Ubuntu Manpage: nbdkit-cache-filter - nbdkit caching filter

Provided by: nbdkit_1.24.1-2ubuntu4_amd64

NAME

       nbdkit-cache-filter - 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
(nbdkit ≥ 1.10)

Limit the size of the cache to "SIZE". See "CACHE MAXIMUM SIZE" below.

cache-on-read=true
(nbdkit ≥ 1.10)

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.

AUTHORS

       Eric Blake

       Richard W.M. Jones

COPYRIGHT

       Copyright (C) 2018-2020 Red Hat Inc.

LICENSE

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

       •   Redistributions of source code must retain the above copyright notice, this list  of  conditions  and
           the following disclaimer.

       •   Redistributions in binary form must reproduce the above copyright notice, this list of conditions and
           the following disclaimer in the documentation and/or other materials provided with the distribution.

       •   Neither  the  name  of  Red  Hat  nor the names of its contributors may be used to endorse or promote
           products derived from this software without specific prior written permission.

       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.

nbdkit-1.24.1                                      2021-01-20                             nbdkit-cache-filter(1)