nbdkit-fua-filter(1)

1nbdkit-fua-filter(1)                NBDKIT                nbdkit-fua-filter(1)
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NAME

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

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

12       "nbdkit-fua-filter" is a filter that intentionally modifies handling of
13       the “Forced Unit Access” (FUA) flag across the NBD protocol.
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15       This filter can be used to disable FUA and flush requests for speed
16       (although this is unsafe).  Also it can be used to test client or
17       server fallbacks, and for evaluating timing differences between proper
18       use of FUA compared to a full flush.
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PARAMETERS

21       The "fuamode" parameter is optional and controls which mode the filter
22       will use.
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24       fuamode=discard
25           (nbdkit ≥ 1.22)
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27           The filter will discard FUA and flush requests.
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29           This mode is unsafe: If the NBD disk contains a filesystem then you
30           will likely lose data in the event of a crash.  It should only be
31           used for ephemeral data which you can easily recreate, such as
32           caches, builds, test data, etc.
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34       fuamode=pass
35           (nbdkit ≥ 1.22)
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37           Pass through FUA and flush requests unchanged.  Turns the filter
38           into a no-op.
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40       fuamode=none
41           FUA support is not advertised to the client.  Clients will not be
42           able to issue FUA write requests, but can send flush commands if
43           the plugin supports it.
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45           This is the default if the "fuamode" parameter is not specified.
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47       fuamode=emulate
48           The filter will emulate FUA support using the plugin’s ".flush"
49           callback, regardless of whether the plugin itself supports more
50           efficient FUA.  It refuses to load if the plugin does not support
51           flush.
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53       fuamode=native
54           The filter will advertise native FUA support to the client and
55           earlier filters in the chain.  This is useful for comparing
56           optimizations of FUA handling when splitting large requests into
57           sub-requests.  It refuses to load if the plugin’s ".can_fua"
58           callback returns "NBDKIT_FUA_NONE".
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60       fuamode=force
61           The filter will request FUA on all write transactions, even when
62           the client did not request it (“write-through” mode).  In turn
63           client flush requests become no-ops.  It refuses to load if the
64           plugin’s ".can_fua" callback returns "NBDKIT_FUA_NONE".
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EXAMPLES

67       ·   Serve the file disk.img discarding all FUA and flush requests.
68           This can greatly improve performance, but you will likely lose data
69           if there is a crash, so it is not safe.
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71            nbdkit --filter=discard file disk.img fuamode=discard
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73       ·   Serve the file disk.img, but force the client to submit explicit
74           flush requests instead of using "NBD_CMD_FLAG_FUA":
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76            nbdkit --filter=fua file disk.img
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78       ·   Observe that the blocksize filter optimizes its handling of the FUA
79           flag based on whether it knows nbdkit will be emulating FUA with a
80           flush, by comparing the log filter output on top of different fua
81           filter modes:
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83            nbdkit --filter=blocksize --filter=log --filter=fua file disk.img \
84              maxlen=4k logfile=fua_emulated fuamode=emulate
85            nbdkit --filter=blocksize --filter=log --filter=fua file disk.img \
86              maxlen=4k logfile=fua_native fuamode=native
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88       ·   Serve the file disk.img in write-through mode, where all writes
89           from the client are immediately flushed to disk as if the client
90           had always requested FUA:
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92            nbdkit --filter=fua file disk.img fuamode=force
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FILES

95       $filterdir/nbdkit-fua-filter.so
96           The filter.
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98           Use "nbdkit --dump-config" to find the location of $filterdir.
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VERSION

101       "nbdkit-fua-filter" first appeared in nbdkit 1.4.
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AUTHORS

110       Eric Blake
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COPYRIGHT

113       Copyright (C) 2018 Red Hat Inc.
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LICENSE

116       Redistribution and use in source and binary forms, with or without
117       modification, are permitted provided that the following conditions are
118       met:
119
120       ·   Redistributions of source code must retain the above copyright
121           notice, this list of conditions and the following disclaimer.
122
123       ·   Redistributions in binary form must reproduce the above copyright
124           notice, this list of conditions and the following disclaimer in the
125           documentation and/or other materials provided with the
126           distribution.
127
128       ·   Neither the name of Red Hat nor the names of its contributors may
129           be used to endorse or promote products derived from this software
130           without specific prior written permission.
131
132       THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
133       EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
134       IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
135       PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
136       LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
137       CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
138       SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
139       BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
140       WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
141       OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
142       ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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146nbdkit-1.24.2                     2021-03-02              nbdkit-fua-filter(1)