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.IX Title "nbdkit-checkwrite-filter 1"
.TH nbdkit-checkwrite-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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.SH NAME
nbdkit\-checkwrite\-filter \- check writes match contents of plugin
.SH SYNOPSIS
.IX Header "SYNOPSIS"
.Vb 1
\& nbdkit \-\-filter=checkwrite PLUGIN
.Ve
.SH DESCRIPTION
.IX Header "DESCRIPTION"
\&\f(CW\*(C`nbdkit\-checkwrite\-filter\*(C'\fR is an nbdkit filter that turns write
operations into checks that what is written matches the existing
content of the plugin.  If the content matches then the write
operation succeeds (doing nothing).  If not then the write operation
fails returning EIO Input/output error.
.PP
The purpose of this filter is to test copying tools as described
below.
.PP
The underlying plugin is opened read-only and is never written to.
All write-like operations are covered by this filter, such as trimming
and zeroing.  This filter tests the logical equivalence of the writes,
ignoring differences in sparseness.
.SS "Using this filter to test copying tools"
.IX Subsection "Using this filter to test copying tools"
You can check that a copying tool is copying data correctly by
creating an nbdkit instance containing some test data, overlaying this
filter, and copying from and to nbdkit at the same time:
.PP
.Vb 2
\& nbdkit \-\-filter=checkwrite data "@32768 1" \e
\&        \-\-run \*(Aqnbdcopy "$uri" "$uri"\*(Aq
.Ve
.PP
.Vb 2
\& nbdkit \-\-filter=checkwrite file disk.img \e
\&        \-\-run \*(Aqnbdcopy "$uri" "$uri"\*(Aq
.Ve
.PP
.Vb 2
\& nbdkit \-\-filter=checkwrite linuxdisk testdir/ \e
\&        \-\-run \*(Aqqemu\-img convert \-n "$uri" "$uri"\*(Aq
.Ve
.PP
If the copying program is buggy then you will see EIO errors and (if
the copying program also handles errors correctly) it should exit with
an error.
.SS "Other plugins for testing NBD tools"
.IX Subsection "Other plugins for testing NBD tools"
If you are doing this kind of testing you will also want to look at
\&\fBnbdkit\-random\-plugin\fR\|(1) and \fBnbdkit\-sparse\-random\-plugin\fR\|(1).
These plugins already have this behaviour built in to them (because it
is more efficient to build it into the plugin) — you do \fBnot\fR need to
use this filter with those plugins.  This filter is useful for testing
with arbitrary plugins.
.SH PARAMETERS
.IX Header "PARAMETERS"
There are no parameters specific to this filter.  Parameters are
passed through to the underlying plugin.
.SH FILES
.IX Header "FILES"
.ie n .IP \fR\fI$filterdir\fR\fI/nbdkit\-checkwrite\-filter.so\fR 4
.el .IP \fR\f(CI$filterdir\fR\fI/nbdkit\-checkwrite\-filter.so\fR 4
.IX Item "$filterdir/nbdkit-checkwrite-filter.so"
The filter.
.Sp
Use \f(CW\*(C`nbdkit \-\-dump\-config\*(C'\fR to find the location of \f(CW$filterdir\fR.
.SH VERSION
.IX Header "VERSION"
\&\f(CW\*(C`nbdkit\-checkwrite\-filter\*(C'\fR first appeared in nbdkit 1.24.
.SH "SEE ALSO"
.IX Header "SEE ALSO"
\&\fBnbdkit\fR\|(1),
\&\fBnbdkit\-random\-plugin\fR\|(1),
\&\fBnbdkit\-sparse\-random\-plugin\fR\|(1),
\&\fBnbdkit\-filter\fR\|(3),
\&\fBnbdkit\-plugin\fR\|(3),
\&\fBnbdcopy\fR\|(1),
\&\fBqemu\-img\fR\|(1).
.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.