# File-backed memory

Important

As of the 18.0.0 Rocky release, the functionality described below is only supported by the libvirt/KVM driver.

The file-backed memory feature in Openstack allows a Nova node to serve guest memory from a file backing store. This mechanism uses the libvirt file memory source, causing guest instance memory to be allocated as files within the libvirt memory backing directory.

Since instance performance will be related to the speed of the backing store, this feature works best when used with very fast block devices or virtual file systems - such as flash or RAM devices.

When configured, `nova-compute` will report the capacity configured for file-backed memory to placement in place of the total system memory capacity. This allows the node to run more instances than would normally fit within system memory.

When available in libvirt and qemu, instance memory will be discarded by qemu at shutdown by calling `madvise(MADV_REMOVE)`, to avoid flushing any dirty memory to the backing store on exit.

To enable file-backed memory, follow the steps below:

1. Configure the backing store

2. Configure Nova Compute for file-backed memory

Important

It is not possible to live migrate from a node running a version of OpenStack that does not support file-backed memory to a node with file backed memory enabled. It is recommended that all Nova compute nodes are upgraded to Rocky before enabling file-backed memory.

## Prerequisites and Limitations

Libvirt

  File-backed memory requires libvirt version 4.0.0 or newer. Discard capability requires libvirt version 4.4.0 or newer.

Qemu

  File-backed memory requires qemu version 2.6.0 or newer. Discard capability requires qemu version 2.10.0 or newer.

Memory overcommit

  File-backed memory is not compatible with memory overcommit. `ram_allocation_ratio` must be set to `1.0` in `nova.conf`, and the host must not be added to a host aggregate with `ram_allocation_ratio` set to anything but `1.0`.

Reserved memory

  When configured, file-backed memory is reported as total system memory to placement, with RAM used as cache. Reserved memory corresponds to disk space not set aside for file-backed memory. `reserved_host_memory_mb` should be set to `0` in `nova.conf`.

Huge pages

  File-backed memory is not compatible with huge pages. Instances with huge pages configured will not start on a host with file-backed memory enabled. It is recommended to use host aggregates to ensure instances configured for huge pages are not placed on hosts with file-backed memory configured.

Handling these limitations could be optimized with a scheduler filter in the future.

## Configure the backing store

Note

`/dev/sdb` and the `ext4` filesystem are used here as an example. This will differ between environments.

Note

`/var/lib/libvirt/qemu/ram` is the default location. The value can be set via `memory_backing_dir` in `/etc/libvirt/qemu.conf`, and the mountpoint must match the value configured there.

By default, Libvirt with qemu/KVM allocates memory within `/var/lib/libvirt/qemu/ram/`. To utilize this, you need to have the backing store mounted at (or above) this location.

1. Create a filesystem on the backing device

       # mkfs.ext4 /dev/sdb

2. Mount the backing device

   Add the backing device to `/etc/fstab` for automatic mounting to `/var/lib/libvirt/qemu/ram`

   Mount the device

       # mount /dev/sdb /var/lib/libvirt/qemu/ram

## Configure Nova Compute for file-backed memory

1. Enable File-backed memory in `nova-compute`

   Configure Nova to utilize file-backed memory with the capacity of the backing store in MiB. 1048576 MiB (1 TiB) is used in this example.

   Edit `/etc/nova/nova.conf`

       [libvirt]
       file_backed_memory=1048576

2. Restart the `nova-compute` service

this page last updated: 2026-06-10 20:07:09

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