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

       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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