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

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