3 @command{qemu-img} @var{command} [@var{command} @var{options}]
7 @c man begin DESCRIPTION
8 qemu-img allows you to create, convert and modify images offline. It can handle
9 all image formats supported by QEMU.
11 @b{Warning:} Never use qemu-img to modify images in use by a running virtual
12 machine or any other process; this may destroy the image. Also, be aware that
13 querying an image that is being modified by another process may encounter
19 The following commands are supported:
21 @include qemu-img-cmds.texi
26 is a disk image filename
28 @item --object @var{objectdef}
30 is a QEMU user creatable object definition. See the @code{qemu(1)} manual
31 page for a description of the object properties. The most common object
32 type is a @code{secret}, which is used to supply passwords and/or encryption
37 Indicates that the @var{filename} parameter is to be interpreted as a
38 full option string, not a plain filename. This parameter is mutually
39 exclusive with the @var{-f} and @var{-F} parameters.
42 is the disk image format. It is guessed automatically in most cases. See below
43 for a description of the supported disk formats.
46 will enumerate information about backing files in a disk image chain. Refer
47 below for further description.
50 is the disk image size in bytes. Optional suffixes @code{k} or @code{K}
51 (kilobyte, 1024) @code{M} (megabyte, 1024k) and @code{G} (gigabyte, 1024M)
52 and T (terabyte, 1024G) are supported. @code{b} is ignored.
55 is the destination disk image filename
58 is the destination format
60 is a comma separated list of format specific options in a
61 name=value format. Use @code{-o ?} for an overview of the options supported
62 by the used format or see the format descriptions below for details.
64 is param used for internal snapshot, format is
65 'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'
66 @item snapshot_id_or_name
67 is deprecated, use snapshot_param instead
70 indicates that target image must be compressed (qcow format only)
72 with or without a command shows help and lists the supported formats
74 display progress bar (compare, convert and rebase commands only).
75 If the @var{-p} option is not used for a command that supports it, the
76 progress is reported when the process receives a @code{SIGUSR1} signal.
78 Quiet mode - do not print any output (except errors). There's no progress bar
79 in case both @var{-q} and @var{-p} options are used.
81 indicates the consecutive number of bytes that must contain only zeros
82 for qemu-img to create a sparse image during conversion. This value is rounded
83 down to the nearest 512 bytes. You may use the common size suffixes like
84 @code{k} for kilobytes.
86 specifies the cache mode that should be used with the (destination) file. See
87 the documentation of the emulator's @code{-drive cache=...} option for allowed
89 @item -T @var{src_cache}
90 specifies the cache mode that should be used with the source file(s). See
91 the documentation of the emulator's @code{-drive cache=...} option for allowed
95 Parameters to snapshot subcommand:
100 is the name of the snapshot to create, apply or delete
102 applies a snapshot (revert disk to saved state)
108 lists all snapshots in the given image
111 Parameters to compare subcommand:
120 Strict mode - fail on different image size or sector allocation
123 Parameters to convert subcommand:
128 Skip the creation of the target volume
134 @item bench [-c @var{count}] [-d @var{depth}] [-f @var{fmt}] [-n] [-q] [-s @var{buffer_size}] [-t @var{cache}] @var{filename}
136 Run a simple sequential read benchmark on the specified image. A total number
137 of @var{count} I/O requests is performed, each @var{buffer_size} bytes in size,
138 and with @var{depth} requests in parallel.
140 If @code{-n} is specified, the native AIO backend is used if possible. On
141 Linux, this option only works if @code{-t none} or @code{-t directsync} is
144 @item check [-f @var{fmt}] [--output=@var{ofmt}] [-r [leaks | all]] [-T @var{src_cache}] @var{filename}
146 Perform a consistency check on the disk image @var{filename}. The command can
147 output in the format @var{ofmt} which is either @code{human} or @code{json}.
149 If @code{-r} is specified, qemu-img tries to repair any inconsistencies found
150 during the check. @code{-r leaks} repairs only cluster leaks, whereas
151 @code{-r all} fixes all kinds of errors, with a higher risk of choosing the
152 wrong fix or hiding corruption that has already occurred.
154 Only the formats @code{qcow2}, @code{qed} and @code{vdi} support
157 In case the image does not have any inconsistencies, check exits with @code{0}.
158 Other exit codes indicate the kind of inconsistency found or if another error
159 occurred. The following table summarizes all exit codes of the check subcommand:
164 Check completed, the image is (now) consistent
166 Check not completed because of internal errors
168 Check completed, image is corrupted
170 Check completed, image has leaked clusters, but is not corrupted
172 Checks are not supported by the image format
176 If @code{-r} is specified, exit codes representing the image state refer to the
177 state after (the attempt at) repairing it. That is, a successful @code{-r all}
178 will yield the exit code 0, independently of the image state before.
180 @item create [-f @var{fmt}] [-o @var{options}] @var{filename} [@var{size}]
182 Create the new disk image @var{filename} of size @var{size} and format
183 @var{fmt}. Depending on the file format, you can add one or more @var{options}
184 that enable additional features of this format.
186 If the option @var{backing_file} is specified, then the image will record
187 only the differences from @var{backing_file}. No size needs to be specified in
188 this case. @var{backing_file} will never be modified unless you use the
189 @code{commit} monitor command (or qemu-img commit).
191 The size can also be specified using the @var{size} option with @code{-o},
192 it doesn't need to be specified separately in this case.
194 @item commit [-q] [-f @var{fmt}] [-t @var{cache}] [-b @var{base}] [-d] [-p] @var{filename}
196 Commit the changes recorded in @var{filename} in its base image or backing file.
197 If the backing file is smaller than the snapshot, then the backing file will be
198 resized to be the same size as the snapshot. If the snapshot is smaller than
199 the backing file, the backing file will not be truncated. If you want the
200 backing file to match the size of the smaller snapshot, you can safely truncate
201 it yourself once the commit operation successfully completes.
203 The image @var{filename} is emptied after the operation has succeeded. If you do
204 not need @var{filename} afterwards and intend to drop it, you may skip emptying
205 @var{filename} by specifying the @code{-d} flag.
207 If the backing chain of the given image file @var{filename} has more than one
208 layer, the backing file into which the changes will be committed may be
209 specified as @var{base} (which has to be part of @var{filename}'s backing
210 chain). If @var{base} is not specified, the immediate backing file of the top
211 image (which is @var{filename}) will be used. For reasons of consistency,
212 explicitly specifying @var{base} will always imply @code{-d} (since emptying an
213 image after committing to an indirect backing file would lead to different data
214 being read from the image due to content in the intermediate backing chain
215 overruling the commit target).
217 @item compare [-f @var{fmt}] [-F @var{fmt}] [-T @var{src_cache}] [-p] [-s] [-q] @var{filename1} @var{filename2}
219 Check if two images have the same content. You can compare images with
220 different format or settings.
222 The format is probed unless you specify it by @var{-f} (used for
223 @var{filename1}) and/or @var{-F} (used for @var{filename2}) option.
225 By default, images with different size are considered identical if the larger
226 image contains only unallocated and/or zeroed sectors in the area after the end
227 of the other image. In addition, if any sector is not allocated in one image
228 and contains only zero bytes in the second one, it is evaluated as equal. You
229 can use Strict mode by specifying the @var{-s} option. When compare runs in
230 Strict mode, it fails in case image size differs or a sector is allocated in
231 one image and is not allocated in the second one.
233 By default, compare prints out a result message. This message displays
234 information that both images are same or the position of the first different
235 byte. In addition, result message can report different image size in case
238 Compare exits with @code{0} in case the images are equal and with @code{1}
239 in case the images differ. Other exit codes mean an error occurred during
240 execution and standard error output should contain an error message.
241 The following table sumarizes all exit codes of the compare subcommand:
250 Error on opening an image
252 Error on checking a sector allocation
254 Error on reading data
258 @item convert [-c] [-p] [-n] [-f @var{fmt}] [-t @var{cache}] [-T @var{src_cache}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_id_or_name}] [-l @var{snapshot_param}] [-S @var{sparse_size}] @var{filename} [@var{filename2} [...]] @var{output_filename}
260 Convert the disk image @var{filename} or a snapshot @var{snapshot_param}(@var{snapshot_id_or_name} is deprecated)
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).
264 Only the formats @code{qcow} and @code{qcow2} support compression. The
265 compression is read-only. It means that if a compressed sector is
266 rewritten, then it is rewritten as uncompressed data.
268 Image conversion is also useful to get smaller image when using a
269 growable format such as @code{qcow}: the empty sectors are detected and
270 suppressed from the destination image.
272 @var{sparse_size} indicates the consecutive number of bytes (defaults to 4k)
273 that must contain only zeros for qemu-img to create a sparse image during
274 conversion. If @var{sparse_size} is 0, the source will not be scanned for
275 unallocated or zero sectors, and the destination image will always be
278 You can use the @var{backing_file} option to force the output image to be
279 created as a copy on write image of the specified base image; the
280 @var{backing_file} should have the same content as the input's base image,
281 however the path, image format, etc may differ.
283 If the @code{-n} option is specified, the target volume creation will be
284 skipped. This is useful for formats such as @code{rbd} if the target
285 volume has already been created with site specific options that cannot
286 be supplied through qemu-img.
288 @item info [-f @var{fmt}] [--output=@var{ofmt}] [--backing-chain] @var{filename}
290 Give information about the disk image @var{filename}. Use it in
291 particular to know the size reserved on disk which can be different
292 from the displayed size. If VM snapshots are stored in the disk image,
293 they are displayed too. The command can output in the format @var{ofmt}
294 which is either @code{human} or @code{json}.
296 If a disk image has a backing file chain, information about each disk image in
297 the chain can be recursively enumerated by using the option @code{--backing-chain}.
299 For instance, if you have an image chain like:
302 base.qcow2 <- snap1.qcow2 <- snap2.qcow2
305 To enumerate information about each disk image in the above chain, starting from top to base, do:
308 qemu-img info --backing-chain snap2.qcow2
311 @item map [-f @var{fmt}] [--output=@var{ofmt}] @var{filename}
313 Dump the metadata of image @var{filename} and its backing file chain.
314 In particular, this commands dumps the allocation state of every sector
315 of @var{filename}, together with the topmost file that allocates it in
316 the backing file chain.
318 Two option formats are possible. The default format (@code{human})
319 only dumps known-nonzero areas of the file. Known-zero parts of the
320 file are omitted altogether, and likewise for parts that are not allocated
321 throughout the chain. @command{qemu-img} output will identify a file
322 from where the data can be read, and the offset in the file. Each line
323 will include four fields, the first three of which are hexadecimal
324 numbers. For example the first line of:
326 Offset Length Mapped to File
327 0 0x20000 0x50000 /tmp/overlay.qcow2
328 0x100000 0x10000 0x95380000 /tmp/backing.qcow2
331 means that 0x20000 (131072) bytes starting at offset 0 in the image are
332 available in /tmp/overlay.qcow2 (opened in @code{raw} format) starting
333 at offset 0x50000 (327680). Data that is compressed, encrypted, or
334 otherwise not available in raw format will cause an error if @code{human}
335 format is in use. Note that file names can include newlines, thus it is
336 not safe to parse this output format in scripts.
338 The alternative format @code{json} will return an array of dictionaries
339 in JSON format. It will include similar information in
340 the @code{start}, @code{length}, @code{offset} fields;
341 it will also include other more specific information:
344 whether the sectors contain actual data or not (boolean field @code{data};
345 if false, the sectors are either unallocated or stored as optimized
349 whether the data is known to read as zero (boolean field @code{zero});
352 in order to make the output shorter, the target file is expressed as
353 a @code{depth}; for example, a depth of 2 refers to the backing file
354 of the backing file of @var{filename}.
357 In JSON format, the @code{offset} field is optional; it is absent in
358 cases where @code{human} format would omit the entry or exit with an error.
359 If @code{data} is false and the @code{offset} field is present, the
360 corresponding sectors in the file are not yet in use, but they are
363 For more information, consult @file{include/block/block.h} in QEMU's
366 @item snapshot [-l | -a @var{snapshot} | -c @var{snapshot} | -d @var{snapshot} ] @var{filename}
368 List, apply, create or delete snapshots in image @var{filename}.
370 @item rebase [-f @var{fmt}] [-t @var{cache}] [-T @var{src_cache}] [-p] [-u] -b @var{backing_file} [-F @var{backing_fmt}] @var{filename}
372 Changes the backing file of an image. Only the formats @code{qcow2} and
373 @code{qed} support changing the backing file.
375 The backing file is changed to @var{backing_file} and (if the image format of
376 @var{filename} supports this) the backing file format is changed to
377 @var{backing_fmt}. If @var{backing_file} is specified as ``'' (the empty
378 string), then the image is rebased onto no backing file (i.e. it will exist
379 independently of any backing file).
381 @var{cache} specifies the cache mode to be used for @var{filename}, whereas
382 @var{src_cache} specifies the cache mode for reading backing files.
384 There are two different modes in which @code{rebase} can operate:
387 This is the default mode and performs a real rebase operation. The new backing
388 file may differ from the old one and qemu-img rebase will take care of keeping
389 the guest-visible content of @var{filename} unchanged.
391 In order to achieve this, any clusters that differ between @var{backing_file}
392 and the old backing file of @var{filename} are merged into @var{filename}
393 before actually changing the backing file.
395 Note that the safe mode is an expensive operation, comparable to converting
396 an image. It only works if the old backing file still exists.
399 qemu-img uses the unsafe mode if @code{-u} is specified. In this mode, only the
400 backing file name and format of @var{filename} is changed without any checks
401 on the file contents. The user must take care of specifying the correct new
402 backing file, or the guest-visible content of the image will be corrupted.
404 This mode is useful for renaming or moving the backing file to somewhere else.
405 It can be used without an accessible old backing file, i.e. you can use it to
406 fix an image whose backing file has already been moved/renamed.
409 You can use @code{rebase} to perform a ``diff'' operation on two
410 disk images. This can be useful when you have copied or cloned
411 a guest, and you want to get back to a thin image on top of a
412 template or base image.
414 Say that @code{base.img} has been cloned as @code{modified.img} by
415 copying it, and that the @code{modified.img} guest has run so there
416 are now some changes compared to @code{base.img}. To construct a thin
417 image called @code{diff.qcow2} that contains just the differences, do:
420 qemu-img create -f qcow2 -b modified.img diff.qcow2
421 qemu-img rebase -b base.img diff.qcow2
424 At this point, @code{modified.img} can be discarded, since
425 @code{base.img + diff.qcow2} contains the same information.
427 @item resize @var{filename} [+ | -]@var{size}
429 Change the disk image as if it had been created with @var{size}.
431 Before using this command to shrink a disk image, you MUST use file system and
432 partitioning tools inside the VM to reduce allocated file systems and partition
433 sizes accordingly. Failure to do so will result in data loss!
435 After using this command to grow a disk image, you must use file system and
436 partitioning tools inside the VM to actually begin using the new space on the
439 @item amend [-p] [-f @var{fmt}] [-t @var{cache}] -o @var{options} @var{filename}
441 Amends the image format specific @var{options} for the image file
442 @var{filename}. Not all file formats support this operation.
448 Supported image file formats:
453 Raw disk image format (default). This format has the advantage of
454 being simple and easily exportable to all other emulators. If your
455 file system supports @emph{holes} (for example in ext2 or ext3 on
456 Linux or NTFS on Windows), then only the written sectors will reserve
457 space. Use @code{qemu-img info} to know the real size used by the
458 image or @code{ls -ls} on Unix/Linux.
463 Preallocation mode (allowed values: @code{off}, @code{falloc}, @code{full}).
464 @code{falloc} mode preallocates space for image by calling posix_fallocate().
465 @code{full} mode preallocates space for image by writing zeros to underlying
470 QEMU image format, the most versatile format. Use it to have smaller
471 images (useful if your filesystem does not supports holes, for example
472 on Windows), optional AES encryption, zlib based compression and
473 support of multiple VM snapshots.
478 Determines the qcow2 version to use. @code{compat=0.10} uses the
479 traditional image format that can be read by any QEMU since 0.10.
480 @code{compat=1.1} enables image format extensions that only QEMU 1.1 and
481 newer understand (this is the default). Amongst others, this includes zero
482 clusters, which allow efficient copy-on-read for sparse images.
485 File name of a base image (see @option{create} subcommand)
487 Image format of the base image
489 If this option is set to @code{on}, the image is encrypted with 128-bit AES-CBC.
491 The use of encryption in qcow and qcow2 images is considered to be flawed by
492 modern cryptography standards, suffering from a number of design problems:
495 @item The AES-CBC cipher is used with predictable initialization vectors based
496 on the sector number. This makes it vulnerable to chosen plaintext attacks
497 which can reveal the existence of encrypted data.
498 @item The user passphrase is directly used as the encryption key. A poorly
499 chosen or short passphrase will compromise the security of the encryption.
500 @item In the event of the passphrase being compromised there is no way to
501 change the passphrase to protect data in any qcow images. The files must
502 be cloned, using a different encryption passphrase in the new file. The
503 original file must then be securely erased using a program like shred,
504 though even this is ineffective with many modern storage technologies.
507 Use of qcow / qcow2 encryption is thus strongly discouraged. Users are
508 recommended to use an alternative encryption technology such as the
509 Linux dm-crypt / LUKS system.
512 Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster
513 sizes can improve the image file size whereas larger cluster sizes generally
514 provide better performance.
517 Preallocation mode (allowed values: @code{off}, @code{metadata}, @code{falloc},
518 @code{full}). An image with preallocated metadata is initially larger but can
519 improve performance when the image needs to grow. @code{falloc} and @code{full}
520 preallocations are like the same options of @code{raw} format, but sets up
524 If this option is set to @code{on}, reference count updates are postponed with
525 the goal of avoiding metadata I/O and improving performance. This is
526 particularly interesting with @option{cache=writethrough} which doesn't batch
527 metadata updates. The tradeoff is that after a host crash, the reference count
528 tables must be rebuilt, i.e. on the next open an (automatic) @code{qemu-img
529 check -r all} is required, which may take some time.
531 This option can only be enabled if @code{compat=1.1} is specified.
534 If this option is set to @code{on}, it will turn off COW of the file. It's only
535 valid on btrfs, no effect on other file systems.
537 Btrfs has low performance when hosting a VM image file, even more when the guest
538 on the VM also using btrfs as file system. Turning off COW is a way to mitigate
539 this bad performance. Generally there are two ways to turn off COW on btrfs:
540 a) Disable it by mounting with nodatacow, then all newly created files will be
541 NOCOW. b) For an empty file, add the NOCOW file attribute. That's what this option
544 Note: this option is only valid to new or empty files. If there is an existing
545 file which is COW and has data blocks already, it couldn't be changed to NOCOW
546 by setting @code{nocow=on}. One can issue @code{lsattr filename} to check if
547 the NOCOW flag is set or not (Capital 'C' is NOCOW flag).
552 QEMU also supports various other image file formats for compatibility with
553 older QEMU versions or other hypervisors, including VMDK, VDI, VHD (vpc), VHDX,
554 qcow1 and QED. For a full list of supported formats see @code{qemu-img --help}.
555 For a more detailed description of these formats, see the QEMU Emulation User
558 The main purpose of the block drivers for these formats is image conversion.
559 For running VMs, it is recommended to convert the disk images to either raw or
560 qcow2 in order to achieve good performance.
566 @setfilename qemu-img
567 @settitle QEMU disk image utility
570 The HTML documentation of QEMU for more precise information and Linux
571 user mode emulator invocation.