NAME

       nbdkit-checkwrite-filter - check writes match contents of plugin

SYNOPSIS

        nbdkit --filter=checkwrite PLUGIN

DESCRIPTION

       "nbdkit-checkwrite-filter" 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.

       The purpose of this filter is to test copying tools as described below.

       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.

   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:

        nbdkit -U - --filter=checkwrite data "@32768 1" \
               --run 'nbdcopy "$uri" "$uri"'

        nbdkit -U - --filter=checkwrite file disk.img \
               --run 'nbdcopy "$uri" "$uri"'

       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.

   Other plugins for testing NBD tools
       If you are doing this kind of testing you will also want to look at
       nbdkit-random-plugin(1) and nbdkit-sparse-random-plugin(1).  These plugins already have
       this behaviour built in to them (because it is more efficient to build it into the plugin)
       — you do not need to use this filter with those plugins.  This filter is useful for
       testing with arbitrary plugins.

PARAMETERS

       There are no parameters specific to this filter.  Parameters are passed through to the
       underlying plugin.

FILES

       $filterdir/nbdkit-checkwrite-filter.so
           The filter.

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

VERSION

       "nbdkit-checkwrite-filter" first appeared in nbdkit 1.24.

SEE ALSO

       nbdkit(1), nbdkit-random-plugin(1), nbdkit-sparse-random-plugin(1), nbdkit-filter(3),
       nbdkit-plugin(3), nbdcopy(1).

AUTHORS

       Richard W.M. Jones

COPYRIGHT

       Copyright (C) 2020 Red Hat Inc.

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.

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