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1@example
2@c man begin SYNOPSIS
10985131 3@command{qemu-img} [@var{standard} @var{options}] @var{command} [@var{command} @var{options}]
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4@c man end
5@end example
6
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7@c man begin DESCRIPTION
8qemu-img allows you to create, convert and modify images offline. It can handle
9all image formats supported by QEMU.
10
11@b{Warning:} Never use qemu-img to modify images in use by a running virtual
12machine or any other process; this may destroy the image. Also, be aware that
13querying an image that is being modified by another process may encounter
14inconsistent state.
15@c man end
16
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17@c man begin OPTIONS
18
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19Standard options:
20@table @option
21@item -h, --help
22Display this help and exit
23@item -V, --version
24Display version information and exit
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25@item -T, --trace [[enable=]@var{pattern}][,events=@var{file}][,file=@var{file}]
26@findex --trace
27@include qemu-option-trace.texi
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28@end table
29
acd935ef 30The following commands are supported:
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31
32@include qemu-img-cmds.texi
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33
34Command parameters:
35@table @var
36@item filename
37 is a disk image filename
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38
39@item --object @var{objectdef}
40
41is a QEMU user creatable object definition. See the @code{qemu(1)} manual
42page for a description of the object properties. The most common object
43type is a @code{secret}, which is used to supply passwords and/or encryption
44keys.
45
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46@item --image-opts
47
305b4c60 48Indicates that the source @var{filename} parameter is to be interpreted as a
eb769f74 49full option string, not a plain filename. This parameter is mutually
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50exclusive with the @var{-f} parameter.
51
52@item --target-image-opts
53
54Indicates that the @var{output_filename} parameter(s) are to be interpreted as
55a full option string, not a plain filename. This parameter is mutually
56exclusive with the @var{-O} parameters. It is currently required to also use
57the @var{-n} parameter to skip image creation. This restriction may be relaxed
58in a future release.
eb769f74 59
5fafdf24 60@item fmt
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61is the disk image format. It is guessed automatically in most cases. See below
62for a description of the supported disk formats.
acd935ef 63
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64@item --backing-chain
65will enumerate information about backing files in a disk image chain. Refer
66below for further description.
67
5fafdf24 68@item size
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69is the disk image size in bytes. Optional suffixes @code{k} or @code{K}
70(kilobyte, 1024) @code{M} (megabyte, 1024k) and @code{G} (gigabyte, 1024M)
71and T (terabyte, 1024G) are supported. @code{b} is ignored.
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72
73@item output_filename
5fafdf24 74is the destination disk image filename
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75
76@item output_fmt
77 is the destination format
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78@item options
79is a comma separated list of format specific options in a
80name=value format. Use @code{-o ?} for an overview of the options supported
3e032364 81by the used format or see the format descriptions below for details.
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82@item snapshot_param
83is param used for internal snapshot, format is
84'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'
85@item snapshot_id_or_name
86is deprecated, use snapshot_param instead
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87
88@item -c
89indicates that target image must be compressed (qcow format only)
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90@item -h
91with or without a command shows help and lists the supported formats
aaf55b47 92@item -p
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93display progress bar (compare, convert and rebase commands only).
94If the @var{-p} option is not used for a command that supports it, the
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95progress is reported when the process receives a @code{SIGUSR1} or
96@code{SIGINFO} signal.
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97@item -q
98Quiet mode - do not print any output (except errors). There's no progress bar
99in case both @var{-q} and @var{-p} options are used.
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100@item -S @var{size}
101indicates the consecutive number of bytes that must contain only zeros
102for qemu-img to create a sparse image during conversion. This value is rounded
103down to the nearest 512 bytes. You may use the common size suffixes like
104@code{k} for kilobytes.
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105@item -t @var{cache}
106specifies the cache mode that should be used with the (destination) file. See
107the documentation of the emulator's @code{-drive cache=...} option for allowed
108values.
40055951 109@item -T @var{src_cache}
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110specifies the cache mode that should be used with the source file(s). See
111the documentation of the emulator's @code{-drive cache=...} option for allowed
112values.
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113@end table
114
115Parameters to snapshot subcommand:
116
117@table @option
118
119@item snapshot
120is the name of the snapshot to create, apply or delete
121@item -a
122applies a snapshot (revert disk to saved state)
123@item -c
124creates a snapshot
125@item -d
126deletes a snapshot
127@item -l
128lists all snapshots in the given image
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129@end table
130
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131Parameters to compare subcommand:
132
133@table @option
134
135@item -f
136First image format
137@item -F
138Second image format
139@item -s
b6af0975 140Strict mode - fail on different image size or sector allocation
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141@end table
142
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143Parameters to convert subcommand:
144
145@table @option
146
147@item -n
148Skip the creation of the target volume
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149@item -m
150Number of parallel coroutines for the convert process
151@item -W
152Allow out-of-order writes to the destination. This option improves performance,
153but is only recommended for preallocated devices like host devices or other
154raw block devices.
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155@end table
156
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157Parameters to dd subcommand:
158
159@table @option
160
161@item bs=@var{block_size}
162defines the block size
163@item count=@var{blocks}
164sets the number of input blocks to copy
165@item if=@var{input}
166sets the input file
167@item of=@var{output}
168sets the output file
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169@item skip=@var{blocks}
170sets the number of input blocks to skip
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171@end table
172
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173Command description:
174
175@table @option
55d539c8 176@item bench [-c @var{count}] [-d @var{depth}] [-f @var{fmt}] [--flush-interval=@var{flush_interval}] [-n] [--no-drain] [-o @var{offset}] [--pattern=@var{pattern}] [-q] [-s @var{buffer_size}] [-S @var{step_size}] [-t @var{cache}] [-w] @var{filename}
b6133b8c 177
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178Run a simple sequential I/O benchmark on the specified image. If @code{-w} is
179specified, a write test is performed, otherwise a read test is performed.
180
181A total number of @var{count} I/O requests is performed, each @var{buffer_size}
d3199a31 182bytes in size, and with @var{depth} requests in parallel. The first request
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183starts at the position given by @var{offset}, each following request increases
184the current position by @var{step_size}. If @var{step_size} is not given,
185@var{buffer_size} is used for its value.
b6133b8c 186
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187If @var{flush_interval} is specified for a write test, the request queue is
188drained and a flush is issued before new writes are made whenever the number of
189remaining requests is a multiple of @var{flush_interval}. If additionally
190@code{--no-drain} is specified, a flush is issued without draining the request
191queue first.
192
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193If @code{-n} is specified, the native AIO backend is used if possible. On
194Linux, this option only works if @code{-t none} or @code{-t directsync} is
195specified as well.
196
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197For write tests, by default a buffer filled with zeros is written. This can be
198overridden with a pattern byte specified by @var{pattern}.
199
40055951 200@item check [-f @var{fmt}] [--output=@var{ofmt}] [-r [leaks | all]] [-T @var{src_cache}] @var{filename}
e6184690 201
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202Perform a consistency check on the disk image @var{filename}. The command can
203output in the format @var{ofmt} which is either @code{human} or @code{json}.
e6184690 204
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205If @code{-r} is specified, qemu-img tries to repair any inconsistencies found
206during the check. @code{-r leaks} repairs only cluster leaks, whereas
207@code{-r all} fixes all kinds of errors, with a higher risk of choosing the
0546b8c2 208wrong fix or hiding corruption that has already occurred.
4534ff54 209
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210Only the formats @code{qcow2}, @code{qed} and @code{vdi} support
211consistency checks.
212
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213In case the image does not have any inconsistencies, check exits with @code{0}.
214Other exit codes indicate the kind of inconsistency found or if another error
215occurred. The following table summarizes all exit codes of the check subcommand:
216
217@table @option
218
219@item 0
220Check completed, the image is (now) consistent
221@item 1
222Check not completed because of internal errors
223@item 2
224Check completed, image is corrupted
225@item 3
226Check completed, image has leaked clusters, but is not corrupted
227@item 63
228Checks are not supported by the image format
229
230@end table
231
232If @code{-r} is specified, exit codes representing the image state refer to the
233state after (the attempt at) repairing it. That is, a successful @code{-r all}
234will yield the exit code 0, independently of the image state before.
235
6e6e55f5 236@item create [-f @var{fmt}] [-b @var{backing_file}] [-F @var{backing_fmt}] [-u] [-o @var{options}] @var{filename} [@var{size}]
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237
238Create the new disk image @var{filename} of size @var{size} and format
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239@var{fmt}. Depending on the file format, you can add one or more @var{options}
240that enable additional features of this format.
acd935ef 241
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242If the option @var{backing_file} is specified, then the image will record
243only the differences from @var{backing_file}. No size needs to be specified in
244this case. @var{backing_file} will never be modified unless you use the
245@code{commit} monitor command (or qemu-img commit).
acd935ef 246
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247Note that a given backing file will be opened to check that it is valid. Use
248the @code{-u} option to enable unsafe backing file mode, which means that the
249image will be created even if the associated backing file cannot be opened. A
250matching backing file must be created or additional options be used to make the
251backing file specification valid when you want to use an image created this
252way.
253
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254The size can also be specified using the @var{size} option with @code{-o},
255it doesn't need to be specified separately in this case.
256
1b22bffd 257@item commit [-q] [-f @var{fmt}] [-t @var{cache}] [-b @var{base}] [-d] [-p] @var{filename}
acd935ef 258
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259Commit the changes recorded in @var{filename} in its base image or backing file.
260If the backing file is smaller than the snapshot, then the backing file will be
261resized to be the same size as the snapshot. If the snapshot is smaller than
262the backing file, the backing file will not be truncated. If you want the
263backing file to match the size of the smaller snapshot, you can safely truncate
264it yourself once the commit operation successfully completes.
acd935ef 265
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266The image @var{filename} is emptied after the operation has succeeded. If you do
267not need @var{filename} afterwards and intend to drop it, you may skip emptying
268@var{filename} by specifying the @code{-d} flag.
269
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270If the backing chain of the given image file @var{filename} has more than one
271layer, the backing file into which the changes will be committed may be
272specified as @var{base} (which has to be part of @var{filename}'s backing
273chain). If @var{base} is not specified, the immediate backing file of the top
274image (which is @var{filename}) will be used. For reasons of consistency,
275explicitly specifying @var{base} will always imply @code{-d} (since emptying an
276image after committing to an indirect backing file would lead to different data
277being read from the image due to content in the intermediate backing chain
278overruling the commit target).
279
40055951 280@item compare [-f @var{fmt}] [-F @var{fmt}] [-T @var{src_cache}] [-p] [-s] [-q] @var{filename1} @var{filename2}
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281
282Check if two images have the same content. You can compare images with
283different format or settings.
284
285The format is probed unless you specify it by @var{-f} (used for
286@var{filename1}) and/or @var{-F} (used for @var{filename2}) option.
287
288By default, images with different size are considered identical if the larger
289image contains only unallocated and/or zeroed sectors in the area after the end
290of the other image. In addition, if any sector is not allocated in one image
291and contains only zero bytes in the second one, it is evaluated as equal. You
292can use Strict mode by specifying the @var{-s} option. When compare runs in
293Strict mode, it fails in case image size differs or a sector is allocated in
294one image and is not allocated in the second one.
295
296By default, compare prints out a result message. This message displays
297information that both images are same or the position of the first different
298byte. In addition, result message can report different image size in case
299Strict mode is used.
300
301Compare exits with @code{0} in case the images are equal and with @code{1}
302in case the images differ. Other exit codes mean an error occurred during
303execution and standard error output should contain an error message.
304The following table sumarizes all exit codes of the compare subcommand:
305
306@table @option
307
308@item 0
309Images are identical
310@item 1
311Images differ
312@item 2
313Error on opening an image
314@item 3
315Error on checking a sector allocation
316@item 4
317Error on reading data
318
319@end table
320
2b4c0a20 321@item convert [-c] [-p] [-n] [-f @var{fmt}] [-t @var{cache}] [-T @var{src_cache}] [-O @var{output_fmt}] [-B @var{backing_file}] [-o @var{options}] [-s @var{snapshot_id_or_name}] [-l @var{snapshot_param}] [-m @var{num_coroutines}] [-W] [-S @var{sparse_size}] @var{filename} [@var{filename2} [...]] @var{output_filename}
acd935ef 322
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323Convert the disk image @var{filename} or a snapshot @var{snapshot_param}(@var{snapshot_id_or_name} is deprecated)
324to disk image @var{output_filename} using format @var{output_fmt}. It can be optionally compressed (@code{-c}
eff44266 325option) or use any format specific options like encryption (@code{-o} option).
acd935ef 326
8063d0fe 327Only the formats @code{qcow} and @code{qcow2} support compression. The
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328compression is read-only. It means that if a compressed sector is
329rewritten, then it is rewritten as uncompressed data.
330
acd935ef 331Image conversion is also useful to get smaller image when using a
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332growable format such as @code{qcow}: the empty sectors are detected and
333suppressed from the destination image.
acd935ef 334
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335@var{sparse_size} indicates the consecutive number of bytes (defaults to 4k)
336that must contain only zeros for qemu-img to create a sparse image during
337conversion. If @var{sparse_size} is 0, the source will not be scanned for
338unallocated or zero sectors, and the destination image will always be
339fully allocated.
340
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341You can use the @var{backing_file} option to force the output image to be
342created as a copy on write image of the specified base image; the
343@var{backing_file} should have the same content as the input's base image,
344however the path, image format, etc may differ.
345
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346If the @code{-n} option is specified, the target volume creation will be
347skipped. This is useful for formats such as @code{rbd} if the target
348volume has already been created with site specific options that cannot
349be supplied through qemu-img.
350
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351Out of order writes can be enabled with @code{-W} to improve performance.
352This is only recommended for preallocated devices like host devices or other
353raw block devices. Out of order write does not work in combination with
354creating compressed images.
355
356@var{num_coroutines} specifies how many coroutines work in parallel during
357the convert process (defaults to 8).
358
f7c15533 359@item dd [-f @var{fmt}] [-O @var{output_fmt}] [bs=@var{block_size}] [count=@var{blocks}] [skip=@var{blocks}] if=@var{input} of=@var{output}
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360
361Dd copies from @var{input} file to @var{output} file converting it from
362@var{fmt} format to @var{output_fmt} format.
363
364The data is by default read and written using blocks of 512 bytes but can be
365modified by specifying @var{block_size}. If count=@var{blocks} is specified
366dd will stop reading input after reading @var{blocks} input blocks.
367
368The size syntax is similar to dd(1)'s size syntax.
369
e5357560 370@item info [-f @var{fmt}] [--output=@var{ofmt}] [--backing-chain] @var{filename}
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371
372Give information about the disk image @var{filename}. Use it in
373particular to know the size reserved on disk which can be different
19d36792 374from the displayed size. If VM snapshots are stored in the disk image,
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375they are displayed too. The command can output in the format @var{ofmt}
376which is either @code{human} or @code{json}.
d2c639d6 377
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378If a disk image has a backing file chain, information about each disk image in
379the chain can be recursively enumerated by using the option @code{--backing-chain}.
380
381For instance, if you have an image chain like:
382
383@example
384base.qcow2 <- snap1.qcow2 <- snap2.qcow2
385@end example
386
387To enumerate information about each disk image in the above chain, starting from top to base, do:
388
389@example
390qemu-img info --backing-chain snap2.qcow2
391@end example
392
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393@item map [-f @var{fmt}] [--output=@var{ofmt}] @var{filename}
394
395Dump the metadata of image @var{filename} and its backing file chain.
396In particular, this commands dumps the allocation state of every sector
397of @var{filename}, together with the topmost file that allocates it in
398the backing file chain.
399
400Two option formats are possible. The default format (@code{human})
401only dumps known-nonzero areas of the file. Known-zero parts of the
402file are omitted altogether, and likewise for parts that are not allocated
403throughout the chain. @command{qemu-img} output will identify a file
404from where the data can be read, and the offset in the file. Each line
405will include four fields, the first three of which are hexadecimal
406numbers. For example the first line of:
407@example
408Offset Length Mapped to File
4090 0x20000 0x50000 /tmp/overlay.qcow2
4100x100000 0x10000 0x95380000 /tmp/backing.qcow2
411@end example
412@noindent
413means that 0x20000 (131072) bytes starting at offset 0 in the image are
414available in /tmp/overlay.qcow2 (opened in @code{raw} format) starting
415at offset 0x50000 (327680). Data that is compressed, encrypted, or
416otherwise not available in raw format will cause an error if @code{human}
417format is in use. Note that file names can include newlines, thus it is
418not safe to parse this output format in scripts.
419
420The alternative format @code{json} will return an array of dictionaries
421in JSON format. It will include similar information in
422the @code{start}, @code{length}, @code{offset} fields;
423it will also include other more specific information:
424@itemize @minus
425@item
426whether the sectors contain actual data or not (boolean field @code{data};
427if false, the sectors are either unallocated or stored as optimized
428all-zero clusters);
429
430@item
431whether the data is known to read as zero (boolean field @code{zero});
432
433@item
434in order to make the output shorter, the target file is expressed as
435a @code{depth}; for example, a depth of 2 refers to the backing file
436of the backing file of @var{filename}.
437@end itemize
438
439In JSON format, the @code{offset} field is optional; it is absent in
440cases where @code{human} format would omit the entry or exit with an error.
441If @code{data} is false and the @code{offset} field is present, the
442corresponding sectors in the file are not yet in use, but they are
443preallocated.
444
445For more information, consult @file{include/block/block.h} in QEMU's
446source code.
447
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448@item measure [--output=@var{ofmt}] [-O @var{output_fmt}] [-o @var{options}] [--size @var{N} | [--object @var{objectdef}] [--image-opts] [-f @var{fmt}] [-l @var{snapshot_param}] @var{filename}]
449
450Calculate the file size required for a new image. This information can be used
451to size logical volumes or SAN LUNs appropriately for the image that will be
452placed in them. The values reported are guaranteed to be large enough to fit
453the image. The command can output in the format @var{ofmt} which is either
454@code{human} or @code{json}.
455
456If the size @var{N} is given then act as if creating a new empty image file
457using @command{qemu-img create}. If @var{filename} is given then act as if
458converting an existing image file using @command{qemu-img convert}. The format
459of the new file is given by @var{output_fmt} while the format of an existing
460file is given by @var{fmt}.
461
462A snapshot in an existing image can be specified using @var{snapshot_param}.
463
464The following fields are reported:
465@example
466required size: 524288
467fully allocated size: 1074069504
468@end example
469
470The @code{required size} is the file size of the new image. It may be smaller
471than the virtual disk size if the image format supports compact representation.
472
473The @code{fully allocated size} is the file size of the new image once data has
474been written to all sectors. This is the maximum size that the image file can
475occupy with the exception of internal snapshots, dirty bitmaps, vmstate data,
476and other advanced image format features.
477
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478@item snapshot [-l | -a @var{snapshot} | -c @var{snapshot} | -d @var{snapshot} ] @var{filename}
479
480List, apply, create or delete snapshots in image @var{filename}.
ae6b0ed6 481
40055951 482@item rebase [-f @var{fmt}] [-t @var{cache}] [-T @var{src_cache}] [-p] [-u] -b @var{backing_file} [-F @var{backing_fmt}] @var{filename}
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483
484Changes the backing file of an image. Only the formats @code{qcow2} and
485@code{qed} support changing the backing file.
486
487The backing file is changed to @var{backing_file} and (if the image format of
488@var{filename} supports this) the backing file format is changed to
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489@var{backing_fmt}. If @var{backing_file} is specified as ``'' (the empty
490string), then the image is rebased onto no backing file (i.e. it will exist
491independently of any backing file).
e6184690 492
40055951 493@var{cache} specifies the cache mode to be used for @var{filename}, whereas
3ba6796d 494@var{src_cache} specifies the cache mode for reading backing files.
40055951 495
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496There are two different modes in which @code{rebase} can operate:
497@table @option
498@item Safe mode
499This is the default mode and performs a real rebase operation. The new backing
500file may differ from the old one and qemu-img rebase will take care of keeping
501the guest-visible content of @var{filename} unchanged.
502
503In order to achieve this, any clusters that differ between @var{backing_file}
504and the old backing file of @var{filename} are merged into @var{filename}
505before actually changing the backing file.
506
507Note that the safe mode is an expensive operation, comparable to converting
508an image. It only works if the old backing file still exists.
509
510@item Unsafe mode
511qemu-img uses the unsafe mode if @code{-u} is specified. In this mode, only the
512backing file name and format of @var{filename} is changed without any checks
513on the file contents. The user must take care of specifying the correct new
514backing file, or the guest-visible content of the image will be corrupted.
515
516This mode is useful for renaming or moving the backing file to somewhere else.
517It can be used without an accessible old backing file, i.e. you can use it to
518fix an image whose backing file has already been moved/renamed.
519@end table
520
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521You can use @code{rebase} to perform a ``diff'' operation on two
522disk images. This can be useful when you have copied or cloned
523a guest, and you want to get back to a thin image on top of a
524template or base image.
525
526Say that @code{base.img} has been cloned as @code{modified.img} by
527copying it, and that the @code{modified.img} guest has run so there
528are now some changes compared to @code{base.img}. To construct a thin
529image called @code{diff.qcow2} that contains just the differences, do:
530
531@example
532qemu-img create -f qcow2 -b modified.img diff.qcow2
533qemu-img rebase -b base.img diff.qcow2
534@end example
535
536At this point, @code{modified.img} can be discarded, since
537@code{base.img + diff.qcow2} contains the same information.
538
dc5f690b 539@item resize [--preallocation=@var{prealloc}] @var{filename} [+ | -]@var{size}
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540
541Change the disk image as if it had been created with @var{size}.
542
543Before using this command to shrink a disk image, you MUST use file system and
544partitioning tools inside the VM to reduce allocated file systems and partition
545sizes accordingly. Failure to do so will result in data loss!
546
547After using this command to grow a disk image, you must use file system and
548partitioning tools inside the VM to actually begin using the new space on the
549device.
6f176b48 550
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551When growing an image, the @code{--preallocation} option may be used to specify
552how the additional image area should be allocated on the host. See the format
553description in the @code{NOTES} section which values are allowed. Using this
554option may result in slightly more data being allocated than necessary.
555
76a3a34d 556@item amend [-p] [-f @var{fmt}] [-t @var{cache}] -o @var{options} @var{filename}
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557
558Amends the image format specific @var{options} for the image file
559@var{filename}. Not all file formats support this operation.
acd935ef 560@end table
d3067b02 561@c man end
acd935ef 562
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563@ignore
564@c man begin NOTES
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565Supported image file formats:
566
567@table @option
568@item raw
569
570Raw disk image format (default). This format has the advantage of
571being simple and easily exportable to all other emulators. If your
572file system supports @emph{holes} (for example in ext2 or ext3 on
573Linux or NTFS on Windows), then only the written sectors will reserve
574space. Use @code{qemu-img info} to know the real size used by the
575image or @code{ls -ls} on Unix/Linux.
576
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577Supported options:
578@table @code
579@item preallocation
580Preallocation mode (allowed values: @code{off}, @code{falloc}, @code{full}).
581@code{falloc} mode preallocates space for image by calling posix_fallocate().
582@code{full} mode preallocates space for image by writing zeros to underlying
583storage.
584@end table
585
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586@item qcow2
587QEMU image format, the most versatile format. Use it to have smaller
588images (useful if your filesystem does not supports holes, for example
589on Windows), optional AES encryption, zlib based compression and
590support of multiple VM snapshots.
8063d0fe 591
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592Supported options:
593@table @code
d3067b02 594@item compat
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595Determines the qcow2 version to use. @code{compat=0.10} uses the
596traditional image format that can be read by any QEMU since 0.10.
d3067b02 597@code{compat=1.1} enables image format extensions that only QEMU 1.1 and
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598newer understand (this is the default). Amongst others, this includes zero
599clusters, which allow efficient copy-on-read for sparse images.
d3067b02 600
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601@item backing_file
602File name of a base image (see @option{create} subcommand)
603@item backing_fmt
604Image format of the base image
605@item encryption
136cd19d 606If this option is set to @code{on}, the image is encrypted with 128-bit AES-CBC.
3e032364 607
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608The use of encryption in qcow and qcow2 images is considered to be flawed by
609modern cryptography standards, suffering from a number of design problems:
610
611@itemize @minus
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612@item
613The AES-CBC cipher is used with predictable initialization vectors based
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614on the sector number. This makes it vulnerable to chosen plaintext attacks
615which can reveal the existence of encrypted data.
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616@item
617The user passphrase is directly used as the encryption key. A poorly
136cd19d 618chosen or short passphrase will compromise the security of the encryption.
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619@item
620In the event of the passphrase being compromised there is no way to
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621change the passphrase to protect data in any qcow images. The files must
622be cloned, using a different encryption passphrase in the new file. The
623original file must then be securely erased using a program like shred,
624though even this is ineffective with many modern storage technologies.
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625@item
626Initialization vectors used to encrypt sectors are based on the
627guest virtual sector number, instead of the host physical sector. When
628a disk image has multiple internal snapshots this means that data in
629multiple physical sectors is encrypted with the same initialization
630vector. With the CBC mode, this opens the possibility of watermarking
631attacks if the attack can collect multiple sectors encrypted with the
632same IV and some predictable data. Having multiple qcow2 images with
633the same passphrase also exposes this weakness since the passphrase
634is directly used as the key.
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635@end itemize
636
637Use of qcow / qcow2 encryption is thus strongly discouraged. Users are
638recommended to use an alternative encryption technology such as the
639Linux dm-crypt / LUKS system.
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640
641@item cluster_size
642Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster
643sizes can improve the image file size whereas larger cluster sizes generally
644provide better performance.
645
646@item preallocation
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647Preallocation mode (allowed values: @code{off}, @code{metadata}, @code{falloc},
648@code{full}). An image with preallocated metadata is initially larger but can
649improve performance when the image needs to grow. @code{falloc} and @code{full}
650preallocations are like the same options of @code{raw} format, but sets up
651metadata also.
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653@item lazy_refcounts
654If this option is set to @code{on}, reference count updates are postponed with
655the goal of avoiding metadata I/O and improving performance. This is
656particularly interesting with @option{cache=writethrough} which doesn't batch
657metadata updates. The tradeoff is that after a host crash, the reference count
658tables must be rebuilt, i.e. on the next open an (automatic) @code{qemu-img
659check -r all} is required, which may take some time.
3e032364 660
d3067b02 661This option can only be enabled if @code{compat=1.1} is specified.
f085800e 662
4ab15590 663@item nocow
bc3a7f90 664If this option is set to @code{on}, it will turn off COW of the file. It's only
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665valid on btrfs, no effect on other file systems.
666
667Btrfs has low performance when hosting a VM image file, even more when the guest
668on the VM also using btrfs as file system. Turning off COW is a way to mitigate
669this bad performance. Generally there are two ways to turn off COW on btrfs:
670a) Disable it by mounting with nodatacow, then all newly created files will be
671NOCOW. b) For an empty file, add the NOCOW file attribute. That's what this option
672does.
673
674Note: this option is only valid to new or empty files. If there is an existing
675file which is COW and has data blocks already, it couldn't be changed to NOCOW
676by setting @code{nocow=on}. One can issue @code{lsattr filename} to check if
bc3a7f90 677the NOCOW flag is set or not (Capital 'C' is NOCOW flag).
4ab15590 678
f085800e 679@end table
3e032364 680
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681@item Other
682QEMU also supports various other image file formats for compatibility with
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683older QEMU versions or other hypervisors, including VMDK, VDI, VHD (vpc), VHDX,
684qcow1 and QED. For a full list of supported formats see @code{qemu-img --help}.
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685For a more detailed description of these formats, see the QEMU Emulation User
686Documentation.
3e032364 687
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688The main purpose of the block drivers for these formats is image conversion.
689For running VMs, it is recommended to convert the disk images to either raw or
690qcow2 in order to achieve good performance.
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691@end table
692
693
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694@c man end
695
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696@setfilename qemu-img
697@settitle QEMU disk image utility
698
699@c man begin SEEALSO
700The HTML documentation of QEMU for more precise information and Linux
701user mode emulator invocation.
702@c man end
703
704@c man begin AUTHOR
705Fabrice Bellard
706@c man end
707
708@end ignore