nbdkit-fua-filter - Man Page

nbdkit FUA filter

Synopsis

 nbdkit --filter=fua plugin [fuamode=MODE] [plugin-args...]

Description

nbdkit-fua-filter is a filter that intentionally modifies handling of the 'Forced Unit Access' (FUA) flag across the NBD protocol.  It is mainly useful for testing client or server fallbacks, and for evaluating timing differences between proper use of FUA compared to a full flush.

Parameters

fuamode=none
fuamode=emulate
fuamode=native
fuamode=force

Optional, controls which mode the filter will use.  Mode none (default) means that FUA support is not advertised to the client. Mode emulate causes the filter to emulate FUA support using the plugin's flush callback, regardless of whether the plugin itself supports more efficient FUA (or refuses to load if the plugin's can_flush callback returns false).  Mode native causes the filter to advertise native FUA support to earlier filters, useful for comparing optimizations of FUA handling when splitting large requests into sub-requests (or refuses to load if the plugin's can_fua callback returns NBDKIT_FUA_NONE).  Mode force causes the filter to request FUA on all write transactions, even when the client did not request it, and in turn treats client flush requests as a no-op (or refuses to load if the plugin's can_fua callback returns NBDKIT_FUA_NONE).

Examples

Serve the file disk.img, but force the client to submit explicit flush requests instead of using NBD_CMD_FLAG_FUA:

 nbdkit --filter=fua file disk.img

Observe that the blocksize filter optimizes its handling of the FUA flag based on whether it knows nbdkit will be emulating FUA with a flush, by comparing the log filter output on top of different fua filter modes:

 nbdkit --filter=blocksize --filter=log --filter=fua file disk.img \
   maxlen=4k logfile=fua_emulated fuamode=emulate
 nbdkit --filter=blocksize --filter=log --filter=fua file disk.img \
   maxlen=4k logfile=fua_native fuamode=native

Serve the file disk.img in write-through mode, where all writes from the client are immediately flushed to disk as if the client had always requested FUA:

 nbdkit --filter=fua file fuamode=force disk.img

Files

$filterdir/nbdkit-fua-filter.so

The filter.

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

Version

nbdkit-fua-filter first appeared in nbdkit 1.2.

See Also

nbdkit(1), nbdkit-file-plugin(1), nbdkit-filter(3), nbdkit-blocksize-filter(1), nbdkit-log-filter(1), nbdkit-nocache-filter(1), nbdkit-noextents-filter(1), nbdkit-noparallel-filter(1), nbdkit-nozero-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-nocache-filter(1), nbdkit-noextents-filter(1), nbdkit-noparallel-filter(1), nbdkit-nozero-filter(1), nbdkit-release-notes-1.4(1).

2020-03-31 nbdkit-1.19.6 NBDKIT