RBD Persistent Write-back Cache¶

Persistent Write-back Cache¶

The persistent write-back cache provides a persistent, fault-tolerant write-back cache for librbd-based RBD clients.

This cache uses a log-ordered write-back design which maintains checkpoints internally so that writes that get flushed back to the cluster are always crash consistent. Even if the client cache is lost entirely, the disk image is still consistent but the data will appear to be stale.

This cache can be used with PMEM or SSD as a cache device.

Usage¶

The persistent write-back cache manages the cache data in a persistent device. It looks for and creates cache files in a configured directory, and then caches data in the file.

The persistent write-back cache can’t be enabled without the exclusive-lock feature. It tries to enable the write-back cache only when the exclusive lock is acquired.

The cache provides two different persistence modes. In persistent-on-write mode, the writes are completed only when they are persisted to the cache device and will be readable after a crash. In persistent-on-flush mode, the writes are completed as soon as it no longer needs the caller’s data buffer to complete the writes, but does not guarantee that writes will be readable after a crash. The data is persisted to the cache device when a flush request is received.

Initially it defaults to the persistent-on-write mode and it switches to persistent-on-flush mode after the first flush request is received.

Enable Persistent Write-back Cache¶

To enable the persistent write-back cache, the following Ceph settings need to be enabled.:

rbd persistent cache mode = {cache-mode}
rbd plugins = pwl_cache

Value of {cache-mode} can be rwl, ssd or disabled. By default the cache is disabled.

Here are some cache configuration settings:

rbd_persistent_cache_path A file folder to cache data. This folder must have DAX enabled (see DAX) when using rwl mode to avoid performance degradation.
rbd_persistent_cache_size The cache size per image. The minimum cache size is 1 GB.

The above configurations can be set per-host, per-pool, per-image etc. Eg, to set per-host, add the overrides to the appropriate section in the host’s ceph.conf file. To set per-pool, per-image, etc, please refer to the rbd config commands.

Cache Status¶

The persistent write-back cache is enabled when the exclusive lock is acquired, and it is closed when the exclusive lock is released. To check the cache status, users may use the command rbd status.

rbd status {pool-name}/{image-name}

The status of the cache is shown, including present, clean, cache size and the location as well as some basic metrics.

For example:

$ rbd status rbd/foo
Watchers:
        watcher=10.10.0.102:0/1061883624 client.25496 cookie=140338056493088
Persistent cache state:
        host: sceph9
        path: /mnt/nvme0/rbd-pwl.rbd.101e5824ad9a.pool
        size: 1 GiB
        mode: ssd
        stats_timestamp: Sun Apr 10 13:26:32 2022
        present: true   empty: false    clean: false
        allocated: 509 MiB
        cached: 501 MiB
        dirty: 338 MiB
        free: 515 MiB
        hits_full: 1450 / 61%
        hits_partial: 0 / 0%
        misses: 924
        hit_bytes: 192 MiB / 66%
        miss_bytes: 97 MiB

Flush Cache¶

To flush a cache file with rbd, specify the persistent-cache flush command, the pool name and the image name.

rbd persistent-cache flush {pool-name}/{image-name}

If the application dies unexpectedly, this command can also be used to flush the cache back to OSDs.

For example:

$ rbd persistent-cache flush rbd/foo

Invalidate Cache¶

To invalidate (discard) a cache file with rbd, specify the persistent-cache invalidate command, the pool name and the image name.

rbd persistent-cache invalidate {pool-name}/{image-name}

The command removes the cache metadata of the corresponding image, disables the cache feature and deletes the local cache file if it exists.

For example:

$ rbd persistent-cache invalidate rbd/foo