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)
7

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.
19
20       Note that by default, the NBD protocol does not guarantee that the use
21       of FUA from one connection will be visible from another connection
22       unless the server advertised NBD_FLAG_MULTI_CONN.  You may wish to
23       combine this filter with nbdkit-multi-conn-filter(1) if you plan on
24       making multiple connections to the plugin.
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PARAMETERS

27       The "fuamode" parameter is optional and controls which mode the filter
28       will use.
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30       fuamode=discard
31           (nbdkit ≥ 1.22)
32
33           The filter will discard FUA and flush requests.
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35           This mode is unsafe: If the NBD disk contains a filesystem then you
36           will likely lose data in the event of a crash.  It should only be
37           used for ephemeral data which you can easily recreate, such as
38           caches, builds, test data, etc.
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40       fuamode=pass
41           (nbdkit ≥ 1.22)
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43           Pass through FUA and flush requests unchanged.  Turns the filter
44           into a no-op.
45
46       fuamode=none
47           FUA support is not advertised to the client.  Clients will not be
48           able to issue FUA write requests, but can send flush commands if
49           the plugin supports it.
50
51           This is the default if the "fuamode" parameter is not specified.
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53       fuamode=emulate
54           The filter will emulate FUA support using the plugin’s ".flush"
55           callback, regardless of whether the plugin itself supports more
56           efficient FUA.  It refuses to load if the plugin does not support
57           flush.
58
59       fuamode=native
60           The filter will advertise native FUA support to the client and
61           earlier filters in the chain.  This is useful for comparing
62           optimizations of FUA handling when splitting large requests into
63           sub-requests.  It refuses to load if the plugin’s ".can_fua"
64           callback returns "NBDKIT_FUA_NONE".
65
66       fuamode=force
67           The filter will request FUA on all write transactions, even when
68           the client did not request it (“write-through” mode).  In turn
69           client flush requests become no-ops.  It refuses to load if the
70           plugin’s ".can_fua" callback returns "NBDKIT_FUA_NONE".
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EXAMPLES

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

101       $filterdir/nbdkit-fua-filter.so
102           The filter.
103
104           Use "nbdkit --dump-config" to find the location of $filterdir.
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VERSION

107       "nbdkit-fua-filter" first appeared in nbdkit 1.4.
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SEE ALSO

110       nbdkit(1), nbdkit-file-plugin(1), nbdkit-filter(3),
111       nbdkit-blocksize-filter(1), nbdkit-log-filter(1),
112       nbdkit-multi-conn-filter(1), nbdkit-nocache-filter(1),
113       nbdkit-noextents-filter(1), nbdkit-noparallel-filter(1),
114       nbdkit-nozero-filter(1).
115

AUTHORS

117       Eric Blake
118
120       Copyright (C) 2018 Red Hat Inc.
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LICENSE

123       Redistribution and use in source and binary forms, with or without
124       modification, are permitted provided that the following conditions are
125       met:
126
127       •   Redistributions of source code must retain the above copyright
128           notice, this list of conditions and the following disclaimer.
129
130       •   Redistributions in binary form must reproduce the above copyright
131           notice, this list of conditions and the following disclaimer in the
132           documentation and/or other materials provided with the
133           distribution.
134
135       •   Neither the name of Red Hat nor the names of its contributors may
136           be used to endorse or promote products derived from this software
137           without specific prior written permission.
138
139       THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
140       EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
141       IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
142       PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
143       LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
144       CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
145       SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
146       BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
147       WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
148       OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
149       ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
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153nbdkit-1.28.2                     2021-11-09              nbdkit-fua-filter(1)
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