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
           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

COPYRIGHT

       Copyright (C) 2018-2019 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:

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           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.

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