nbdkit-nozero-filter - nbdkit nozero filter
nbdkit --filter=nozero plugin [plugin-args...] \
[zeromode=MODE] [fastzeromode=MODE]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.
The parameters zeromode and fastzeromode are optional and control which mode the filter will use.
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.
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 zero callback.
(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.
(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.
(nbdkit ≥ 1.16)
Support for fast zeroing is not advertised to the client.
(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.
(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.
(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).
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.imgServe 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.imgServe 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.imgThe filter.
Use nbdkit --dump-config to find the location of $filterdir.
nbdkit-nozero-filter first appeared in nbdkit 1.4.
nbdkit(1), nbdkit-file-plugin(1), nbdkit-filter(3), nbdkit-fua-filter(1), nbdkit-multi-conn-filter(1), nbdkit-nocache-filter(1), nbdkit-noparallel-filter(1), nbdkit-noextents-filter(1).
Eric Blake
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