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 Note that the use of this filter rounds the image size down to a
30 multiple of the caching granularity (the larger of 4096 or the
31 "f_bsize" field of fstatvfs(3)), to ease the implementation. If you
32 need to round the image size up instead to access the last few bytes,
33 combine this filter with nbdkit-truncate-filter(1).
34 This filter only caches image contents. To cache image metadata, use
36 nbdkit-cacheextents-filter(1) between this filter and the plugin. To
37 accelerate sequential reads, use nbdkit-readahead-filter(1) instead.
38
40 cache=writeback
41 Store writes in the cache. They are not written to the plugin
42 unless an explicit flush is done by the client.
43 This is the default caching mode, and is safe if your client issues
45 flush requests correctly (which is true for modern Linux and other
46 well-written NBD clients).
47 cache=writethrough
49 Always force writes through to the plugin.
50 This makes the cache less effective, but is necessary if your
52 client does not issue correct flush requests.
53 cache=unsafe
55 Ignore flush requests. Never write to the plugin unless the cache
56 grows too large.
57 This is dangerous and can cause data loss, but this may be
59 acceptable if you only use it for testing or with data that you
60 don't care about or can cheaply reconstruct.
61 cache-max-size=SIZE
63 cache-high-threshold=N
64 cache-low-threshold=N
65 (nbdkit ≥ 1.10)
66 Limit the size of the cache to "SIZE". See "CACHE MAXIMUM SIZE"
68 below.
69 cache-on-read=true
71 (nbdkit ≥ 1.10)
72 Cache read requests as well as write and cache requests. Any time
74 a block is read from the plugin, it is saved in the cache (if there
75 is sufficient space) so the same data can be served more quickly
76 later.
77 Note that if the underlying data served by the plugin can be
79 modified by some other means (eg. something else can write to a
80 file which is being served by nbdkit-file-plugin(1)), this option
81 will cause nbdkit to serve stale data because reads won't always go
82 through to the plugin.
83 cache-on-read=false
85 Do not cache read requests (this is the default).
86
88 By default the cache can grow to any size (although not larger than the
89 virtual size of the underlying plugin) and you have to ensure there is
90 sufficient space in $TMPDIR for it.
91 Using the parameters "cache-max-size", "cache-high-threshold" and
93 "cache-low-threshold" you can limit the maximum size of the cache.
94 This requires kernel and filesystem support (for fallocate(2)
96 "FALLOC_FL_PUNCH_HOLE"), so it may not work on all platforms.
97 Some examples:
99 "cache-max-size=1G"
101 The cache is limited to around 1 gigabyte.
102 "cache-max-size=1G cache-high-threshold=95 cache-low-threshold=80"
104 Once the cache size reaches 972M (95% of 1G), blocks are reclaimed
105 from the cache until the size is reduced to 819M (80% of 1G).
106 The way this works is once the size of the cache exceeds
108 "SIZE" ✕ the high threshold, the filter works to reduce the size of the
109 cache until it is less than "SIZE" ✕ the low threshold. Once the size
110 is below the low threshold, no more reclaim work is done until the size
111 exceeds the high threshold again.
112 The default thresholds are high 95% and low 80%. You must set
114 0 < low < high. The thresholds are expressed as integer percentages of
115 "cache-max-size".
116 Least recently used blocks are discarded first.
118
120 "TMPDIR"
121 The cache is stored in a temporary file located in /var/tmp by
122 default. You can override this location by setting the "TMPDIR"
123 environment variable before starting nbdkit.
124
126 $filterdir/nbdkit-cache-filter.so
127 The filter.
128 Use "nbdkit --dump-config" to find the location of $filterdir.
130
132 "nbdkit-cache-filter" first appeared in nbdkit 1.2.
133
135 nbdkit(1), nbdkit-file-plugin(1), nbdkit-cacheextents-filter(1),
136 nbdkit-readahead-filter(1), nbdkit-truncate-filter(1),
137 nbdkit-filter(3), qemu-img(1).
138
140 Eric Blake
141 Richard W.M. Jones
143
145 Copyright (C) 2018-2020 Red Hat Inc.
146
148 Redistribution and use in source and binary forms, with or without
149 modification, are permitted provided that the following conditions are
150 met:
151 · Redistributions of source code must retain the above copyright
153 notice, this list of conditions and the following disclaimer.
154 · Redistributions in binary form must reproduce the above copyright
156 notice, this list of conditions and the following disclaimer in the
157 documentation and/or other materials provided with the
158 distribution.
159 · Neither the name of Red Hat nor the names of its contributors may
161 be used to endorse or promote products derived from this software
162 without specific prior written permission.
163 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
165 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
166 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
167 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
168 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
169 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
170 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
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172 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
173 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
174 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
175nbdkit-1.24.2 2021-03-02 nbdkit-cache-filter(1)