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qemu-img.texi 32KB

  1. @example
  2. @c man begin SYNOPSIS
  3. @command{qemu-img} [@var{standard} @var{options}] @var{command} [@var{command} @var{options}]
  4. @c man end
  5. @end example
  6. @c man begin DESCRIPTION
  7. qemu-img allows you to create, convert and modify images offline. It can handle
  8. all image formats supported by QEMU.
  9. @b{Warning:} Never use qemu-img to modify images in use by a running virtual
  10. machine or any other process; this may destroy the image. Also, be aware that
  11. querying an image that is being modified by another process may encounter
  12. inconsistent state.
  13. @c man end
  14. @c man begin OPTIONS
  15. Standard options:
  16. @table @option
  17. @item -h, --help
  18. Display this help and exit
  19. @item -V, --version
  20. Display version information and exit
  21. @item -T, --trace [[enable=]@var{pattern}][,events=@var{file}][,file=@var{file}]
  22. @findex --trace
  23. @include qemu-option-trace.texi
  24. @end table
  25. The following commands are supported:
  26. @include qemu-img-cmds.texi
  27. Command parameters:
  28. @table @var
  29. @item filename
  30. is a disk image filename
  31. @item fmt
  32. is the disk image format. It is guessed automatically in most cases. See below
  33. for a description of the supported disk formats.
  34. @item size
  35. is the disk image size in bytes. Optional suffixes @code{k} or @code{K}
  36. (kilobyte, 1024) @code{M} (megabyte, 1024k) and @code{G} (gigabyte, 1024M)
  37. and T (terabyte, 1024G) are supported. @code{b} is ignored.
  38. @item output_filename
  39. is the destination disk image filename
  40. @item output_fmt
  41. is the destination format
  42. @item options
  43. is a comma separated list of format specific options in a
  44. name=value format. Use @code{-o ?} for an overview of the options supported
  45. by the used format or see the format descriptions below for details.
  46. @item snapshot_param
  47. is param used for internal snapshot, format is
  48. '[ID],[NAME]' or '[ID_OR_NAME]'
  49. @end table
  50. @table @option
  51. @item --object @var{objectdef}
  52. is a QEMU user creatable object definition. See the @code{qemu(1)} manual
  53. page for a description of the object properties. The most common object
  54. type is a @code{secret}, which is used to supply passwords and/or encryption
  55. keys.
  56. @item --image-opts
  57. Indicates that the source @var{filename} parameter is to be interpreted as a
  58. full option string, not a plain filename. This parameter is mutually
  59. exclusive with the @var{-f} parameter.
  60. @item --target-image-opts
  61. Indicates that the @var{output_filename} parameter(s) are to be interpreted as
  62. a full option string, not a plain filename. This parameter is mutually
  63. exclusive with the @var{-O} parameters. It is currently required to also use
  64. the @var{-n} parameter to skip image creation. This restriction may be relaxed
  65. in a future release.
  66. @item --force-share (-U)
  67. If specified, @code{qemu-img} will open the image in shared mode, allowing
  68. other QEMU processes to open it in write mode. For example, this can be used to
  69. get the image information (with 'info' subcommand) when the image is used by a
  70. running guest. Note that this could produce inconsistent results because of
  71. concurrent metadata changes, etc. This option is only allowed when opening
  72. images in read-only mode.
  73. @item --backing-chain
  74. will enumerate information about backing files in a disk image chain. Refer
  75. below for further description.
  76. @item -c
  77. indicates that target image must be compressed (qcow format only)
  78. @item -h
  79. with or without a command shows help and lists the supported formats
  80. @item -p
  81. display progress bar (compare, convert and rebase commands only).
  82. If the @var{-p} option is not used for a command that supports it, the
  83. progress is reported when the process receives a @code{SIGUSR1} or
  84. @code{SIGINFO} signal.
  85. @item -q
  86. Quiet mode - do not print any output (except errors). There's no progress bar
  87. in case both @var{-q} and @var{-p} options are used.
  88. @item -S @var{size}
  89. indicates the consecutive number of bytes that must contain only zeros
  90. for qemu-img to create a sparse image during conversion. This value is rounded
  91. down to the nearest 512 bytes. You may use the common size suffixes like
  92. @code{k} for kilobytes.
  93. @item -t @var{cache}
  94. specifies the cache mode that should be used with the (destination) file. See
  95. the documentation of the emulator's @code{-drive cache=...} option for allowed
  96. values.
  97. @item -T @var{src_cache}
  98. specifies the cache mode that should be used with the source file(s). See
  99. the documentation of the emulator's @code{-drive cache=...} option for allowed
  100. values.
  101. @end table
  102. Parameters to snapshot subcommand:
  103. @table @option
  104. @item snapshot
  105. is the name of the snapshot to create, apply or delete
  106. @item -a
  107. applies a snapshot (revert disk to saved state)
  108. @item -c
  109. creates a snapshot
  110. @item -d
  111. deletes a snapshot
  112. @item -l
  113. lists all snapshots in the given image
  114. @end table
  115. Parameters to compare subcommand:
  116. @table @option
  117. @item -f
  118. First image format
  119. @item -F
  120. Second image format
  121. @item -s
  122. Strict mode - fail on different image size or sector allocation
  123. @end table
  124. Parameters to convert subcommand:
  125. @table @option
  126. @item -n
  127. Skip the creation of the target volume
  128. @item -m
  129. Number of parallel coroutines for the convert process
  130. @item -W
  131. Allow out-of-order writes to the destination. This option improves performance,
  132. but is only recommended for preallocated devices like host devices or other
  133. raw block devices.
  134. @item -C
  135. Try to use copy offloading to move data from source image to target. This may
  136. improve performance if the data is remote, such as with NFS or iSCSI backends,
  137. but will not automatically sparsify zero sectors, and may result in a fully
  138. allocated target image depending on the host support for getting allocation
  139. information.
  140. @item --salvage
  141. Try to ignore I/O errors when reading. Unless in quiet mode (@code{-q}), errors
  142. will still be printed. Areas that cannot be read from the source will be
  143. treated as containing only zeroes.
  144. @end table
  145. Parameters to dd subcommand:
  146. @table @option
  147. @item bs=@var{block_size}
  148. defines the block size
  149. @item count=@var{blocks}
  150. sets the number of input blocks to copy
  151. @item if=@var{input}
  152. sets the input file
  153. @item of=@var{output}
  154. sets the output file
  155. @item skip=@var{blocks}
  156. sets the number of input blocks to skip
  157. @end table
  158. Command description:
  159. @table @option
  160. @item amend [--object @var{objectdef}] [--image-opts] [-p] [-q] [-f @var{fmt}] [-t @var{cache}] -o @var{options} @var{filename}
  161. Amends the image format specific @var{options} for the image file
  162. @var{filename}. Not all file formats support this operation.
  163. @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] [-U] @var{filename}
  164. Run a simple sequential I/O benchmark on the specified image. If @code{-w} is
  165. specified, a write test is performed, otherwise a read test is performed.
  166. A total number of @var{count} I/O requests is performed, each @var{buffer_size}
  167. bytes in size, and with @var{depth} requests in parallel. The first request
  168. starts at the position given by @var{offset}, each following request increases
  169. the current position by @var{step_size}. If @var{step_size} is not given,
  170. @var{buffer_size} is used for its value.
  171. If @var{flush_interval} is specified for a write test, the request queue is
  172. drained and a flush is issued before new writes are made whenever the number of
  173. remaining requests is a multiple of @var{flush_interval}. If additionally
  174. @code{--no-drain} is specified, a flush is issued without draining the request
  175. queue first.
  176. If @code{-n} is specified, the native AIO backend is used if possible. On
  177. Linux, this option only works if @code{-t none} or @code{-t directsync} is
  178. specified as well.
  179. For write tests, by default a buffer filled with zeros is written. This can be
  180. overridden with a pattern byte specified by @var{pattern}.
  181. @item check [--object @var{objectdef}] [--image-opts] [-q] [-f @var{fmt}] [--output=@var{ofmt}] [-r [leaks | all]] [-T @var{src_cache}] [-U] @var{filename}
  182. Perform a consistency check on the disk image @var{filename}. The command can
  183. output in the format @var{ofmt} which is either @code{human} or @code{json}.
  184. The JSON output is an object of QAPI type @code{ImageCheck}.
  185. If @code{-r} is specified, qemu-img tries to repair any inconsistencies found
  186. during the check. @code{-r leaks} repairs only cluster leaks, whereas
  187. @code{-r all} fixes all kinds of errors, with a higher risk of choosing the
  188. wrong fix or hiding corruption that has already occurred.
  189. Only the formats @code{qcow2}, @code{qed} and @code{vdi} support
  190. consistency checks.
  191. In case the image does not have any inconsistencies, check exits with @code{0}.
  192. Other exit codes indicate the kind of inconsistency found or if another error
  193. occurred. The following table summarizes all exit codes of the check subcommand:
  194. @table @option
  195. @item 0
  196. Check completed, the image is (now) consistent
  197. @item 1
  198. Check not completed because of internal errors
  199. @item 2
  200. Check completed, image is corrupted
  201. @item 3
  202. Check completed, image has leaked clusters, but is not corrupted
  203. @item 63
  204. Checks are not supported by the image format
  205. @end table
  206. If @code{-r} is specified, exit codes representing the image state refer to the
  207. state after (the attempt at) repairing it. That is, a successful @code{-r all}
  208. will yield the exit code 0, independently of the image state before.
  209. @item commit [--object @var{objectdef}] [--image-opts] [-q] [-f @var{fmt}] [-t @var{cache}] [-b @var{base}] [-d] [-p] @var{filename}
  210. Commit the changes recorded in @var{filename} in its base image or backing file.
  211. If the backing file is smaller than the snapshot, then the backing file will be
  212. resized to be the same size as the snapshot. If the snapshot is smaller than
  213. the backing file, the backing file will not be truncated. If you want the
  214. backing file to match the size of the smaller snapshot, you can safely truncate
  215. it yourself once the commit operation successfully completes.
  216. The image @var{filename} is emptied after the operation has succeeded. If you do
  217. not need @var{filename} afterwards and intend to drop it, you may skip emptying
  218. @var{filename} by specifying the @code{-d} flag.
  219. If the backing chain of the given image file @var{filename} has more than one
  220. layer, the backing file into which the changes will be committed may be
  221. specified as @var{base} (which has to be part of @var{filename}'s backing
  222. chain). If @var{base} is not specified, the immediate backing file of the top
  223. image (which is @var{filename}) will be used. Note that after a commit operation
  224. all images between @var{base} and the top image will be invalid and may return
  225. garbage data when read. For this reason, @code{-b} implies @code{-d} (so that
  226. the top image stays valid).
  227. @item compare [--object @var{objectdef}] [--image-opts] [-f @var{fmt}] [-F @var{fmt}] [-T @var{src_cache}] [-p] [-q] [-s] [-U] @var{filename1} @var{filename2}
  228. Check if two images have the same content. You can compare images with
  229. different format or settings.
  230. The format is probed unless you specify it by @var{-f} (used for
  231. @var{filename1}) and/or @var{-F} (used for @var{filename2}) option.
  232. By default, images with different size are considered identical if the larger
  233. image contains only unallocated and/or zeroed sectors in the area after the end
  234. of the other image. In addition, if any sector is not allocated in one image
  235. and contains only zero bytes in the second one, it is evaluated as equal. You
  236. can use Strict mode by specifying the @var{-s} option. When compare runs in
  237. Strict mode, it fails in case image size differs or a sector is allocated in
  238. one image and is not allocated in the second one.
  239. By default, compare prints out a result message. This message displays
  240. information that both images are same or the position of the first different
  241. byte. In addition, result message can report different image size in case
  242. Strict mode is used.
  243. Compare exits with @code{0} in case the images are equal and with @code{1}
  244. in case the images differ. Other exit codes mean an error occurred during
  245. execution and standard error output should contain an error message.
  246. The following table sumarizes all exit codes of the compare subcommand:
  247. @table @option
  248. @item 0
  249. Images are identical
  250. @item 1
  251. Images differ
  252. @item 2
  253. Error on opening an image
  254. @item 3
  255. Error on checking a sector allocation
  256. @item 4
  257. Error on reading data
  258. @end table
  259. @item convert [--object @var{objectdef}] [--image-opts] [--target-image-opts] [-U] [-C] [-c] [-p] [-q] [-n] [-f @var{fmt}] [-t @var{cache}] [-T @var{src_cache}] [-O @var{output_fmt}] [-B @var{backing_file}] [-o @var{options}] [-l @var{snapshot_param}] [-S @var{sparse_size}] [-m @var{num_coroutines}] [-W] @var{filename} [@var{filename2} [...]] @var{output_filename}
  260. Convert the disk image @var{filename} or a snapshot @var{snapshot_param}
  261. to disk image @var{output_filename} using format @var{output_fmt}. It can be optionally compressed (@code{-c}
  262. option) or use any format specific options like encryption (@code{-o} option).
  263. Only the formats @code{qcow} and @code{qcow2} support compression. The
  264. compression is read-only. It means that if a compressed sector is
  265. rewritten, then it is rewritten as uncompressed data.
  266. Image conversion is also useful to get smaller image when using a
  267. growable format such as @code{qcow}: the empty sectors are detected and
  268. suppressed from the destination image.
  269. @var{sparse_size} indicates the consecutive number of bytes (defaults to 4k)
  270. that must contain only zeros for qemu-img to create a sparse image during
  271. conversion. If @var{sparse_size} is 0, the source will not be scanned for
  272. unallocated or zero sectors, and the destination image will always be
  273. fully allocated.
  274. You can use the @var{backing_file} option to force the output image to be
  275. created as a copy on write image of the specified base image; the
  276. @var{backing_file} should have the same content as the input's base image,
  277. however the path, image format, etc may differ.
  278. If a relative path name is given, the backing file is looked up relative to
  279. the directory containing @var{output_filename}.
  280. If the @code{-n} option is specified, the target volume creation will be
  281. skipped. This is useful for formats such as @code{rbd} if the target
  282. volume has already been created with site specific options that cannot
  283. be supplied through qemu-img.
  284. Out of order writes can be enabled with @code{-W} to improve performance.
  285. This is only recommended for preallocated devices like host devices or other
  286. raw block devices. Out of order write does not work in combination with
  287. creating compressed images.
  288. @var{num_coroutines} specifies how many coroutines work in parallel during
  289. the convert process (defaults to 8).
  290. @item create [--object @var{objectdef}] [-q] [-f @var{fmt}] [-b @var{backing_file}] [-F @var{backing_fmt}] [-u] [-o @var{options}] @var{filename} [@var{size}]
  291. Create the new disk image @var{filename} of size @var{size} and format
  292. @var{fmt}. Depending on the file format, you can add one or more @var{options}
  293. that enable additional features of this format.
  294. If the option @var{backing_file} is specified, then the image will record
  295. only the differences from @var{backing_file}. No size needs to be specified in
  296. this case. @var{backing_file} will never be modified unless you use the
  297. @code{commit} monitor command (or qemu-img commit).
  298. If a relative path name is given, the backing file is looked up relative to
  299. the directory containing @var{filename}.
  300. Note that a given backing file will be opened to check that it is valid. Use
  301. the @code{-u} option to enable unsafe backing file mode, which means that the
  302. image will be created even if the associated backing file cannot be opened. A
  303. matching backing file must be created or additional options be used to make the
  304. backing file specification valid when you want to use an image created this
  305. way.
  306. The size can also be specified using the @var{size} option with @code{-o},
  307. it doesn't need to be specified separately in this case.
  308. @item dd [--image-opts] [-U] [-f @var{fmt}] [-O @var{output_fmt}] [bs=@var{block_size}] [count=@var{blocks}] [skip=@var{blocks}] if=@var{input} of=@var{output}
  309. Dd copies from @var{input} file to @var{output} file converting it from
  310. @var{fmt} format to @var{output_fmt} format.
  311. The data is by default read and written using blocks of 512 bytes but can be
  312. modified by specifying @var{block_size}. If count=@var{blocks} is specified
  313. dd will stop reading input after reading @var{blocks} input blocks.
  314. The size syntax is similar to dd(1)'s size syntax.
  315. @item info [--object @var{objectdef}] [--image-opts] [-f @var{fmt}] [--output=@var{ofmt}] [--backing-chain] [-U] @var{filename}
  316. Give information about the disk image @var{filename}. Use it in
  317. particular to know the size reserved on disk which can be different
  318. from the displayed size. If VM snapshots are stored in the disk image,
  319. they are displayed too.
  320. If a disk image has a backing file chain, information about each disk image in
  321. the chain can be recursively enumerated by using the option @code{--backing-chain}.
  322. For instance, if you have an image chain like:
  323. @example
  324. base.qcow2 <- snap1.qcow2 <- snap2.qcow2
  325. @end example
  326. To enumerate information about each disk image in the above chain, starting from top to base, do:
  327. @example
  328. qemu-img info --backing-chain snap2.qcow2
  329. @end example
  330. The command can output in the format @var{ofmt} which is either @code{human} or
  331. @code{json}. The JSON output is an object of QAPI type @code{ImageInfo}; with
  332. @code{--backing-chain}, it is an array of @code{ImageInfo} objects.
  333. @code{--output=human} reports the following information (for every image in the
  334. chain):
  335. @table @var
  336. @item image
  337. The image file name
  338. @item file format
  339. The image format
  340. @item virtual size
  341. The size of the guest disk
  342. @item disk size
  343. How much space the image file occupies on the host file system (may be shown as
  344. 0 if this information is unavailable, e.g. because there is no file system)
  345. @item cluster_size
  346. Cluster size of the image format, if applicable
  347. @item encrypted
  348. Whether the image is encrypted (only present if so)
  349. @item cleanly shut down
  350. This is shown as @code{no} if the image is dirty and will have to be
  351. auto-repaired the next time it is opened in qemu.
  352. @item backing file
  353. The backing file name, if present
  354. @item backing file format
  355. The format of the backing file, if the image enforces it
  356. @item Snapshot list
  357. A list of all internal snapshots
  358. @item Format specific information
  359. Further information whose structure depends on the image format. This section
  360. is a textual representation of the respective @code{ImageInfoSpecific*} QAPI
  361. object (e.g. @code{ImageInfoSpecificQCow2} for qcow2 images).
  362. @end table
  363. @item map [--object @var{objectdef}] [--image-opts] [-f @var{fmt}] [--output=@var{ofmt}] [-U] @var{filename}
  364. Dump the metadata of image @var{filename} and its backing file chain.
  365. In particular, this commands dumps the allocation state of every sector
  366. of @var{filename}, together with the topmost file that allocates it in
  367. the backing file chain.
  368. Two option formats are possible. The default format (@code{human})
  369. only dumps known-nonzero areas of the file. Known-zero parts of the
  370. file are omitted altogether, and likewise for parts that are not allocated
  371. throughout the chain. @command{qemu-img} output will identify a file
  372. from where the data can be read, and the offset in the file. Each line
  373. will include four fields, the first three of which are hexadecimal
  374. numbers. For example the first line of:
  375. @example
  376. Offset Length Mapped to File
  377. 0 0x20000 0x50000 /tmp/overlay.qcow2
  378. 0x100000 0x10000 0x95380000 /tmp/backing.qcow2
  379. @end example
  380. @noindent
  381. means that 0x20000 (131072) bytes starting at offset 0 in the image are
  382. available in /tmp/overlay.qcow2 (opened in @code{raw} format) starting
  383. at offset 0x50000 (327680). Data that is compressed, encrypted, or
  384. otherwise not available in raw format will cause an error if @code{human}
  385. format is in use. Note that file names can include newlines, thus it is
  386. not safe to parse this output format in scripts.
  387. The alternative format @code{json} will return an array of dictionaries
  388. in JSON format. It will include similar information in
  389. the @code{start}, @code{length}, @code{offset} fields;
  390. it will also include other more specific information:
  391. @itemize @minus
  392. @item
  393. whether the sectors contain actual data or not (boolean field @code{data};
  394. if false, the sectors are either unallocated or stored as optimized
  395. all-zero clusters);
  396. @item
  397. whether the data is known to read as zero (boolean field @code{zero});
  398. @item
  399. in order to make the output shorter, the target file is expressed as
  400. a @code{depth}; for example, a depth of 2 refers to the backing file
  401. of the backing file of @var{filename}.
  402. @end itemize
  403. In JSON format, the @code{offset} field is optional; it is absent in
  404. cases where @code{human} format would omit the entry or exit with an error.
  405. If @code{data} is false and the @code{offset} field is present, the
  406. corresponding sectors in the file are not yet in use, but they are
  407. preallocated.
  408. For more information, consult @file{include/block/block.h} in QEMU's
  409. source code.
  410. @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}]
  411. Calculate the file size required for a new image. This information can be used
  412. to size logical volumes or SAN LUNs appropriately for the image that will be
  413. placed in them. The values reported are guaranteed to be large enough to fit
  414. the image. The command can output in the format @var{ofmt} which is either
  415. @code{human} or @code{json}. The JSON output is an object of QAPI type
  416. @code{BlockMeasureInfo}.
  417. If the size @var{N} is given then act as if creating a new empty image file
  418. using @command{qemu-img create}. If @var{filename} is given then act as if
  419. converting an existing image file using @command{qemu-img convert}. The format
  420. of the new file is given by @var{output_fmt} while the format of an existing
  421. file is given by @var{fmt}.
  422. A snapshot in an existing image can be specified using @var{snapshot_param}.
  423. The following fields are reported:
  424. @example
  425. required size: 524288
  426. fully allocated size: 1074069504
  427. @end example
  428. The @code{required size} is the file size of the new image. It may be smaller
  429. than the virtual disk size if the image format supports compact representation.
  430. The @code{fully allocated size} is the file size of the new image once data has
  431. been written to all sectors. This is the maximum size that the image file can
  432. occupy with the exception of internal snapshots, dirty bitmaps, vmstate data,
  433. and other advanced image format features.
  434. @item snapshot [--object @var{objectdef}] [--image-opts] [-U] [-q] [-l | -a @var{snapshot} | -c @var{snapshot} | -d @var{snapshot}] @var{filename}
  435. List, apply, create or delete snapshots in image @var{filename}.
  436. @item rebase [--object @var{objectdef}] [--image-opts] [-U] [-q] [-f @var{fmt}] [-t @var{cache}] [-T @var{src_cache}] [-p] [-u] -b @var{backing_file} [-F @var{backing_fmt}] @var{filename}
  437. Changes the backing file of an image. Only the formats @code{qcow2} and
  438. @code{qed} support changing the backing file.
  439. The backing file is changed to @var{backing_file} and (if the image format of
  440. @var{filename} supports this) the backing file format is changed to
  441. @var{backing_fmt}. If @var{backing_file} is specified as ``'' (the empty
  442. string), then the image is rebased onto no backing file (i.e. it will exist
  443. independently of any backing file).
  444. If a relative path name is given, the backing file is looked up relative to
  445. the directory containing @var{filename}.
  446. @var{cache} specifies the cache mode to be used for @var{filename}, whereas
  447. @var{src_cache} specifies the cache mode for reading backing files.
  448. There are two different modes in which @code{rebase} can operate:
  449. @table @option
  450. @item Safe mode
  451. This is the default mode and performs a real rebase operation. The new backing
  452. file may differ from the old one and qemu-img rebase will take care of keeping
  453. the guest-visible content of @var{filename} unchanged.
  454. In order to achieve this, any clusters that differ between @var{backing_file}
  455. and the old backing file of @var{filename} are merged into @var{filename}
  456. before actually changing the backing file.
  457. Note that the safe mode is an expensive operation, comparable to converting
  458. an image. It only works if the old backing file still exists.
  459. @item Unsafe mode
  460. qemu-img uses the unsafe mode if @code{-u} is specified. In this mode, only the
  461. backing file name and format of @var{filename} is changed without any checks
  462. on the file contents. The user must take care of specifying the correct new
  463. backing file, or the guest-visible content of the image will be corrupted.
  464. This mode is useful for renaming or moving the backing file to somewhere else.
  465. It can be used without an accessible old backing file, i.e. you can use it to
  466. fix an image whose backing file has already been moved/renamed.
  467. @end table
  468. You can use @code{rebase} to perform a ``diff'' operation on two
  469. disk images. This can be useful when you have copied or cloned
  470. a guest, and you want to get back to a thin image on top of a
  471. template or base image.
  472. Say that @code{base.img} has been cloned as @code{modified.img} by
  473. copying it, and that the @code{modified.img} guest has run so there
  474. are now some changes compared to @code{base.img}. To construct a thin
  475. image called @code{diff.qcow2} that contains just the differences, do:
  476. @example
  477. qemu-img create -f qcow2 -b modified.img diff.qcow2
  478. qemu-img rebase -b base.img diff.qcow2
  479. @end example
  480. At this point, @code{modified.img} can be discarded, since
  481. @code{base.img + diff.qcow2} contains the same information.
  482. @item resize [--object @var{objectdef}] [--image-opts] [-f @var{fmt}] [--preallocation=@var{prealloc}] [-q] [--shrink] @var{filename} [+ | -]@var{size}
  483. Change the disk image as if it had been created with @var{size}.
  484. Before using this command to shrink a disk image, you MUST use file system and
  485. partitioning tools inside the VM to reduce allocated file systems and partition
  486. sizes accordingly. Failure to do so will result in data loss!
  487. When shrinking images, the @code{--shrink} option must be given. This informs
  488. qemu-img that the user acknowledges all loss of data beyond the truncated
  489. image's end.
  490. After using this command to grow a disk image, you must use file system and
  491. partitioning tools inside the VM to actually begin using the new space on the
  492. device.
  493. When growing an image, the @code{--preallocation} option may be used to specify
  494. how the additional image area should be allocated on the host. See the format
  495. description in the @code{NOTES} section which values are allowed. Using this
  496. option may result in slightly more data being allocated than necessary.
  497. @end table
  498. @c man end
  499. @ignore
  500. @c man begin NOTES
  501. Supported image file formats:
  502. @table @option
  503. @item raw
  504. Raw disk image format (default). This format has the advantage of
  505. being simple and easily exportable to all other emulators. If your
  506. file system supports @emph{holes} (for example in ext2 or ext3 on
  507. Linux or NTFS on Windows), then only the written sectors will reserve
  508. space. Use @code{qemu-img info} to know the real size used by the
  509. image or @code{ls -ls} on Unix/Linux.
  510. Supported options:
  511. @table @code
  512. @item preallocation
  513. Preallocation mode (allowed values: @code{off}, @code{falloc}, @code{full}).
  514. @code{falloc} mode preallocates space for image by calling posix_fallocate().
  515. @code{full} mode preallocates space for image by writing zeros to underlying
  516. storage.
  517. @end table
  518. @item qcow2
  519. QEMU image format, the most versatile format. Use it to have smaller
  520. images (useful if your filesystem does not supports holes, for example
  521. on Windows), optional AES encryption, zlib based compression and
  522. support of multiple VM snapshots.
  523. Supported options:
  524. @table @code
  525. @item compat
  526. Determines the qcow2 version to use. @code{compat=0.10} uses the
  527. traditional image format that can be read by any QEMU since 0.10.
  528. @code{compat=1.1} enables image format extensions that only QEMU 1.1 and
  529. newer understand (this is the default). Amongst others, this includes zero
  530. clusters, which allow efficient copy-on-read for sparse images.
  531. @item backing_file
  532. File name of a base image (see @option{create} subcommand)
  533. @item backing_fmt
  534. Image format of the base image
  535. @item encryption
  536. If this option is set to @code{on}, the image is encrypted with 128-bit AES-CBC.
  537. The use of encryption in qcow and qcow2 images is considered to be flawed by
  538. modern cryptography standards, suffering from a number of design problems:
  539. @itemize @minus
  540. @item
  541. The AES-CBC cipher is used with predictable initialization vectors based
  542. on the sector number. This makes it vulnerable to chosen plaintext attacks
  543. which can reveal the existence of encrypted data.
  544. @item
  545. The user passphrase is directly used as the encryption key. A poorly
  546. chosen or short passphrase will compromise the security of the encryption.
  547. @item
  548. In the event of the passphrase being compromised there is no way to
  549. change the passphrase to protect data in any qcow images. The files must
  550. be cloned, using a different encryption passphrase in the new file. The
  551. original file must then be securely erased using a program like shred,
  552. though even this is ineffective with many modern storage technologies.
  553. @item
  554. Initialization vectors used to encrypt sectors are based on the
  555. guest virtual sector number, instead of the host physical sector. When
  556. a disk image has multiple internal snapshots this means that data in
  557. multiple physical sectors is encrypted with the same initialization
  558. vector. With the CBC mode, this opens the possibility of watermarking
  559. attacks if the attack can collect multiple sectors encrypted with the
  560. same IV and some predictable data. Having multiple qcow2 images with
  561. the same passphrase also exposes this weakness since the passphrase
  562. is directly used as the key.
  563. @end itemize
  564. Use of qcow / qcow2 encryption is thus strongly discouraged. Users are
  565. recommended to use an alternative encryption technology such as the
  566. Linux dm-crypt / LUKS system.
  567. @item cluster_size
  568. Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster
  569. sizes can improve the image file size whereas larger cluster sizes generally
  570. provide better performance.
  571. @item preallocation
  572. Preallocation mode (allowed values: @code{off}, @code{metadata}, @code{falloc},
  573. @code{full}). An image with preallocated metadata is initially larger but can
  574. improve performance when the image needs to grow. @code{falloc} and @code{full}
  575. preallocations are like the same options of @code{raw} format, but sets up
  576. metadata also.
  577. @item lazy_refcounts
  578. If this option is set to @code{on}, reference count updates are postponed with
  579. the goal of avoiding metadata I/O and improving performance. This is
  580. particularly interesting with @option{cache=writethrough} which doesn't batch
  581. metadata updates. The tradeoff is that after a host crash, the reference count
  582. tables must be rebuilt, i.e. on the next open an (automatic) @code{qemu-img
  583. check -r all} is required, which may take some time.
  584. This option can only be enabled if @code{compat=1.1} is specified.
  585. @item nocow
  586. If this option is set to @code{on}, it will turn off COW of the file. It's only
  587. valid on btrfs, no effect on other file systems.
  588. Btrfs has low performance when hosting a VM image file, even more when the guest
  589. on the VM also using btrfs as file system. Turning off COW is a way to mitigate
  590. this bad performance. Generally there are two ways to turn off COW on btrfs:
  591. a) Disable it by mounting with nodatacow, then all newly created files will be
  592. NOCOW. b) For an empty file, add the NOCOW file attribute. That's what this option
  593. does.
  594. Note: this option is only valid to new or empty files. If there is an existing
  595. file which is COW and has data blocks already, it couldn't be changed to NOCOW
  596. by setting @code{nocow=on}. One can issue @code{lsattr filename} to check if
  597. the NOCOW flag is set or not (Capital 'C' is NOCOW flag).
  598. @end table
  599. @item Other
  600. QEMU also supports various other image file formats for compatibility with
  601. older QEMU versions or other hypervisors, including VMDK, VDI, VHD (vpc), VHDX,
  602. qcow1 and QED. For a full list of supported formats see @code{qemu-img --help}.
  603. For a more detailed description of these formats, see the QEMU Emulation User
  604. Documentation.
  605. The main purpose of the block drivers for these formats is image conversion.
  606. For running VMs, it is recommended to convert the disk images to either raw or
  607. qcow2 in order to achieve good performance.
  608. @end table
  609. @c man end
  610. @setfilename qemu-img
  611. @settitle QEMU disk image utility
  612. @c man begin SEEALSO
  613. The HTML documentation of QEMU for more precise information and Linux
  614. user mode emulator invocation.
  615. @c man end
  616. @c man begin AUTHOR
  617. Fabrice Bellard
  618. @c man end
  619. @end ignore