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.IX Title "nbdkit-blocksize-policy-filter 1"
.TH nbdkit-blocksize-policy-filter 1 2024-04-21 nbdkit-1.38.1 NBDKIT
.\" For nroff, turn off justification.  Always turn off hyphenation; it makes
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.nh
.SH NAME
nbdkit\-blocksize\-policy\-filter \- set minimum, preferred and
maximum block size, and apply error policy
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 6
\& nbdkit \-\-filter=blocksize\-policy PLUGIN
\&        [blocksize\-error\-policy=allow|error]
\&        [blocksize\-minimum=N]
\&        [blocksize\-preferred=N]
\&        [blocksize\-maximum=N]
\&        [blocksize\-write\-disconnect=N]
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
\&\f(CW\*(C`nbdkit\-blocksize\-policy\-filter\*(C'\fR is an \fBnbdkit\fR\|(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.
.PP
For more information about block size constraints, see section
"Block size constraints" in
https://github.com/NetworkBlockDevice/nbd/blob/master/doc/proto.md.
.PP
The simplest usage is to place this filter on top of any plugin which
does not advertise block size constraints, and set the
\&\f(CW\*(C`blocksize\-minimum\*(C'\fR, \f(CW\*(C`blocksize\-preferred\*(C'\fR and \f(CW\*(C`blocksize\-maximum\*(C'\fR
parameters with the desired constraints.  For example:
.PP
.Vb 2
\& nbdkit \-\-filter=blocksize\-policy memory 1G \e
\&        blocksize\-preferred=32K
.Ve
.PP
would adjust \fBnbdkit\-memory\-plugin\fR\|(1) so that clients should
prefer 32K requests.  You can query the NBD server advertised constraints
using \fBnbdinfo\fR\|(1):
.PP
.Vb 5
\& $ nbdinfo nbd://localhost
\& [...]
\&     block_size_minimum: 1
\&     block_size_preferred: 32768
\&     block_size_maximum: 4294967295
.Ve
.PP
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 \f(CW\*(C`blocksize\-error\-policy=error\*(C'\fR to reject these requests in the
filter with an EINVAL error.  The plugin will not see them.
.PP
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 \f(CW\*(C`blocksize\-write\-disconnect=32M\*(C'\fR
to emulate the behavior of other servers.
.SS "Combining with \fBnbdkit\-blocksize\-filter\fP\|(1)"
.IX Subsection "Combining with nbdkit-blocksize-filter"
A related filter is \fBnbdkit\-blocksize\-filter\fR\|(1).  That filter can
split and combine requests for plugins that cannot handle requests
under or over a particular size.
.PP
Both filters may be used together like this (note that the order of
the filters is important):
.PP
.Vb 5
\&  nbdkit \-\-filter=blocksize\-policy \e
\&         \-\-filter=blocksize \e
\&         PLUGIN ... \e
\&         blocksize\-error\-policy=allow \e
\&         blocksize\-minimum=64K minblock=64K
.Ve
.PP
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.
.SH PARAMETERS
.IX Header "PARAMETERS"
.IP \fBblocksize\-error\-policy=allow\fR 4
.IX Item "blocksize-error-policy=allow"
.PD 0
.IP \fBblocksize\-error\-policy=error\fR 4
.IX Item "blocksize-error-policy=error"
.PD
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, \f(CW\*(C`blocksize\-error\-policy\*(C'\fR chooses what the filter
will do.  The default (and also nbdkit's default) is \f(CW\*(C`allow\*(C'\fR which
means pass the request through to the plugin.
.Sp
Use \f(CW\*(C`error\*(C'\fR to return an EINVAL error back to the client.  The plugin
will not see the badly formed request in this case.
.IP \fBblocksize\-write\-disconnect=\fRN 4
.IX Item "blocksize-write-disconnect=N"
(nbdkit ≥ 1.34)
.Sp
If a client sends a write request which is larger than the specified
\&\fIsize\fR (using the usual size modifiers like \f(CW\*(C`32M\*(C'\fR), 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.
.IP \fBblocksize\-minimum=\fRN 4
.IX Item "blocksize-minimum=N"
.PD 0
.IP \fBblocksize\-preferred=\fRN 4
.IX Item "blocksize-preferred=N"
.IP \fBblocksize\-maximum=\fRN 4
.IX Item "blocksize-maximum=N"
.PD
Advertise minimum, preferred and/or maximum block size to the client.
Well-behaved clients should obey these constraints.
.Sp
For each parameter, you can specify it as a size (using the usual
modifiers like \f(CW\*(C`4K\*(C'\fR).
.Sp
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.
.SH FILES
.IX Header "FILES"
.ie n .IP \fR\fI$filterdir\fR\fI/nbdkit\-blocksize\-policy\-filter.so\fR 4
.el .IP \fR\f(CI$filterdir\fR\fI/nbdkit\-blocksize\-policy\-filter.so\fR 4
.IX Item "$filterdir/nbdkit-blocksize-policy-filter.so"
The filter.
.Sp
Use \f(CW\*(C`nbdkit \-\-dump\-config\*(C'\fR to find the location of \f(CW$filterdir\fR.
.SH VERSION
.IX Header "VERSION"
\&\f(CW\*(C`nbdkit\-limit\-filter\*(C'\fR first appeared in nbdkit 1.30.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbdkit\fR\|(1),
\&\fBnbdkit\-blocksize\-filter\fR\|(1),
\&\fBnbdkit\-filter\fR\|(3),
\&\fBnbdkit\-plugin\fR\|(3).
.SH AUTHORS
.IX Header "AUTHORS"
Richard W.M. Jones
.SH COPYRIGHT
.IX Header "COPYRIGHT"
Copyright Red Hat
.SH LICENSE
.IX Header "LICENSE"
Redistribution and use in source and binary forms, with or without
modification, are permitted provided that the following conditions are
met:
.IP \(bu 4
Redistributions of source code must retain the above copyright
notice, this list of conditions and the following disclaimer.
.IP \(bu 4
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.
.IP \(bu 4
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.
.PP
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 SUBSTITUTE GOODS OR SERVICES; LOSS OF
USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT
OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
SUCH DAMAGE.