NAME

       nbdkit-fua-filter - modify nbdkit flush and Forced Unit Access (FUA)

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.

       This filter can be used to disable FUA and flush requests for speed (although this is unsafe).  Also it
       can be used to test client or server fallbacks, and for evaluating timing differences between proper use
       of FUA compared to a full flush.

PARAMETERS

       The "fuamode" parameter is optional and controls which mode the filter will use.

       fuamode=discard
           (nbdkit ≥ 1.22)

           The filter will discard FUA and flush requests.

           This  mode  is  unsafe:  If  the NBD disk contains a filesystem then you will likely lose data in the
           event of a crash.  It should only be used for ephemeral data which you can easily recreate,  such  as
           caches, builds, test data, etc.

       fuamode=pass
           (nbdkit ≥ 1.22)

           Pass through FUA and flush requests unchanged.  Turns the filter into a no-op.

       fuamode=none
           FUA  support  is not advertised to the client.  Clients will not be able to issue FUA write requests,
           but can send flush commands if the plugin supports it.

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

       fuamode=emulate
           The filter will emulate FUA support using the plugin’s ".flush" callback, regardless of  whether  the
           plugin itself supports more efficient FUA.  It refuses to load if the plugin does not support flush.

       fuamode=native
           The filter will advertise native FUA support to the client and earlier filters in the chain.  This is
           useful  for  comparing optimizations of FUA handling when splitting large requests into sub-requests.
           It refuses to load if the plugin’s ".can_fua" callback returns "NBDKIT_FUA_NONE".

       fuamode=force
           The filter will request FUA on all write transactions, even  when  the  client  did  not  request  it
           (“write-through”  mode).   In  turn  client  flush requests become no-ops.  It refuses to load if the
           plugin’s ".can_fua" callback returns "NBDKIT_FUA_NONE".

EXAMPLES

       •   Serve the file disk.img discarding all FUA and flush requests.  This can greatly improve performance,
           but you will likely lose data if there is a crash, so it is not safe.

            nbdkit --filter=discard file disk.img fuamode=discard

       •   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 disk.img fuamode=force

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 1.4.

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

COPYRIGHT

       Copyright (C) 2018 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.

       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.

nbdkit-1.24.1                                      2021-01-20                               nbdkit-fua-filter(1)