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

6       nbdkit-cache-filter - nbdkit caching filter
7

SYNOPSIS

9        nbdkit --filter=cache plugin [plugin-args...]
10                                     [cache=writeback|writethrough|unsafe]
11                                     [cache-min-block-size=SIZE]
12                                     [cache-max-size=SIZE]
13                                     [cache-high-threshold=N]
14                                     [cache-low-threshold=N]
15                                     [cache-on-read=true|false|/PATH]
16

DESCRIPTION

18       "nbdkit-cache-filter" is a filter that adds caching on top of a plugin.
19       This is useful if a plugin is slow or expensive to use, because nbdkit
20       will try to minimize requests to the plugin by caching previous
21       requests.
22
23       Note that many NBD clients are able to do caching, and because the
24       caching happens on the client side it will usually be more effective
25       than caching inside the server.  This filter can be used if the client
26       does not have effective caching, or (with "cache=unsafe") to defeat
27       flush requests from the client (which is unsafe and can cause data
28       loss, as the name suggests).
29
30       This filter only caches image contents.  To cache image metadata, use
31       nbdkit-cacheextents-filter(1) between this filter and the plugin.  To
32       accelerate sequential reads, use nbdkit-readahead-filter(1) or
33       nbdkit-scan-filter(1) on top of this filter.
34

PARAMETERS

36       cache=writeback
37           Store writes in the cache.  They are not written to the plugin
38           unless an explicit flush is done by the client.
39
40           This is the default caching mode, and is safe if your client issues
41           flush requests correctly (which is true for modern Linux and other
42           well-written NBD clients).
43
44       cache=writethrough
45           Always force writes through to the plugin.
46
47           This makes the cache less effective, but is necessary if your
48           client does not issue correct flush requests.
49
50       cache=unsafe
51           Ignore flush requests.  Never write to the plugin unless the cache
52           grows too large.
53
54           This is dangerous and can cause data loss, but this may be
55           acceptable if you only use it for testing or with data that you
56           don't care about or can cheaply reconstruct.
57
58       cache-min-block-size=SIZE
59           Set the minimum block size used by the cache.  This must be a power
60           of 2 and ≥ 4096.
61
62           The default is 64K, or the block size of the filesystem which
63           contains the temporary file storing the cache (whichever is
64           larger).
65
66       cache-max-size=SIZE
67       cache-high-threshold=N
68       cache-low-threshold=N
69           (nbdkit ≥ 1.10)
70
71           Limit the size of the cache to "SIZE".  See "CACHE MAXIMUM SIZE"
72           below.
73
74       cache-on-read=true
75           (nbdkit ≥ 1.10)
76
77           Cache read requests as well as write and cache requests.  Any time
78           a block is read from the plugin, it is saved in the cache (if there
79           is sufficient space) so the same data can be served more quickly
80           later.
81
82           Note that if the underlying data served by the plugin can be
83           modified by some other means (eg. something else can write to a
84           file which is being served by nbdkit-file-plugin(1)), this option
85           will cause nbdkit to serve stale data because reads won't always go
86           through to the plugin.
87
88       cache-on-read=false
89           Do not cache read requests (this is the default).
90
91       cache-on-read=/PATH
92           (nbdkit ≥ 1.28)
93
94           When /PATH (which must be an absolute path) exists, this behaves
95           like "cache-on-read=true", and when it does not exist like
96           "cache-on-read=false".  This allows you to control the cache-on-
97           read behaviour while nbdkit is running.
98

CACHE MAXIMUM SIZE

100       By default the cache can grow to any size (although not larger than the
101       virtual size of the underlying plugin) and you have to ensure there is
102       sufficient space in $TMPDIR for it.
103
104       Using the parameters "cache-max-size", "cache-high-threshold" and
105       "cache-low-threshold" you can limit the maximum size of the cache.
106
107       This requires kernel and filesystem support (for fallocate(2)
108       "FALLOC_FL_PUNCH_HOLE"), so it may not work on all platforms.
109
110       Some examples:
111
112       "cache-max-size=1G"
113           The cache is limited to around 1 gigabyte.
114
115       "cache-max-size=1G cache-high-threshold=95 cache-low-threshold=80"
116           Once the cache size reaches 972M (95% of 1G), blocks are reclaimed
117           from the cache until the size is reduced to 819M (80% of 1G).
118
119       The way this works is once the size of the cache exceeds
120       "SIZE" ✕ the high threshold, the filter works to reduce the size of the
121       cache until it is less than "SIZE" ✕ the low threshold.  Once the size
122       is below the low threshold, no more reclaim work is done until the size
123       exceeds the high threshold again.
124
125       The default thresholds are high 95% and low 80%.  You must set
126       0 < low < high.  The thresholds are expressed as integer percentages of
127       "cache-max-size".
128
129       Least recently used blocks are discarded first.
130

ENVIRONMENT VARIABLES

132       "TMPDIR"
133           The cache is stored in a temporary file located in /var/tmp by
134           default.  You can override this location by setting the "TMPDIR"
135           environment variable before starting nbdkit.
136

FILES

138       $filterdir/nbdkit-cache-filter.so
139           The filter.
140
141           Use "nbdkit --dump-config" to find the location of $filterdir.
142

VERSION

144       "nbdkit-cache-filter" first appeared in nbdkit 1.2.
145

SEE ALSO

147       nbdkit(1), nbdkit-file-plugin(1), nbdkit-cacheextents-filter(1),
148       nbdkit-cow-filter(1), nbdkit-readahead-filter(1), nbdkit-filter(3),
149       qemu-img(1).
150

AUTHORS

152       Eric Blake
153
154       Richard W.M. Jones
155
157       Copyright Red Hat
158

LICENSE

160       Redistribution and use in source and binary forms, with or without
161       modification, are permitted provided that the following conditions are
162       met:
163
164       •   Redistributions of source code must retain the above copyright
165           notice, this list of conditions and the following disclaimer.
166
167       •   Redistributions in binary form must reproduce the above copyright
168           notice, this list of conditions and the following disclaimer in the
169           documentation and/or other materials provided with the
170           distribution.
171
172       •   Neither the name of Red Hat nor the names of its contributors may
173           be used to endorse or promote products derived from this software
174           without specific prior written permission.
175
176       THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
177       EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
178       IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
179       PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
180       LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
181       CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
182       SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
183       BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
184       WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
185       OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
186       ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
187
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190nbdkit-1.36.2                     2023-11-26            nbdkit-cache-filter(1)
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