzhash,19,23,21,4095` (default) results in a coarse-grained deduplication and creates a much smaller number of chunks and thus uses less resources. This is good for relatively big data volumes and if the machine has a relatively low amount of free RAM and disk space.

`--chunker-params=fixed,4194304` results in fixed 4 MiB-sized block deduplication and is more efficient than the previous example when used with for block devices (like disks, partitions, LVM LVs) or raw disk image files.

`--chunker-params=fixed,4096,512` results in fixed 4 KiB-sized blocks, but the first header block will only be 512B long. This might be useful to dedup files with 1 header + N fixed size data blocks. Be careful not to produce too many chunks (for example, using a small block size for huge files).

If you have already created some archives in a repository and then change chunker parameters, this of course impacts deduplication as the chunks will be cut differently.

In the worst case (all files are big and were touched in between backups), this will store all content into the repository again.

Usually, it is not that bad though:

- usually most files are not touched, so it will just re-use the old chunks it already has in the repo

- files smaller than the (both old and new) minimum chunk size result in only one chunk anyway, so the resulting chunks are the same and deduplication will apply

If you switch chunker parameters to save resources for an existing repository that already has some backup archives, you will see an increasing effect over time, when more and more files have been touched and stored again using the bigger chunk size and all references to the smaller, older chunks have been removed (by deleting / pruning archives).

If you want to see an immediate, significant effect on resource usage, you should start a new repository when changing chunker parameters.

For more details, see Chunks.

## `--noatime / --noctime`

You can use these `borg create` options to not store the respective timestamp into the archive, in case you do not really need it.

Besides saving a little space by omitting the timestamp, it might also affect metadata stream deduplication: if only this timestamp changes between backups and is stored into the metadata stream, the metadata stream chunks will not deduplicate just because of that.

## `--nobsdflags / --noflags`

You can use this to avoid querying and storing (or extracting and setting) flags — in case you don’t need them or if they are broken for your filesystem.

On Linux, dealing with the flags needs some additional syscalls. Especially when dealing with lots of small files, this causes a noticeable overhead, so you can use this option also for speeding up operations.

## `--umask`

borg uses a safe default umask of 077 (that means the files borg creates have only permissions for owner, but no permissions for group and others) - so there should rarely be a need to change the default behaviour.

This option only affects the process to which it is given. Thus, when you run borg in client/server mode and you want to change the behaviour on the server side, you need to use `borg serve --umask=XXX ...` as a ssh forced command in `authorized_keys`. The `--umask` value given on the client side is not transferred to the server side.

Also, if you choose to use the `--umask` option, always be consistent and use the same umask value so you do not create a mixup of permissions in a borg repository or with other files borg creates.

## `--read-special`

The `--read-special` option is special - you do not want to use it for normal full-filesystem backups, but rather after carefully picking some targets for it.

The option `--read-special` triggers special treatment for block and char device files as well as FIFOs. Instead of storing them as such a device (or FIFO), they will get opened, their content will be read and in the backup archive they will show up like a regular file.

Symlinks will also get special treatment if (and only if) they point to such a special file: instead of storing them as a symlink, the target special file will get processed as described above.

One intended use case of this is backing up the contents of one or multiple block devices, like e.g. LVM snapshots or inactive LVs or disk partitions.

You need to be careful about what you include when using `--read-special`, e.g. if you include `/dev/zero`, your backup will never terminate.

Restoring such files’ content is currently only supported one at a time via `--stdout` option (and you have to redirect stdout to where ever it shall go, maybe directly into an existing device file of your choice or indirectly via `dd`).

To some extent, mounting a backup archive with the backups of special files via `borg mount` and then loop-mounting the image files from inside the mount point will work. If you plan to access a lot of data in there, it likely will scale and perform better if you do not work via the FUSE mount.

### Example

Imagine you have made some snapshots of logical volumes (LVs) you want to back up.

Note

For some scenarios, this is a good method to get “crash-like” consistency (I call it crash-like because it is the same as you would get if you just hit the reset button or your machine would abruptly and completely crash). This is better than no consistency at all and a good method for some use cases, but likely not good enough if you have databases running.

Then you create a backup archive of all these snapshots. The backup process will see a “frozen” state of the logical volumes, while the processes working in the original volumes continue changing the data stored there.

You also add the output of `lvdisplay` to your backup, so you can see the LV sizes in case you ever need to recreate and restore them.

After the backup has completed, you remove the snapshots again.

    $ # create snapshots here
    $ lvdisplay > lvdisplay.txt
    $ borg create --read-special arch lvdisplay.txt /dev/vg0/*-snapshot
    $ # remove snapshots here

Now, let’s see how to restore some LVs from such a backup.

    $ borg extract arch lvdisplay.txt
    $ # create empty LVs with correct sizes here (look into lvdisplay.txt).
    $ # we assume that you created an empty root and home LV and overwrite it now:
    $ borg extract --stdout arch dev/vg0/root-snapshot > /dev/vg0/root
    $ borg extract --stdout arch dev/vg0/home-snapshot > /dev/vg0/home

## Separate compaction

Borg does not auto-compact the segment files in the repository at commit time (at the end of each repository-writing command) any more (since borg 1.2.0).

This has some notable consequences:

- repository space is not freed immediately when deleting / pruning archives

- commands finish quicker

- repository is more robust and might be easier to recover after damages (as it contains data in a more sequential manner, historic manifests, multiple commits - until you run `borg compact`)

- user can choose when to run compaction (it should be done regularly, but not necessarily after each single borg command)

- user can choose from where to invoke `borg compact` to do the compaction (from client or from server, it does not need a key)

- less repo sync data traffic in case you create a copy of your repository by using a sync tool (like rsync, rclone, …)

You can manually run compaction by invoking the `borg compact` command.

## SSH batch mode

When running Borg using an automated script, `ssh` might still ask for a password, even if there is an SSH key for the target server. Use this to make scripts more robust:

    export BORG_RSH='ssh -oBatchMode=yes'