1nbdkit-cache-filter(1) NBDKIT nbdkit-cache-filter(1)
2
6 nbdkit-cache-filter - nbdkit caching filter
7
9 nbdkit --filter=cache plugin [cache=writeback|writethrough|unsafe]
10 [cache-max-size=SIZE]
11 [cache-high-threshold=N]
12 [cache-low-threshold=N]
13 [cache-on-read=true|false]
14 [plugin-args...]
15
17 "nbdkit-cache-filter" is a filter that adds caching on top of a plugin.
18 This is useful if a plugin is slow or expensive to use, because nbdkit
19 will try to minimize requests to the plugin by caching previous
20 requests.
21 Note that many NBD clients are able to do caching, and because the
23 caching happens on the client side it will usually be more effective
24 than caching inside the server. This filter can be used if the client
25 does not have effective caching, or (with "cache=unsafe") to defeat
26 flush requests from the client (which is unsafe and can cause data
27 loss, as the name suggests).
28 This filter only caches image contents. To cache image metadata, use
30 nbdkit-cacheextents-filter(1) between this filter and the plugin. To
31 accelerate sequential reads, use nbdkit-readahead-filter(1) instead.
32
34 cache=writeback
35 Store writes in the cache. They are not written to the plugin
36 unless an explicit flush is done by the client.
37 This is the default caching mode, and is safe if your client issues
39 flush requests correctly (which is true for modern Linux and other
40 well-written NBD clients).
41 cache=writethrough
43 Always force writes through to the plugin.
44 This makes the cache less effective, but is necessary if your
46 client does not issue correct flush requests.
47 cache=unsafe
49 Ignore flush requests. Never write to the plugin unless the cache
50 grows too large.
51 This is dangerous and can cause data loss, but this may be
53 acceptable if you only use it for testing or with data that you
54 don't care about or can cheaply reconstruct.
55 cache-max-size=SIZE
57 cache-high-threshold=N
58 cache-low-threshold=N
59 (nbdkit ≥ 1.10)
60 Limit the size of the cache to "SIZE". See "CACHE MAXIMUM SIZE"
62 below.
63 cache-on-read=true
65 (nbdkit ≥ 1.10)
66 Cache read requests as well as write and cache requests. Any time
68 a block is read from the plugin, it is saved in the cache (if there
69 is sufficient space) so the same data can be served more quickly
70 later.
71 Note that if the underlying data served by the plugin can be
73 modified by some other means (eg. something else can write to a
74 file which is being served by nbdkit-file-plugin(1)), this option
75 will cause nbdkit to serve stale data because reads won't always go
76 through to the plugin.
77 cache-on-read=false
79 Do not cache read requests (this is the default).
80
82 By default the cache can grow to any size (although not larger than the
83 virtual size of the underlying plugin) and you have to ensure there is
84 sufficient space in $TMPDIR for it.
85 Using the parameters "cache-max-size", "cache-high-threshold" and
87 "cache-low-threshold" you can limit the maximum size of the cache.
88 This requires kernel and filesystem support (for fallocate(2)
90 "FALLOC_FL_PUNCH_HOLE"), so it may not work on all platforms.
91 Some examples:
93 "cache-max-size=1G"
95 The cache is limited to around 1 gigabyte.
96 "cache-max-size=1G cache-high-threshold=95 cache-low-threshold=80"
98 Once the cache size reaches 972M (95% of 1G), blocks are reclaimed
99 from the cache until the size is reduced to 819M (80% of 1G).
100 The way this works is once the size of the cache exceeds
102 "SIZE" ✕ the high threshold, the filter works to reduce the size of the
103 cache until it is less than "SIZE" ✕ the low threshold. Once the size
104 is below the low threshold, no more reclaim work is done until the size
105 exceeds the high threshold again.
106 The default thresholds are high 95% and low 80%. You must set
108 0 < low < high. The thresholds are expressed as integer percentages of
109 "cache-max-size".
110 Least recently used blocks are discarded first.
112
114 "TMPDIR"
115 The cache is stored in a temporary file located in /var/tmp by
116 default. You can override this location by setting the "TMPDIR"
117 environment variable before starting nbdkit.
118
120 $filterdir/nbdkit-cache-filter.so
121 The filter.
122 Use "nbdkit --dump-config" to find the location of $filterdir.
124
126 "nbdkit-cache-filter" first appeared in nbdkit 1.2.
127
129 nbdkit(1), nbdkit-file-plugin(1), nbdkit-cacheextents-filter(1),
130 nbdkit-readahead-filter(1), nbdkit-filter(3), qemu-img(1).
131
133 Eric Blake
134 Richard W.M. Jones
136
138 Copyright (C) 2018-2021 Red Hat Inc.
139
141 Redistribution and use in source and binary forms, with or without
142 modification, are permitted provided that the following conditions are
143 met:
144 • Redistributions of source code must retain the above copyright
146 notice, this list of conditions and the following disclaimer.
147 • Redistributions in binary form must reproduce the above copyright
149 notice, this list of conditions and the following disclaimer in the
150 documentation and/or other materials provided with the
151 distribution.
152 • Neither the name of Red Hat nor the names of its contributors may
154 be used to endorse or promote products derived from this software
155 without specific prior written permission.
156 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
158 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
159 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
160 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
161 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
162 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
163 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
164 BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
165 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
166 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
167 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
168nbdkit-1.25.8 2021-05-25 nbdkit-cache-filter(1)