NAME

       nbdkit-fua-filter - 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

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.

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