nbdkit-cache-filter - Man Page

nbdkit caching filter

Synopsis

 nbdkit --filter=cache plugin [cache=writeback|writethrough|unsafe]
                              [cache-max-size=SIZE]
                              [cache-high-threshold=N]
                              [cache-low-threshold=N]
                              [cache-on-read=true|false]
                              [plugin-args...]

Description

nbdkit-cache-filter is a filter that adds caching on top of a plugin.  This is useful if a plugin is slow or expensive to use, because nbdkit will try to minimize requests to the plugin by caching previous requests.

Note that many NBD clients are able to do caching, and because the caching happens on the client side it will usually be more effective than caching inside the server.  This filter can be used if the client does not have effective caching, or (with cache=unsafe) to defeat flush requests from the client (which is unsafe and can cause data loss, as the name suggests).

Note that the use of this filter rounds the image size down to a multiple of the caching granularity (the larger of 4096 or the f_bsize field of fstatvfs(3)), to ease the implementation. If you need to round the image size up instead to access the last few bytes, combine this filter with nbdkit-truncate-filter(1).

This filter only caches image contents.  To cache image metadata, use nbdkit-cacheextents-filter(1) between this filter and the plugin. To accelerate sequential reads, use nbdkit-readahead-filter(1) instead.

Parameters

cache=writeback

Store writes in the cache.  They are not written to the plugin unless an explicit flush is done by the client.

This is the default caching mode, and is safe if your client issues flush requests correctly (which is true for modern Linux and other well-written NBD clients).

cache=writethrough

Always force writes through to the plugin.

This makes the cache less effective, but is necessary if your client does not issue correct flush requests.

cache=unsafe

Ignore flush requests.  Never write to the plugin unless the cache grows too large.

This is dangerous and can cause data loss, but this may be acceptable if you only use it for testing or with data that you don't care about or can cheaply reconstruct.

cache-max-size=SIZE
cache-high-threshold=N
cache-low-threshold=N

Limit the size of the cache to SIZE.  See “Cache Maximum Size” below.

cache-on-read=true

Cache read requests as well as write and cache requests.  Any time a block is read from the plugin, it is saved in the cache (if there is sufficient space) so the same data can be served more quickly later.

Note that if the underlying data served by the plugin can be modified by some other means (eg. something else can write to a file which is being served by nbdkit-file-plugin(1)), this option will cause nbdkit to serve stale data because reads won't always go through to the plugin.

cache-on-read=false

Do not cache read requests (this is the default).

Cache Maximum Size

By default the cache can grow to any size (although not larger than the virtual size of the underlying plugin) and you have to ensure there is sufficient space in $TMPDIR for it.

Using the parameters cache-max-size, cache-high-threshold and cache-low-threshold you can limit the maximum size of the cache.

This requires kernel and filesystem support (for fallocate(2) FALLOC_FL_PUNCH_HOLE), so it may not work on all platforms.

Some examples:

cache-max-size=1G

The cache is limited to around 1 gigabyte.

cache-max-size=1G cache-high-threshold=95 cache-low-threshold=80

Once the cache size reaches 972M (95% of 1G), blocks are reclaimed from the cache until the size is reduced to 819M (80% of 1G).

The way this works is once the size of the cache exceeds SIZE ✕ the high threshold, the filter works to reduce the size of the cache until it is less than SIZE ✕ the low threshold. Once the size is below the low threshold, no more reclaim work is done until the size exceeds the high threshold again.

The default thresholds are high 95% and low 80%.  You must set 0 < low < high.  The thresholds are expressed as integer percentages of cache-max-size.

Least recently used blocks are discarded first.

Environment Variables

TMPDIR

The cache is stored in a temporary file located in /var/tmp by default.  You can override this location by setting the TMPDIR environment variable before starting nbdkit.

Files

$filterdir/nbdkit-cache-filter.so

The filter.

Use nbdkit --dump-config to find the location of $filterdir.

Version

nbdkit-cache-filter first appeared in nbdkit 1.2.

See Also

nbdkit(1), nbdkit-file-plugin(1), nbdkit-cacheextents-filter(1), nbdkit-readahead-filter(1), nbdkit-truncate-filter(1), nbdkit-filter(3), qemu-img(1).

Authors

Eric Blake

Richard W.M. Jones

License

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Referenced By

nbdkit(1), nbdkit-cacheextents-filter(1), nbdkit-filter(3), nbdkit-nocache-filter(1), nbdkit-readahead-filter(1), nbdkit-release-notes-1.10(1), nbdkit-release-notes-1.14(1), nbdkit-truncate-filter(1).

2020-06-29 nbdkit-1.21.13