nbdkit-nozero-filter - Man Page

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

License

Redistribution and use in source and binary forms, with or without modification, are permitted provided that the following conditions are met:

THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

Referenced By

nbdkit(1), nbdkit-filter(3), nbdkit-fua-filter(1), nbdkit-nocache-filter(1), nbdkit-noextents-filter(1), nbdkit-noparallel-filter(1), nbdkit-plugin(3), nbdkit-release-notes-1.4(1).

2020-07-18 nbdkit-1.21.19