3 @command{qemu-img} [@var{standard} @var{options}] @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
22 Display this help and exit
24 Display version information and exit
27 The following commands are supported:
29 @include qemu-img-cmds.texi
34 is a disk image filename
36 @item --object @var{objectdef}
38 is a QEMU user creatable object definition. See the @code{qemu(1)} manual
39 page for a description of the object properties. The most common object
40 type is a @code{secret}, which is used to supply passwords and/or encryption
45 Indicates that the @var{filename} parameter is to be interpreted as a
46 full option string, not a plain filename. This parameter is mutually
47 exclusive with the @var{-f} and @var{-F} parameters.
50 is the disk image format. It is guessed automatically in most cases. See below
51 for a description of the supported disk formats.
54 will enumerate information about backing files in a disk image chain. Refer
55 below for further description.
58 is the disk image size in bytes. Optional suffixes @code{k} or @code{K}
59 (kilobyte, 1024) @code{M} (megabyte, 1024k) and @code{G} (gigabyte, 1024M)
60 and T (terabyte, 1024G) are supported. @code{b} is ignored.
63 is the destination disk image filename
66 is the destination format
68 is a comma separated list of format specific options in a
69 name=value format. Use @code{-o ?} for an overview of the options supported
70 by the used format or see the format descriptions below for details.
72 is param used for internal snapshot, format is
73 'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'
74 @item snapshot_id_or_name
75 is deprecated, use snapshot_param instead
78 indicates that target image must be compressed (qcow format only)
80 with or without a command shows help and lists the supported formats
82 display progress bar (compare, convert and rebase commands only).
83 If the @var{-p} option is not used for a command that supports it, the
84 progress is reported when the process receives a @code{SIGUSR1} signal.
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.
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.
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
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
103 Parameters to snapshot subcommand:
108 is the name of the snapshot to create, apply or delete
110 applies a snapshot (revert disk to saved state)
116 lists all snapshots in the given image
119 Parameters to compare subcommand:
128 Strict mode - fail on different image size or sector allocation
131 Parameters to convert subcommand:
136 Skip the creation of the target volume
142 @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}
144 Run a simple sequential I/O benchmark on the specified image. If @code{-w} is
145 specified, a write test is performed, otherwise a read test is performed.
147 A total number of @var{count} I/O requests is performed, each @var{buffer_size}
148 bytes in size, and with @var{depth} requests in parallel. The first request
149 starts at the position given by @var{offset}, each following request increases
150 the current position by @var{step_size}. If @var{step_size} is not given,
151 @var{buffer_size} is used for its value.
153 If @var{flush_interval} is specified for a write test, the request queue is
154 drained and a flush is issued before new writes are made whenever the number of
155 remaining requests is a multiple of @var{flush_interval}. If additionally
156 @code{--no-drain} is specified, a flush is issued without draining the request
159 If @code{-n} is specified, the native AIO backend is used if possible. On
160 Linux, this option only works if @code{-t none} or @code{-t directsync} is
163 For write tests, by default a buffer filled with zeros is written. This can be
164 overridden with a pattern byte specified by @var{pattern}.
166 @item check [-f @var{fmt}] [--output=@var{ofmt}] [-r [leaks | all]] [-T @var{src_cache}] @var{filename}
168 Perform a consistency check on the disk image @var{filename}. The command can
169 output in the format @var{ofmt} which is either @code{human} or @code{json}.
171 If @code{-r} is specified, qemu-img tries to repair any inconsistencies found
172 during the check. @code{-r leaks} repairs only cluster leaks, whereas
173 @code{-r all} fixes all kinds of errors, with a higher risk of choosing the
174 wrong fix or hiding corruption that has already occurred.
176 Only the formats @code{qcow2}, @code{qed} and @code{vdi} support
179 In case the image does not have any inconsistencies, check exits with @code{0}.
180 Other exit codes indicate the kind of inconsistency found or if another error
181 occurred. The following table summarizes all exit codes of the check subcommand:
186 Check completed, the image is (now) consistent
188 Check not completed because of internal errors
190 Check completed, image is corrupted
192 Check completed, image has leaked clusters, but is not corrupted
194 Checks are not supported by the image format
198 If @code{-r} is specified, exit codes representing the image state refer to the
199 state after (the attempt at) repairing it. That is, a successful @code{-r all}
200 will yield the exit code 0, independently of the image state before.
202 @item create [-f @var{fmt}] [-o @var{options}] @var{filename} [@var{size}]
204 Create the new disk image @var{filename} of size @var{size} and format
205 @var{fmt}. Depending on the file format, you can add one or more @var{options}
206 that enable additional features of this format.
208 If the option @var{backing_file} is specified, then the image will record
209 only the differences from @var{backing_file}. No size needs to be specified in
210 this case. @var{backing_file} will never be modified unless you use the
211 @code{commit} monitor command (or qemu-img commit).
213 The size can also be specified using the @var{size} option with @code{-o},
214 it doesn't need to be specified separately in this case.
216 @item commit [-q] [-f @var{fmt}] [-t @var{cache}] [-b @var{base}] [-d] [-p] @var{filename}
218 Commit the changes recorded in @var{filename} in its base image or backing file.
219 If the backing file is smaller than the snapshot, then the backing file will be
220 resized to be the same size as the snapshot. If the snapshot is smaller than
221 the backing file, the backing file will not be truncated. If you want the
222 backing file to match the size of the smaller snapshot, you can safely truncate
223 it yourself once the commit operation successfully completes.
225 The image @var{filename} is emptied after the operation has succeeded. If you do
226 not need @var{filename} afterwards and intend to drop it, you may skip emptying
227 @var{filename} by specifying the @code{-d} flag.
229 If the backing chain of the given image file @var{filename} has more than one
230 layer, the backing file into which the changes will be committed may be
231 specified as @var{base} (which has to be part of @var{filename}'s backing
232 chain). If @var{base} is not specified, the immediate backing file of the top
233 image (which is @var{filename}) will be used. For reasons of consistency,
234 explicitly specifying @var{base} will always imply @code{-d} (since emptying an
235 image after committing to an indirect backing file would lead to different data
236 being read from the image due to content in the intermediate backing chain
237 overruling the commit target).
239 @item compare [-f @var{fmt}] [-F @var{fmt}] [-T @var{src_cache}] [-p] [-s] [-q] @var{filename1} @var{filename2}
241 Check if two images have the same content. You can compare images with
242 different format or settings.
244 The format is probed unless you specify it by @var{-f} (used for
245 @var{filename1}) and/or @var{-F} (used for @var{filename2}) option.
247 By default, images with different size are considered identical if the larger
248 image contains only unallocated and/or zeroed sectors in the area after the end
249 of the other image. In addition, if any sector is not allocated in one image
250 and contains only zero bytes in the second one, it is evaluated as equal. You
251 can use Strict mode by specifying the @var{-s} option. When compare runs in
252 Strict mode, it fails in case image size differs or a sector is allocated in
253 one image and is not allocated in the second one.
255 By default, compare prints out a result message. This message displays
256 information that both images are same or the position of the first different
257 byte. In addition, result message can report different image size in case
260 Compare exits with @code{0} in case the images are equal and with @code{1}
261 in case the images differ. Other exit codes mean an error occurred during
262 execution and standard error output should contain an error message.
263 The following table sumarizes all exit codes of the compare subcommand:
272 Error on opening an image
274 Error on checking a sector allocation
276 Error on reading data
280 @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}
282 Convert the disk image @var{filename} or a snapshot @var{snapshot_param}(@var{snapshot_id_or_name} is deprecated)
283 to disk image @var{output_filename} using format @var{output_fmt}. It can be optionally compressed (@code{-c}
284 option) or use any format specific options like encryption (@code{-o} option).
286 Only the formats @code{qcow} and @code{qcow2} support compression. The
287 compression is read-only. It means that if a compressed sector is
288 rewritten, then it is rewritten as uncompressed data.
290 Image conversion is also useful to get smaller image when using a
291 growable format such as @code{qcow}: the empty sectors are detected and
292 suppressed from the destination image.
294 @var{sparse_size} indicates the consecutive number of bytes (defaults to 4k)
295 that must contain only zeros for qemu-img to create a sparse image during
296 conversion. If @var{sparse_size} is 0, the source will not be scanned for
297 unallocated or zero sectors, and the destination image will always be
300 You can use the @var{backing_file} option to force the output image to be
301 created as a copy on write image of the specified base image; the
302 @var{backing_file} should have the same content as the input's base image,
303 however the path, image format, etc may differ.
305 If the @code{-n} option is specified, the target volume creation will be
306 skipped. This is useful for formats such as @code{rbd} if the target
307 volume has already been created with site specific options that cannot
308 be supplied through qemu-img.
310 @item info [-f @var{fmt}] [--output=@var{ofmt}] [--backing-chain] @var{filename}
312 Give information about the disk image @var{filename}. Use it in
313 particular to know the size reserved on disk which can be different
314 from the displayed size. If VM snapshots are stored in the disk image,
315 they are displayed too. The command can output in the format @var{ofmt}
316 which is either @code{human} or @code{json}.
318 If a disk image has a backing file chain, information about each disk image in
319 the chain can be recursively enumerated by using the option @code{--backing-chain}.
321 For instance, if you have an image chain like:
324 base.qcow2 <- snap1.qcow2 <- snap2.qcow2
327 To enumerate information about each disk image in the above chain, starting from top to base, do:
330 qemu-img info --backing-chain snap2.qcow2
333 @item map [-f @var{fmt}] [--output=@var{ofmt}] @var{filename}
335 Dump the metadata of image @var{filename} and its backing file chain.
336 In particular, this commands dumps the allocation state of every sector
337 of @var{filename}, together with the topmost file that allocates it in
338 the backing file chain.
340 Two option formats are possible. The default format (@code{human})
341 only dumps known-nonzero areas of the file. Known-zero parts of the
342 file are omitted altogether, and likewise for parts that are not allocated
343 throughout the chain. @command{qemu-img} output will identify a file
344 from where the data can be read, and the offset in the file. Each line
345 will include four fields, the first three of which are hexadecimal
346 numbers. For example the first line of:
348 Offset Length Mapped to File
349 0 0x20000 0x50000 /tmp/overlay.qcow2
350 0x100000 0x10000 0x95380000 /tmp/backing.qcow2
353 means that 0x20000 (131072) bytes starting at offset 0 in the image are
354 available in /tmp/overlay.qcow2 (opened in @code{raw} format) starting
355 at offset 0x50000 (327680). Data that is compressed, encrypted, or
356 otherwise not available in raw format will cause an error if @code{human}
357 format is in use. Note that file names can include newlines, thus it is
358 not safe to parse this output format in scripts.
360 The alternative format @code{json} will return an array of dictionaries
361 in JSON format. It will include similar information in
362 the @code{start}, @code{length}, @code{offset} fields;
363 it will also include other more specific information:
366 whether the sectors contain actual data or not (boolean field @code{data};
367 if false, the sectors are either unallocated or stored as optimized
371 whether the data is known to read as zero (boolean field @code{zero});
374 in order to make the output shorter, the target file is expressed as
375 a @code{depth}; for example, a depth of 2 refers to the backing file
376 of the backing file of @var{filename}.
379 In JSON format, the @code{offset} field is optional; it is absent in
380 cases where @code{human} format would omit the entry or exit with an error.
381 If @code{data} is false and the @code{offset} field is present, the
382 corresponding sectors in the file are not yet in use, but they are
385 For more information, consult @file{include/block/block.h} in QEMU's
388 @item snapshot [-l | -a @var{snapshot} | -c @var{snapshot} | -d @var{snapshot} ] @var{filename}
390 List, apply, create or delete snapshots in image @var{filename}.
392 @item rebase [-f @var{fmt}] [-t @var{cache}] [-T @var{src_cache}] [-p] [-u] -b @var{backing_file} [-F @var{backing_fmt}] @var{filename}
394 Changes the backing file of an image. Only the formats @code{qcow2} and
395 @code{qed} support changing the backing file.
397 The backing file is changed to @var{backing_file} and (if the image format of
398 @var{filename} supports this) the backing file format is changed to
399 @var{backing_fmt}. If @var{backing_file} is specified as ``'' (the empty
400 string), then the image is rebased onto no backing file (i.e. it will exist
401 independently of any backing file).
403 @var{cache} specifies the cache mode to be used for @var{filename}, whereas
404 @var{src_cache} specifies the cache mode for reading backing files.
406 There are two different modes in which @code{rebase} can operate:
409 This is the default mode and performs a real rebase operation. The new backing
410 file may differ from the old one and qemu-img rebase will take care of keeping
411 the guest-visible content of @var{filename} unchanged.
413 In order to achieve this, any clusters that differ between @var{backing_file}
414 and the old backing file of @var{filename} are merged into @var{filename}
415 before actually changing the backing file.
417 Note that the safe mode is an expensive operation, comparable to converting
418 an image. It only works if the old backing file still exists.
421 qemu-img uses the unsafe mode if @code{-u} is specified. In this mode, only the
422 backing file name and format of @var{filename} is changed without any checks
423 on the file contents. The user must take care of specifying the correct new
424 backing file, or the guest-visible content of the image will be corrupted.
426 This mode is useful for renaming or moving the backing file to somewhere else.
427 It can be used without an accessible old backing file, i.e. you can use it to
428 fix an image whose backing file has already been moved/renamed.
431 You can use @code{rebase} to perform a ``diff'' operation on two
432 disk images. This can be useful when you have copied or cloned
433 a guest, and you want to get back to a thin image on top of a
434 template or base image.
436 Say that @code{base.img} has been cloned as @code{modified.img} by
437 copying it, and that the @code{modified.img} guest has run so there
438 are now some changes compared to @code{base.img}. To construct a thin
439 image called @code{diff.qcow2} that contains just the differences, do:
442 qemu-img create -f qcow2 -b modified.img diff.qcow2
443 qemu-img rebase -b base.img diff.qcow2
446 At this point, @code{modified.img} can be discarded, since
447 @code{base.img + diff.qcow2} contains the same information.
449 @item resize @var{filename} [+ | -]@var{size}
451 Change the disk image as if it had been created with @var{size}.
453 Before using this command to shrink a disk image, you MUST use file system and
454 partitioning tools inside the VM to reduce allocated file systems and partition
455 sizes accordingly. Failure to do so will result in data loss!
457 After using this command to grow a disk image, you must use file system and
458 partitioning tools inside the VM to actually begin using the new space on the
461 @item amend [-p] [-f @var{fmt}] [-t @var{cache}] -o @var{options} @var{filename}
463 Amends the image format specific @var{options} for the image file
464 @var{filename}. Not all file formats support this operation.
470 Supported image file formats:
475 Raw disk image format (default). This format has the advantage of
476 being simple and easily exportable to all other emulators. If your
477 file system supports @emph{holes} (for example in ext2 or ext3 on
478 Linux or NTFS on Windows), then only the written sectors will reserve
479 space. Use @code{qemu-img info} to know the real size used by the
480 image or @code{ls -ls} on Unix/Linux.
485 Preallocation mode (allowed values: @code{off}, @code{falloc}, @code{full}).
486 @code{falloc} mode preallocates space for image by calling posix_fallocate().
487 @code{full} mode preallocates space for image by writing zeros to underlying
492 QEMU image format, the most versatile format. Use it to have smaller
493 images (useful if your filesystem does not supports holes, for example
494 on Windows), optional AES encryption, zlib based compression and
495 support of multiple VM snapshots.
500 Determines the qcow2 version to use. @code{compat=0.10} uses the
501 traditional image format that can be read by any QEMU since 0.10.
502 @code{compat=1.1} enables image format extensions that only QEMU 1.1 and
503 newer understand (this is the default). Amongst others, this includes zero
504 clusters, which allow efficient copy-on-read for sparse images.
507 File name of a base image (see @option{create} subcommand)
509 Image format of the base image
511 If this option is set to @code{on}, the image is encrypted with 128-bit AES-CBC.
513 The use of encryption in qcow and qcow2 images is considered to be flawed by
514 modern cryptography standards, suffering from a number of design problems:
517 @item The AES-CBC cipher is used with predictable initialization vectors based
518 on the sector number. This makes it vulnerable to chosen plaintext attacks
519 which can reveal the existence of encrypted data.
520 @item The user passphrase is directly used as the encryption key. A poorly
521 chosen or short passphrase will compromise the security of the encryption.
522 @item In the event of the passphrase being compromised there is no way to
523 change the passphrase to protect data in any qcow images. The files must
524 be cloned, using a different encryption passphrase in the new file. The
525 original file must then be securely erased using a program like shred,
526 though even this is ineffective with many modern storage technologies.
529 Use of qcow / qcow2 encryption is thus strongly discouraged. Users are
530 recommended to use an alternative encryption technology such as the
531 Linux dm-crypt / LUKS system.
534 Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster
535 sizes can improve the image file size whereas larger cluster sizes generally
536 provide better performance.
539 Preallocation mode (allowed values: @code{off}, @code{metadata}, @code{falloc},
540 @code{full}). An image with preallocated metadata is initially larger but can
541 improve performance when the image needs to grow. @code{falloc} and @code{full}
542 preallocations are like the same options of @code{raw} format, but sets up
546 If this option is set to @code{on}, reference count updates are postponed with
547 the goal of avoiding metadata I/O and improving performance. This is
548 particularly interesting with @option{cache=writethrough} which doesn't batch
549 metadata updates. The tradeoff is that after a host crash, the reference count
550 tables must be rebuilt, i.e. on the next open an (automatic) @code{qemu-img
551 check -r all} is required, which may take some time.
553 This option can only be enabled if @code{compat=1.1} is specified.
556 If this option is set to @code{on}, it will turn off COW of the file. It's only
557 valid on btrfs, no effect on other file systems.
559 Btrfs has low performance when hosting a VM image file, even more when the guest
560 on the VM also using btrfs as file system. Turning off COW is a way to mitigate
561 this bad performance. Generally there are two ways to turn off COW on btrfs:
562 a) Disable it by mounting with nodatacow, then all newly created files will be
563 NOCOW. b) For an empty file, add the NOCOW file attribute. That's what this option
566 Note: this option is only valid to new or empty files. If there is an existing
567 file which is COW and has data blocks already, it couldn't be changed to NOCOW
568 by setting @code{nocow=on}. One can issue @code{lsattr filename} to check if
569 the NOCOW flag is set or not (Capital 'C' is NOCOW flag).
574 QEMU also supports various other image file formats for compatibility with
575 older QEMU versions or other hypervisors, including VMDK, VDI, VHD (vpc), VHDX,
576 qcow1 and QED. For a full list of supported formats see @code{qemu-img --help}.
577 For a more detailed description of these formats, see the QEMU Emulation User
580 The main purpose of the block drivers for these formats is image conversion.
581 For running VMs, it is recommended to convert the disk images to either raw or
582 qcow2 in order to achieve good performance.
588 @setfilename qemu-img
589 @settitle QEMU disk image utility
592 The HTML documentation of QEMU for more precise information and Linux
593 user mode emulator invocation.