NAME

       nbdkit-nozero-filter - nbdkit nozero filter

SYNOPSIS

        nbdkit --filter=nozero plugin [plugin-args...] \
          [zeromode=MODE] [fastzeromode=MODE]

DESCRIPTION

       "nbdkit-nozero-filter" is a filter that intentionally disables efficient handling of
       sparse file holes (ranges of all-zero bytes) across the NBD protocol.  It is mainly useful
       for evaluating timing differences between naive vs. sparse-aware connections, and for
       testing client or server fallbacks.

PARAMETERS

       zeromode=none
       zeromode=emulate
       zeromode=notrim
       zeromode=plugin
           Optional, controls which mode the filter will use.  Mode none (default) means that
           zero support is not advertised to the client. Mode emulate means that zero support is
           emulated by the filter using the plugin's "pwrite" callback, regardless of whether the
           plugin itself implemented the "zero" callback with a more efficient way to write
           zeros. Since nbdkit ≥ 1.13.4, mode notrim means that zero requests are forwarded on to
           the plugin, except that the plugin will never see the NBDKIT_MAY_TRIM flag, to
           determine if the client permitting trimming during zero operations makes a difference.
           Since nbdkit ≥ 1.15.0, mode plugin leaves normal zero requests up to the plugin,
           useful when combined with "fastzeromode" for experimenting with the effects of fast
           zero requests.  It is an error to request notrim or plugin if the plugin does not
           support the "zero" callback.

       fastzeromode=none
       fastzeromode=slow
       fastzeromode=ignore
       fastzeromode=default
           Optional since nbdkit ≥ 1.15.0, controls whether fast zeroes are advertised to the
           client, and if so, how the filter will react to a client fast zero request.  Mode none
           avoids advertising fast zero support.  Mode slow advertises fast zero support
           unconditionally, but treats all fast zero requests as an immediate "ENOTSUP" failure
           rather than performing a fallback.  Mode ignore advertises fast zero support, but
           treats all client fast zero requests as if the flag had not been used (this behavior
           is typically contrary to the NBD specification, but can be useful for comparison
           against the actual fast zero implementation to see if fast zeroes make a difference).
           Mode default is selected by default; when paired with "zeromode=emulate", fast zeroes
           are advertised but fast zero requests always fail (similar to "slow"); when paired
           with "zeromode=notrim" or "zeromode=plugin", fast zero support is left to the plugin
           (although in the latter case, the nozero filter could be omitted for the same
           behavior).

EXAMPLES

       Serve the file disk.img, but force the client to write zeroes explicitly rather than with
       "NBD_CMD_WRITE_ZEROES":

        nbdkit --filter=nozero file disk.img

       Serve the file disk.img, allowing the client to take advantage of less network traffic via
       "NBD_CMD_WRITE_ZEROES", but fail any fast zero requests up front and force all other zero
       requests to write data explicitly rather than punching any holes:

        nbdkit --filter=nozero file zeromode=emulate disk.img

       Serve the file disk.img, but do not advertise fast zero support to the client even if the
       plugin supports it:

        nbdkit --filter=nozero file zeromode=plugin fastzeromode=none disk.img

FILES

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

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

VERSION

       "nbdkit-nozero-filter" first appeared in nbdkit 1.2.

SEE ALSO

       nbdkit(1), nbdkit-file-plugin(1), nbdkit-filter(3), nbdkit-fua-filter(1),
       nbdkit-nocache-filter(1), nbdkit-noparallel-filter(1), nbdkit-noextents-filter(1).

AUTHORS

       Eric Blake

COPYRIGHT

       Copyright (C) 2018-2019 Red Hat Inc.

LICENSE

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       ·   Neither the name of Red Hat nor the names of its contributors may be used to endorse
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