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

       The parameters "zeromode" and "fastzeromode" are optional and control which mode the
       filter will use.

       zeromode=none
           Zero support is not advertised to the client; clients must explicitly write any
           regions of zero like any other normal write.

           This is the default if the "zeromode" parameter is not specified.

       zeromode=emulate
           Zero support is advertised, but emulated by the filter by using the plugin's "pwrite"
           callback, regardless of whether the plugin itself has a more efficient C"zero"
           callback.

       zeromode=notrim
           (nbdkit ≥ 1.14)

           Zero requests are forwarded on to the plugin, except that the plugin will never see
           the "NBDKIT_MAY_TRIM" flag.  This can help determine if the client permitting trimming
           during zero operations makes a difference.  It is an error to request this mode if the
           plugin lacks the "zero" callback.

       zeromode=plugin
           (nbdkit ≥ 1.16)

           Zero requests are forwarded on to the plugin, unchanged by the filter; this mode is
           helpful when experimenting with the "fastzeromode" parameter.  It is an error to
           request this mode if the plugin lacks the "zero" callback.

       fastzeromode=none
           (nbdkit ≥ 1.16)

           Support for fast zeroing is not advertised to the client.

       fastzeromode=slow
           (nbdkit ≥ 1.16)

           Fast zero support is advertised to the client, but all fast zero requests result in an
           immediate "ENOTSUP" failure rather than performing any fallback attempts.

       fastzeromode=ignore
           (nbdkit ≥ 1.16)

           This mode is unsafe: Fast zero support is advertised to the client, but all fast zero
           requests behave as if the fast zero flag had not been included.  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.

       fastzeromode=default
           (nbdkit ≥ 1.16)

           This mode is the 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 1.4.

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) 2018-2020 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
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