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:

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           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
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