oracular (1) nbdkit-blocksize-policy-filter.1.gz

Provided by: nbdkit_1.40.4-1ubuntu1_amd64 bug

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]
               [blocksize-write-disconnect=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.

       Normally, nbdkit will accept write requests up to 64M in length, and reply with a gracful error message
       rather than a hard disconnect for a buffer up to twice that large.  But many other servers (for example,
       qemu-nbd) will give a hard disconnect for a write request larger than 32M.  With this filter you can use
       "blocksize-write-disconnect=32M" to emulate the behavior of other servers.

   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-write-disconnect=N
           (nbdkit ≥ 1.34)

           If a client sends a write request which is larger than the specified size (using the usual size
           modifiers like "32M"), abruptly close the connection.  This can be used to emulate qemu's behavior of
           disconnecting for write requests larger than 32M, rather than nbdkit's default of keeping the
           connection alive for write requests up to 128M (although nbdkit does not let the plugin see requests
           larger than 64M).  The write disconnect size is independent of any advertised maximum block size or
           its accompanying error policy.

       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 Red Hat


LICENSE
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