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

Provided by: nbdkit_1.42.2-1ubuntu1_amd64

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.

AUTHORS

       Richard W.M. Jones

COPYRIGHT

       Copyright Red Hat

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
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       ON  ANY  THEORY  OF  LIABILITY,  WHETHER  IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
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       DAMAGE.

nbdkit-1.42.2                                      2025-04-02                  nbdkit-blocksize-policy-filter(1)