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 Limit the size of the cache to "SIZE". See "CACHE MAXIMUM SIZE"
66 below.
67 cache-on-read=true
69 Cache read requests as well as write and cache requests. Any time
70 a block is read from the plugin, it is saved in the cache (if there
71 is sufficient space) so the same data can be served more quickly
72 later.
73 Note that if the underlying data served by the plugin can be
75 modified by some other means (eg. something else can write to a
76 file which is being served by nbdkit-file-plugin(1)), this option
77 will cause nbdkit to serve stale data because reads won't always go
78 through to the plugin.
79 cache-on-read=false
81 Do not cache read requests (this is the default).
82
84 By default the cache can grow to any size (although not larger than the
85 virtual size of the underlying plugin) and you have to ensure there is
86 sufficient space in $TMPDIR for it.
87 Using the parameters "cache-max-size", "cache-high-threshold" and
89 "cache-low-threshold" you can limit the maximum size of the cache.
90 This requires kernel and filesystem support (for fallocate(2)
92 "FALLOC_FL_PUNCH_HOLE"), so it may not work on all platforms.
93 Some examples:
95 "cache-max-size=1G"
97 The cache is limited to around 1 gigabyte.
98 "cache-max-size=1G cache-high-threshold=95 cache-low-threshold=80"
100 Once the cache size reaches 972M (95% of 1G), blocks are reclaimed
101 from the cache until the size is reduced to 819M (80% of 1G).
102 The way this works is once the size of the cache exceeds
104 "SIZE" ✕ the high threshold, the filter works to reduce the size of the
105 cache until it is less than "SIZE" ✕ the low threshold. Once the size
106 is below the low threshold, no more reclaim work is done until the size
107 exceeds the high threshold again.
108 The default thresholds are high 95% and low 80%. You must set
110 0 < low < high. The thresholds are expressed as integer percentages of
111 "cache-max-size".
112 Least recently used blocks are discarded first.
114
116 "TMPDIR"
117 The cache is stored in a temporary file located in /var/tmp by
118 default. You can override this location by setting the "TMPDIR"
119 environment variable before starting nbdkit.
120
122 $filterdir/nbdkit-cache-filter.so
123 The filter.
124 Use "nbdkit --dump-config" to find the location of $filterdir.
126
128 "nbdkit-cache-filter" first appeared in nbdkit 1.2.
129
131 nbdkit(1), nbdkit-file-plugin(1), nbdkit-cacheextents-filter(1),
132 nbdkit-readahead-filter(1), nbdkit-truncate-filter(1),
133 nbdkit-filter(3), qemu-img(1).
134
136 Eric Blake
137 Richard W.M. Jones
139
141 Copyright (C) 2018-2019 Red Hat Inc.
142
144 Redistribution and use in source and binary forms, with or without
145 modification, are permitted provided that the following conditions are
146 met:
147 · Redistributions of source code must retain the above copyright
149 notice, this list of conditions and the following disclaimer.
150 · Redistributions in binary form must reproduce the above copyright
152 notice, this list of conditions and the following disclaimer in the
153 documentation and/or other materials provided with the
154 distribution.
155 · Neither the name of Red Hat nor the names of its contributors may
157 be used to endorse or promote products derived from this software
158 without specific prior written permission.
159 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
161 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
162 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
163 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
164 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
165 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
166 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
167 BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
168 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
169 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
170 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
171nbdkit-1.16.1 2019-12-03 nbdkit-cache-filter(1)