1nbdkit-cow-filter(1) NBDKIT nbdkit-cow-filter(1)
2
6 nbdkit-cow-filter - nbdkit copy-on-write (COW) filter
7
9 nbdkit --filter=cow plugin [plugin-args...]
10 [cow-block-size=N]
11 [cow-on-cache=false|true]
12 [cow-on-read=false|true|/PATH]
13
15 "nbdkit-cow-filter" is a filter that makes a temporary writable copy on
16 top of a plugin. It can also be used to enable writes for plugins
17 which are read-only.
18 The underlying plugin is opened read-only. This filter does not pass
20 any writes or write-like operations (like trim and zero) through to the
21 underlying plugin.
22 Note that anything written is thrown away as soon as nbdkit exits. If
24 you want to save changes, either copy out the whole disk using a tool
25 like nbdcopy(1), or use the method described in "NOTES" below to create
26 a diff.
27 Limitations of the filter include:
29 • The underlying file/device must not be resized.
31 • The underlying plugin must behave “normally”, meaning that it must
33 serve the same data to each client.
34
36 cow-block-size=N
37 Set the block size used by the filter. This has to be a power of
38 two and the minimum block size is 4K. The maximum block size
39 depends on the plugin, but a block size larger than a few megabytes
40 is not usually a good idea.
41 The default is 64K.
43 cow-on-cache=false
45 Do not save data from cache (prefetch) requests in the overlay.
46 This leaves the overlay as small as possible. This is the default.
47 cow-on-cache=true
49 When the client issues a cache (prefetch) request, preemptively
50 save the data from the plugin into the overlay.
51 cow-on-read=false
53 Do not save data from read requests in the overlay. This leaves
54 the overlay as small as possible. This is the default.
55 cow-on-read=true
57 When the client issues a read request, copy the data into the
58 overlay so that the same data can be served more quickly later.
59 cow-on-read=/PATH
61 When /PATH (which must be an absolute path) exists, this behaves
62 like "cow-on-read=true", and when it does not exist like
63 "cow-on-read=false". This allows you to control the "cow-on-read"
64 behaviour while nbdkit is running.
65
67 nbdkit --filter=cow file disk.img
68 Serve the file disk.img, allowing writes, but do not save any changes
69 into the file.
70 nbdkit --filter=cow --filter=xz file disk.xz cow-on-read=true
72 nbdkit-xz-filter(1) only supports read access, but you can provide
73 temporary write access by using the command above. Because xz
74 decompression is slow, using "cow-on-read=true" causes reads to be
75 cached as well as writes, improving performance at the expense of using
76 more temporary space. Note that writes are thrown away when nbdkit
77 exits and do not get saved into the file.
78
80 Creating a diff with qemu-img
81 Although nbdkit-cow-filter itself cannot save the differences, it is
82 possible to do this using an obscure feature of qemu-img(1). nbdkit
83 must remain continuously running during the whole operation, otherwise
84 all changes will be lost.
85 Run nbdkit:
87 nbdkit --filter=cow file disk.img
89 and then connect with a client and make whatever changes you need. At
91 the end, disconnect the client.
92 Run these "qemu-img" commands to construct a qcow2 file containing the
94 differences:
95 qemu-img create -F raw -b nbd:localhost -f qcow2 diff.qcow2
97 qemu-img rebase -F raw -b disk.img -f qcow2 diff.qcow2
98 diff.qcow2 now contains the differences between the base (disk.img) and
100 the changes stored in nbdkit-cow-filter. "nbdkit" can now be killed.
101 Compared to nbd-server -c option
103 All connections to the nbdkit instance see the same view of the disk.
104 This is different from nbd-server(1) -c option where each connection
105 sees its own copy-on-write overlay and simply disconnecting the client
106 throws that away. It also allows us to create diffs as above.
107
109 "TMPDIR"
110 The copy-on-write changes are stored in a temporary file located in
111 /var/tmp by default. You can override this location by setting the
112 "TMPDIR" environment variable before starting nbdkit.
113
115 $filterdir/nbdkit-cow-filter.so
116 The filter.
117 Use "nbdkit --dump-config" to find the location of $filterdir.
119
121 "nbdkit-cow-filter" first appeared in nbdkit 1.2.
122
124 nbdkit(1), nbdkit-file-plugin(1), nbdkit-cache-filter(1),
125 nbdkit-cacheextents-filter(1), nbdkit-xz-filter(1), nbdkit-filter(3),
126 nbdcopy(1), qemu-img(1).
127
129 Eric Blake
130 Richard W.M. Jones
132
134 Copyright Red Hat
135
137 Redistribution and use in source and binary forms, with or without
138 modification, are permitted provided that the following conditions are
139 met:
140 • Redistributions of source code must retain the above copyright
142 notice, this list of conditions and the following disclaimer.
143 • Redistributions in binary form must reproduce the above copyright
145 notice, this list of conditions and the following disclaimer in the
146 documentation and/or other materials provided with the
147 distribution.
148 • Neither the name of Red Hat nor the names of its contributors may
150 be used to endorse or promote products derived from this software
151 without specific prior written permission.
152 THIS SOFTWARE IS PROVIDED BY RED HAT AND CONTRIBUTORS ''AS IS'' AND ANY
154 EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
155 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
156 PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL RED HAT OR CONTRIBUTORS BE
157 LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
158 CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
159 SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR
160 BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY,
161 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR
162 OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF
163 ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
164nbdkit-1.34.4 2023-09-26 nbdkit-cow-filter(1)