NAME

       nbdkit-blocksize-policy-filter - set minimum, preferred and maximum block size, and apply
       error policy

SYNOPSIS

        nbdkit --filter=blocksize-policy PLUGIN
               [blocksize-error-policy=allow|error]
               [blocksize-minimum=N]
               [blocksize-preferred=N]
               [blocksize-maximum=N]

DESCRIPTION

       "nbdkit-blocksize-policy-filter" is an nbdkit(1) filter that can add block size
       constraints to plugins which don't already support them.  It can also enforce an error
       policy for badly behaved clients which do not obey the block size constraints.

       For more information about block size constraints, see section "Block size constraints" in
       https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md.

       The simplest usage is to place this filter on top of any plugin which does not advertise
       block size constraints, and set the "blocksize-minimum", "blocksize-preferred" and
       "blocksize-maximum" parameters with the desired constraints.  For example:

        nbdkit --filter=blocksize-policy memory 1G \
               blocksize-preferred=32K

       would adjust nbdkit-memory-plugin(1) so that clients should prefer 32K requests.  You can
       query the NBD server advertised constraints using nbdinfo(1):

        $ nbdinfo nbd://localhost
        [...]
            block_size_minimum: 1
            block_size_preferred: 32768
            block_size_maximum: 4294967295

       The second part of this filter is adjusting the error policy when badly behaved clients do
       not obey the minimum or maximum request size.  Normally nbdkit permits these requests,
       leaving it up to the plugin whether it rejects the request with an error or tries to
       process the request (eg. trying to split an over-large request or doing a read-modify-
       write for an unaligned write).  With this filter you can use
       "blocksize-error-policy=error" to reject these requests in the filter with an EINVAL
       error.  The plugin will not see them.

   Combining with nbdkit-blocksize-filter(1)
       A related filter is nbdkit-blocksize-filter(1).  That filter can split and combine
       requests for plugins that cannot handle requests under or over a particular size.

       Both filters may be used together like this (note that the order of the filters is
       important):

         nbdkit --filter=blocksize-policy \
                --filter=blocksize \
                PLUGIN ... \
                blocksize-error-policy=allow \
                blocksize-minimum=64K minblock=64K

       This says to advertise a minimum block size of 64K.  Well-behaved clients will obey this.
       Badly behaved clients will send requests < 64K which will be converted to slow 64K read-
       modify-write cycles to the underlying plugin.  In either case the plugin will only see
       requests on 64K (or multiples of 64K) boundaries.

PARAMETERS

       blocksize-error-policy=allow
       blocksize-error-policy=error
           If a client sends a request which is smaller than the permitted minimum size or larger
           than the permitted maximum size, or not aligned to the minimum size,
           "blocksize-error-policy" chooses what the filter will do.  The default (and also
           nbdkit's default) is "allow" which means pass the request through to the plugin.

           Use "error" to return an EINVAL error back to the client.  The plugin will not see the
           badly formed request in this case.

       blocksize-minimum=N
       blocksize-preferred=N
       blocksize-maximum=N
           Advertise minimum, preferred and/or maximum block size to the client.  Well-behaved
           clients should obey these constraints.

           For each parameter, you can specify it as a size (using the usual modifiers like
           "4K").

           If the parameter is omitted then either the constraint advertised by the plugin itself
           is used, or a sensible default for plugins which do not advertise block size
           constraints.

FILES

       $filterdir/nbdkit-blocksize-policy-filter.so
           The filter.

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

VERSION

       "nbdkit-limit-filter" first appeared in nbdkit 1.30.

SEE ALSO

       nbdkit(1), nbdkit-blocksize-filter(1), nbdkit-filter(3), nbdkit-plugin(3).

AUTHORS

       Richard W.M. Jones

COPYRIGHT

       Copyright (C) 2022 Red Hat Inc.

LICENSE

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