DOKK / manpages / debian 12 / nbdkit / nbdkit-checkwrite-filter.1.en
nbdkit-checkwrite-filter(1) NBDKIT nbdkit-checkwrite-filter(1)

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

 nbdkit --filter=checkwrite PLUGIN

"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.

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"'


 nbdkit -U - --filter=checkwrite linuxdisk testdir/ \
        --run 'qemu-img convert -n "$uri" "$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.

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.

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

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

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

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

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

Richard W.M. Jones

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2023-01-04 nbdkit-1.32.5