1 =======================
2 QEMU disk image utility
3 =======================
8 **qemu-img** [*standard options*] *command* [*command options*]
13 qemu-img allows you to create, convert and modify images offline. It can handle
14 all image formats supported by QEMU.
16 **Warning:** Never use qemu-img to modify images in use by a running virtual
17 machine or any other process; this may destroy the image. Also, be aware that
18 querying an image that is being modified by another process may encounter
28 .. option:: -h, --help
30 Display this help and exit
32 .. option:: -V, --version
34 Display version information and exit
36 .. option:: -T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE]
38 .. include:: ../qemu-option-trace.rst.inc
40 The following commands are supported:
42 .. hxtool-doc:: qemu-img-cmds.hx
46 *FILENAME* is a disk image filename.
48 *FMT* is the disk image format. It is guessed automatically in most
49 cases. See below for a description of the supported disk formats.
51 *SIZE* is the disk image size in bytes. Optional suffixes ``k`` or
52 ``K`` (kilobyte, 1024) ``M`` (megabyte, 1024k) and ``G`` (gigabyte,
53 1024M) and T (terabyte, 1024G) are supported. ``b`` is ignored.
55 *OUTPUT_FILENAME* is the destination disk image filename.
57 *OUTPUT_FMT* is the destination format.
59 *OPTIONS* is a comma separated list of format specific options in a
60 name=value format. Use ``-o help`` for an overview of the options supported
61 by the used format or see the format descriptions below for details.
63 *SNAPSHOT_PARAM* is param used for internal snapshot, format is
64 'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'.
67 Note the use of a new 'program'; otherwise Sphinx complains about
68 the -h option appearing both in the above option list and this one.
70 .. program:: qemu-img-common-opts
72 .. option:: --object OBJECTDEF
74 is a QEMU user creatable object definition. See the :manpage:`qemu(1)`
75 manual page for a description of the object properties. The most common
76 object type is a ``secret``, which is used to supply passwords and/or
79 .. option:: --image-opts
81 Indicates that the source *FILENAME* parameter is to be interpreted as a
82 full option string, not a plain filename. This parameter is mutually
83 exclusive with the *-f* parameter.
85 .. option:: --target-image-opts
87 Indicates that the OUTPUT_FILENAME parameter(s) are to be interpreted as
88 a full option string, not a plain filename. This parameter is mutually
89 exclusive with the *-O* parameters. It is currently required to also use
90 the *-n* parameter to skip image creation. This restriction may be relaxed
93 .. option:: --force-share (-U)
95 If specified, ``qemu-img`` will open the image in shared mode, allowing
96 other QEMU processes to open it in write mode. For example, this can be used to
97 get the image information (with 'info' subcommand) when the image is used by a
98 running guest. Note that this could produce inconsistent results because of
99 concurrent metadata changes, etc. This option is only allowed when opening
100 images in read-only mode.
102 .. option:: --backing-chain
104 Will enumerate information about backing files in a disk image chain. Refer
105 below for further description.
109 Indicates that target image must be compressed (qcow/qcow2 and vmdk with
110 streamOptimized subformat only).
112 For qcow2, the compression algorithm can be specified with the ``-o
113 compression_type=...`` option (see below).
117 With or without a command, shows help and lists the supported formats.
121 Display progress bar (compare, convert and rebase commands only).
122 If the *-p* option is not used for a command that supports it, the
123 progress is reported when the process receives a ``SIGUSR1`` or
128 Quiet mode - do not print any output (except errors). There's no progress bar
129 in case both *-q* and *-p* options are used.
133 Indicates the consecutive number of bytes that must contain only zeros
134 for ``qemu-img`` to create a sparse image during conversion. This value is
135 rounded down to the nearest 512 bytes. You may use the common size suffixes
136 like ``k`` for kilobytes.
140 Specifies the cache mode that should be used with the (destination) file. See
141 the documentation of the emulator's ``-drive cache=...`` option for allowed
144 .. option:: -T SRC_CACHE
146 Specifies the cache mode that should be used with the source file(s). See
147 the documentation of the emulator's ``-drive cache=...`` option for allowed
150 Parameters to compare subcommand:
152 .. program:: qemu-img-compare
164 Strict mode - fail on different image size or sector allocation
166 Parameters to convert subcommand:
168 .. program:: qemu-img-convert
170 .. option:: --bitmaps
172 Additionally copy all persistent bitmaps from the top layer of the source
176 Skip the creation of the target volume
180 Number of parallel coroutines for the convert process
184 Allow out-of-order writes to the destination. This option improves performance,
185 but is only recommended for preallocated devices like host devices or other
190 Try to use copy offloading to move data from source image to target. This may
191 improve performance if the data is remote, such as with NFS or iSCSI backends,
192 but will not automatically sparsify zero sectors, and may result in a fully
193 allocated target image depending on the host support for getting allocation
198 Rate limit for the convert process
200 .. option:: --salvage
202 Try to ignore I/O errors when reading. Unless in quiet mode (``-q``), errors
203 will still be printed. Areas that cannot be read from the source will be
204 treated as containing only zeroes.
206 .. option:: --target-is-zero
208 Assume that reading the destination image will always return
209 zeros. This parameter is mutually exclusive with a destination image
210 that has a backing file. It is required to also use the ``-n``
211 parameter to skip image creation.
213 Parameters to dd subcommand:
215 .. program:: qemu-img-dd
217 .. option:: bs=BLOCK_SIZE
219 Defines the block size
221 .. option:: count=BLOCKS
223 Sets the number of input blocks to copy
229 .. option:: of=OUTPUT
233 .. option:: skip=BLOCKS
235 Sets the number of input blocks to skip
237 Parameters to snapshot subcommand:
239 .. program:: qemu-img-snapshot
243 Is the name of the snapshot to create, apply or delete
247 Applies a snapshot (revert disk to saved state)
259 Lists all snapshots in the given image
263 .. program:: qemu-img-commands
265 .. option:: amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE] [--force] -o OPTIONS FILENAME
267 Amends the image format specific *OPTIONS* for the image file
268 *FILENAME*. Not all file formats support this operation.
270 The set of options that can be amended are dependent on the image
271 format, but note that amending the backing chain relationship should
272 instead be performed with ``qemu-img rebase``.
274 --force allows some unsafe operations. Currently for -f luks, it allows to
275 erase the last encryption key, and to overwrite an active encryption key.
277 .. option:: bench [-c COUNT] [-d DEPTH] [-f FMT] [--flush-interval=FLUSH_INTERVAL] [-i AIO] [-n] [--no-drain] [-o OFFSET] [--pattern=PATTERN] [-q] [-s BUFFER_SIZE] [-S STEP_SIZE] [-t CACHE] [-w] [-U] FILENAME
279 Run a simple sequential I/O benchmark on the specified image. If ``-w`` is
280 specified, a write test is performed, otherwise a read test is performed.
282 A total number of *COUNT* I/O requests is performed, each *BUFFER_SIZE*
283 bytes in size, and with *DEPTH* requests in parallel. The first request
284 starts at the position given by *OFFSET*, each following request increases
285 the current position by *STEP_SIZE*. If *STEP_SIZE* is not given,
286 *BUFFER_SIZE* is used for its value.
288 If *FLUSH_INTERVAL* is specified for a write test, the request queue is
289 drained and a flush is issued before new writes are made whenever the number of
290 remaining requests is a multiple of *FLUSH_INTERVAL*. If additionally
291 ``--no-drain`` is specified, a flush is issued without draining the request
294 if ``-i`` is specified, *AIO* option can be used to specify different
295 AIO backends: ``threads``, ``native`` or ``io_uring``.
297 If ``-n`` is specified, the native AIO backend is used if possible. On
298 Linux, this option only works if ``-t none`` or ``-t directsync`` is
301 For write tests, by default a buffer filled with zeros is written. This can be
302 overridden with a pattern byte specified by *PATTERN*.
304 .. option:: bitmap (--merge SOURCE | --add | --remove | --clear | --enable | --disable)... [-b SOURCE_FILE [-F SOURCE_FMT]] [-g GRANULARITY] [--object OBJECTDEF] [--image-opts | -f FMT] FILENAME BITMAP
306 Perform one or more modifications of the persistent bitmap *BITMAP*
307 in the disk image *FILENAME*. The various modifications are:
309 ``--add`` to create *BITMAP*, enabled to record future edits.
311 ``--remove`` to remove *BITMAP*.
313 ``--clear`` to clear *BITMAP*.
315 ``--enable`` to change *BITMAP* to start recording future edits.
317 ``--disable`` to change *BITMAP* to stop recording future edits.
319 ``--merge`` to merge the contents of the *SOURCE* bitmap into *BITMAP*.
321 Additional options include ``-g`` which sets a non-default
322 *GRANULARITY* for ``--add``, and ``-b`` and ``-F`` which select an
323 alternative source file for all *SOURCE* bitmaps used by
326 To see what bitmaps are present in an image, use ``qemu-img info``.
328 .. option:: check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT] [-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME
330 Perform a consistency check on the disk image *FILENAME*. The command can
331 output in the format *OFMT* which is either ``human`` or ``json``.
332 The JSON output is an object of QAPI type ``ImageCheck``.
334 If ``-r`` is specified, qemu-img tries to repair any inconsistencies found
335 during the check. ``-r leaks`` repairs only cluster leaks, whereas
336 ``-r all`` fixes all kinds of errors, with a higher risk of choosing the
337 wrong fix or hiding corruption that has already occurred.
339 Only the formats ``qcow2``, ``qed``, ``parallels``, ``vhdx``, ``vmdk`` and
340 ``vdi`` support consistency checks.
342 In case the image does not have any inconsistencies, check exits with ``0``.
343 Other exit codes indicate the kind of inconsistency found or if another error
344 occurred. The following table summarizes all exit codes of the check subcommand:
347 Check completed, the image is (now) consistent
349 Check not completed because of internal errors
351 Check completed, image is corrupted
353 Check completed, image has leaked clusters, but is not corrupted
355 Checks are not supported by the image format
357 If ``-r`` is specified, exit codes representing the image state refer to the
358 state after (the attempt at) repairing it. That is, a successful ``-r all``
359 will yield the exit code 0, independently of the image state before.
361 .. option:: commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b BASE] [-r RATE_LIMIT] [-d] [-p] FILENAME
363 Commit the changes recorded in *FILENAME* in its base image or backing file.
364 If the backing file is smaller than the snapshot, then the backing file will be
365 resized to be the same size as the snapshot. If the snapshot is smaller than
366 the backing file, the backing file will not be truncated. If you want the
367 backing file to match the size of the smaller snapshot, you can safely truncate
368 it yourself once the commit operation successfully completes.
370 The image *FILENAME* is emptied after the operation has succeeded. If you do
371 not need *FILENAME* afterwards and intend to drop it, you may skip emptying
372 *FILENAME* by specifying the ``-d`` flag.
374 If the backing chain of the given image file *FILENAME* has more than one
375 layer, the backing file into which the changes will be committed may be
376 specified as *BASE* (which has to be part of *FILENAME*'s backing
377 chain). If *BASE* is not specified, the immediate backing file of the top
378 image (which is *FILENAME*) will be used. Note that after a commit operation
379 all images between *BASE* and the top image will be invalid and may return
380 garbage data when read. For this reason, ``-b`` implies ``-d`` (so that
381 the top image stays valid).
383 The rate limit for the commit process is specified by ``-r``.
385 .. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 FILENAME2
387 Check if two images have the same content. You can compare images with
388 different format or settings.
390 The format is probed unless you specify it by ``-f`` (used for
391 *FILENAME1*) and/or ``-F`` (used for *FILENAME2*) option.
393 By default, images with different size are considered identical if the larger
394 image contains only unallocated and/or zeroed sectors in the area after the end
395 of the other image. In addition, if any sector is not allocated in one image
396 and contains only zero bytes in the second one, it is evaluated as equal. You
397 can use Strict mode by specifying the ``-s`` option. When compare runs in
398 Strict mode, it fails in case image size differs or a sector is allocated in
399 one image and is not allocated in the second one.
401 By default, compare prints out a result message. This message displays
402 information that both images are same or the position of the first different
403 byte. In addition, result message can report different image size in case
406 Compare exits with ``0`` in case the images are equal and with ``1``
407 in case the images differ. Other exit codes mean an error occurred during
408 execution and standard error output should contain an error message.
409 The following table sumarizes all exit codes of the compare subcommand:
412 Images are identical (or requested help was printed)
416 Error on opening an image
418 Error on checking a sector allocation
420 Error on reading data
422 .. option:: convert [--object OBJECTDEF] [--image-opts] [--target-image-opts] [--target-is-zero] [--bitmaps [--skip-broken-bitmaps]] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-O OUTPUT_FMT] [-B BACKING_FILE [-F BACKING_FMT]] [-o OPTIONS] [-l SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-r RATE_LIMIT] [-m NUM_COROUTINES] [-W] FILENAME [FILENAME2 [...]] OUTPUT_FILENAME
424 Convert the disk image *FILENAME* or a snapshot *SNAPSHOT_PARAM*
425 to disk image *OUTPUT_FILENAME* using format *OUTPUT_FMT*. It can
426 be optionally compressed (``-c`` option) or use any format specific
427 options like encryption (``-o`` option).
429 Only the formats ``qcow`` and ``qcow2`` support compression. The
430 compression is read-only. It means that if a compressed sector is
431 rewritten, then it is rewritten as uncompressed data.
433 Image conversion is also useful to get smaller image when using a
434 growable format such as ``qcow``: the empty sectors are detected and
435 suppressed from the destination image.
437 *SPARSE_SIZE* indicates the consecutive number of bytes (defaults to 4k)
438 that must contain only zeros for ``qemu-img`` to create a sparse image during
439 conversion. If *SPARSE_SIZE* is 0, the source will not be scanned for
440 unallocated or zero sectors, and the destination image will always be
443 You can use the *BACKING_FILE* option to force the output image to be
444 created as a copy on write image of the specified base image; the
445 *BACKING_FILE* should have the same content as the input's base image,
446 however the path, image format (as given by *BACKING_FMT*), etc may differ.
448 If a relative path name is given, the backing file is looked up relative to
449 the directory containing *OUTPUT_FILENAME*.
451 If the ``-n`` option is specified, the target volume creation will be
452 skipped. This is useful for formats such as ``rbd`` if the target
453 volume has already been created with site specific options that cannot
454 be supplied through ``qemu-img``.
456 Out of order writes can be enabled with ``-W`` to improve performance.
457 This is only recommended for preallocated devices like host devices or other
458 raw block devices. Out of order write does not work in combination with
459 creating compressed images.
461 *NUM_COROUTINES* specifies how many coroutines work in parallel during
462 the convert process (defaults to 8).
464 Use of ``--bitmaps`` requests that any persistent bitmaps present in
465 the original are also copied to the destination. If any bitmap is
466 inconsistent in the source, the conversion will fail unless
467 ``--skip-broken-bitmaps`` is also specified to copy only the
470 .. option:: create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE [-F BACKING_FMT]] [-u] [-o OPTIONS] FILENAME [SIZE]
472 Create the new disk image *FILENAME* of size *SIZE* and format
473 *FMT*. Depending on the file format, you can add one or more *OPTIONS*
474 that enable additional features of this format.
476 If the option *BACKING_FILE* is specified, then the image will record
477 only the differences from *BACKING_FILE*. No size needs to be specified in
478 this case. *BACKING_FILE* will never be modified unless you use the
479 ``commit`` monitor command (or ``qemu-img commit``).
481 If a relative path name is given, the backing file is looked up relative to
482 the directory containing *FILENAME*.
484 Note that a given backing file will be opened to check that it is valid. Use
485 the ``-u`` option to enable unsafe backing file mode, which means that the
486 image will be created even if the associated backing file cannot be opened. A
487 matching backing file must be created or additional options be used to make the
488 backing file specification valid when you want to use an image created this
491 The size can also be specified using the *SIZE* option with ``-o``,
492 it doesn't need to be specified separately in this case.
495 .. option:: dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE] [count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT
497 dd copies from *INPUT* file to *OUTPUT* file converting it from
498 *FMT* format to *OUTPUT_FMT* format.
500 The data is by default read and written using blocks of 512 bytes but can be
501 modified by specifying *BLOCK_SIZE*. If count=\ *BLOCKS* is specified
502 dd will stop reading input after reading *BLOCKS* input blocks.
504 The size syntax is similar to :manpage:`dd(1)`'s size syntax.
506 .. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [-U] FILENAME
508 Give information about the disk image *FILENAME*. Use it in
509 particular to know the size reserved on disk which can be different
510 from the displayed size. If VM snapshots are stored in the disk image,
511 they are displayed too.
513 If a disk image has a backing file chain, information about each disk image in
514 the chain can be recursively enumerated by using the option ``--backing-chain``.
516 For instance, if you have an image chain like:
520 base.qcow2 <- snap1.qcow2 <- snap2.qcow2
522 To enumerate information about each disk image in the above chain, starting from top to base, do:
526 qemu-img info --backing-chain snap2.qcow2
528 The command can output in the format *OFMT* which is either ``human`` or
529 ``json``. The JSON output is an object of QAPI type ``ImageInfo``; with
530 ``--backing-chain``, it is an array of ``ImageInfo`` objects.
532 ``--output=human`` reports the following information (for every image in the
542 The size of the guest disk
545 How much space the image file occupies on the host file system (may be
546 shown as 0 if this information is unavailable, e.g. because there is no
550 Cluster size of the image format, if applicable
553 Whether the image is encrypted (only present if so)
556 This is shown as ``no`` if the image is dirty and will have to be
557 auto-repaired the next time it is opened in qemu.
560 The backing file name, if present
562 *backing file format*
563 The format of the backing file, if the image enforces it
566 A list of all internal snapshots
568 *Format specific information*
569 Further information whose structure depends on the image format. This
570 section is a textual representation of the respective
571 ``ImageInfoSpecific*`` QAPI object (e.g. ``ImageInfoSpecificQCow2``
574 .. option:: map [--object OBJECTDEF] [--image-opts] [-f FMT] [--start-offset=OFFSET] [--max-length=LEN] [--output=OFMT] [-U] FILENAME
576 Dump the metadata of image *FILENAME* and its backing file chain.
577 In particular, this commands dumps the allocation state of every sector
578 of *FILENAME*, together with the topmost file that allocates it in
579 the backing file chain.
581 Two option formats are possible. The default format (``human``)
582 only dumps known-nonzero areas of the file. Known-zero parts of the
583 file are omitted altogether, and likewise for parts that are not allocated
584 throughout the chain. ``qemu-img`` output will identify a file
585 from where the data can be read, and the offset in the file. Each line
586 will include four fields, the first three of which are hexadecimal
587 numbers. For example the first line of:
591 Offset Length Mapped to File
592 0 0x20000 0x50000 /tmp/overlay.qcow2
593 0x100000 0x10000 0x95380000 /tmp/backing.qcow2
595 means that 0x20000 (131072) bytes starting at offset 0 in the image are
596 available in /tmp/overlay.qcow2 (opened in ``raw`` format) starting
597 at offset 0x50000 (327680). Data that is compressed, encrypted, or
598 otherwise not available in raw format will cause an error if ``human``
599 format is in use. Note that file names can include newlines, thus it is
600 not safe to parse this output format in scripts.
602 The alternative format ``json`` will return an array of dictionaries
603 in JSON format. It will include similar information in
604 the ``start``, ``length``, ``offset`` fields;
605 it will also include other more specific information:
607 - boolean field ``data``: true if the sectors contain actual data,
608 false if the sectors are either unallocated or stored as optimized
610 - boolean field ``zero``: true if the data is known to read as zero
611 - boolean field ``present``: true if the data belongs to the backing
612 chain, false if rebasing the backing chain onto a deeper file
613 would pick up data from the deeper file;
614 - integer field ``depth``: the depth within the backing chain at
615 which the data was resolved; for example, a depth of 2 refers to
616 the backing file of the backing file of *FILENAME*.
618 In JSON format, the ``offset`` field is optional; it is absent in
619 cases where ``human`` format would omit the entry or exit with an error.
620 If ``data`` is false and the ``offset`` field is present, the
621 corresponding sectors in the file are not yet in use, but they are
624 For more information, consult ``include/block/block.h`` in QEMU's
627 .. option:: measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--object OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME]
629 Calculate the file size required for a new image. This information
630 can be used to size logical volumes or SAN LUNs appropriately for
631 the image that will be placed in them. The values reported are
632 guaranteed to be large enough to fit the image. The command can
633 output in the format *OFMT* which is either ``human`` or ``json``.
634 The JSON output is an object of QAPI type ``BlockMeasureInfo``.
636 If the size *N* is given then act as if creating a new empty image file
637 using ``qemu-img create``. If *FILENAME* is given then act as if
638 converting an existing image file using ``qemu-img convert``. The format
639 of the new file is given by *OUTPUT_FMT* while the format of an existing
640 file is given by *FMT*.
642 A snapshot in an existing image can be specified using *SNAPSHOT_PARAM*.
644 The following fields are reported:
648 required size: 524288
649 fully allocated size: 1074069504
652 The ``required size`` is the file size of the new image. It may be smaller
653 than the virtual disk size if the image format supports compact representation.
655 The ``fully allocated size`` is the file size of the new image once data has
656 been written to all sectors. This is the maximum size that the image file can
657 occupy with the exception of internal snapshots, dirty bitmaps, vmstate data,
658 and other advanced image format features.
660 The ``bitmaps size`` is the additional size required in order to
661 copy bitmaps from a source image in addition to the guest-visible
662 data; the line is omitted if either source or destination lacks
663 bitmap support, or 0 if bitmaps are supported but there is nothing
666 .. option:: snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAPSHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME
668 List, apply, create or delete snapshots in image *FILENAME*.
670 .. option:: rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-p] [-u] [-c] -b BACKING_FILE [-F BACKING_FMT] FILENAME
672 Changes the backing file of an image. Only the formats ``qcow2`` and
673 ``qed`` support changing the backing file.
675 The backing file is changed to *BACKING_FILE* and (if the image format of
676 *FILENAME* supports this) the backing file format is changed to
677 *BACKING_FMT*. If *BACKING_FILE* is specified as "" (the empty
678 string), then the image is rebased onto no backing file (i.e. it will exist
679 independently of any backing file).
681 If a relative path name is given, the backing file is looked up relative to
682 the directory containing *FILENAME*.
684 *CACHE* specifies the cache mode to be used for *FILENAME*, whereas
685 *SRC_CACHE* specifies the cache mode for reading backing files.
687 There are two different modes in which ``rebase`` can operate:
690 This is the default mode and performs a real rebase operation. The
691 new backing file may differ from the old one and ``qemu-img rebase``
692 will take care of keeping the guest-visible content of *FILENAME*
695 In order to achieve this, any clusters that differ between
696 *BACKING_FILE* and the old backing file of *FILENAME* are merged
697 into *FILENAME* before actually changing the backing file. With the
698 ``-c`` option specified, the clusters which are being merged (but not
699 the entire *FILENAME* image) are compressed when written.
701 Note that the safe mode is an expensive operation, comparable to
702 converting an image. It only works if the old backing file still
706 ``qemu-img`` uses the unsafe mode if ``-u`` is specified. In this
707 mode, only the backing file name and format of *FILENAME* is changed
708 without any checks on the file contents. The user must take care of
709 specifying the correct new backing file, or the guest-visible
710 content of the image will be corrupted.
712 This mode is useful for renaming or moving the backing file to
713 somewhere else. It can be used without an accessible old backing
714 file, i.e. you can use it to fix an image whose backing file has
715 already been moved/renamed.
717 You can use ``rebase`` to perform a "diff" operation on two
718 disk images. This can be useful when you have copied or cloned
719 a guest, and you want to get back to a thin image on top of a
720 template or base image.
722 Say that ``base.img`` has been cloned as ``modified.img`` by
723 copying it, and that the ``modified.img`` guest has run so there
724 are now some changes compared to ``base.img``. To construct a thin
725 image called ``diff.qcow2`` that contains just the differences, do:
729 qemu-img create -f qcow2 -b modified.img diff.qcow2
730 qemu-img rebase -b base.img diff.qcow2
732 At this point, ``modified.img`` can be discarded, since
733 ``base.img + diff.qcow2`` contains the same information.
735 .. option:: resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--preallocation=PREALLOC] [-q] [--shrink] FILENAME [+ | -]SIZE
737 Change the disk image as if it had been created with *SIZE*.
739 Before using this command to shrink a disk image, you MUST use file system and
740 partitioning tools inside the VM to reduce allocated file systems and partition
741 sizes accordingly. Failure to do so will result in data loss!
743 When shrinking images, the ``--shrink`` option must be given. This informs
744 ``qemu-img`` that the user acknowledges all loss of data beyond the truncated
747 After using this command to grow a disk image, you must use file system and
748 partitioning tools inside the VM to actually begin using the new space on the
751 When growing an image, the ``--preallocation`` option may be used to specify
752 how the additional image area should be allocated on the host. See the format
753 description in the :ref:`notes` section which values are allowed. Using this
754 option may result in slightly more data being allocated than necessary.
761 Supported image file formats:
765 Raw disk image format (default). This format has the advantage of
766 being simple and easily exportable to all other emulators. If your
767 file system supports *holes* (for example in ext2 or ext3 on
768 Linux or NTFS on Windows), then only the written sectors will reserve
769 space. Use ``qemu-img info`` to know the real size used by the
770 image or ``ls -ls`` on Unix/Linux.
775 Preallocation mode (allowed values: ``off``, ``falloc``,
776 ``full``). ``falloc`` mode preallocates space for image by
777 calling ``posix_fallocate()``. ``full`` mode preallocates space
778 for image by writing data to underlying storage. This data may or
779 may not be zero, depending on the storage location.
783 QEMU image format, the most versatile format. Use it to have smaller
784 images (useful if your filesystem does not supports holes, for example
785 on Windows), optional AES encryption, zlib or zstd based compression and
786 support of multiple VM snapshots.
791 Determines the qcow2 version to use. ``compat=0.10`` uses the
792 traditional image format that can be read by any QEMU since 0.10.
793 ``compat=1.1`` enables image format extensions that only QEMU 1.1 and
794 newer understand (this is the default). Amongst others, this includes zero
795 clusters, which allow efficient copy-on-read for sparse images.
798 File name of a base image (see ``create`` subcommand)
801 Image format of the base image
804 This option configures which compression algorithm will be used for
805 compressed clusters on the image. Note that setting this option doesn't yet
806 cause the image to actually receive compressed writes. It is most commonly
807 used with the ``-c`` option of ``qemu-img convert``, but can also be used
808 with the ``compress`` filter driver or backup block jobs with compression
811 Valid values are ``zlib`` and ``zstd``. For images that use
812 ``compat=0.10``, only ``zlib`` compression is available.
815 If this option is set to ``on``, the image is encrypted with
818 The use of encryption in qcow and qcow2 images is considered to be
819 flawed by modern cryptography standards, suffering from a number
822 - The AES-CBC cipher is used with predictable initialization
823 vectors based on the sector number. This makes it vulnerable to
824 chosen plaintext attacks which can reveal the existence of
827 - The user passphrase is directly used as the encryption key. A
828 poorly chosen or short passphrase will compromise the security
831 - In the event of the passphrase being compromised there is no way
832 to change the passphrase to protect data in any qcow images. The
833 files must be cloned, using a different encryption passphrase in
834 the new file. The original file must then be securely erased
835 using a program like shred, though even this is ineffective with
836 many modern storage technologies.
838 - Initialization vectors used to encrypt sectors are based on the
839 guest virtual sector number, instead of the host physical
840 sector. When a disk image has multiple internal snapshots this
841 means that data in multiple physical sectors is encrypted with
842 the same initialization vector. With the CBC mode, this opens
843 the possibility of watermarking attacks if the attack can
844 collect multiple sectors encrypted with the same IV and some
845 predictable data. Having multiple qcow2 images with the same
846 passphrase also exposes this weakness since the passphrase is
847 directly used as the key.
849 Use of qcow / qcow2 encryption is thus strongly discouraged. Users are
850 recommended to use an alternative encryption technology such as the
851 Linux dm-crypt / LUKS system.
854 Changes the qcow2 cluster size (must be between 512 and
855 2M). Smaller cluster sizes can improve the image file size whereas
856 larger cluster sizes generally provide better performance.
859 Preallocation mode (allowed values: ``off``, ``metadata``,
860 ``falloc``, ``full``). An image with preallocated metadata is
861 initially larger but can improve performance when the image needs
862 to grow. ``falloc`` and ``full`` preallocations are like the same
863 options of ``raw`` format, but sets up metadata also.
866 If this option is set to ``on``, reference count updates are
867 postponed with the goal of avoiding metadata I/O and improving
868 performance. This is particularly interesting with
869 ``cache=writethrough`` which doesn't batch metadata
870 updates. The tradeoff is that after a host crash, the reference
871 count tables must be rebuilt, i.e. on the next open an (automatic)
872 ``qemu-img check -r all`` is required, which may take some time.
874 This option can only be enabled if ``compat=1.1`` is specified.
877 If this option is set to ``on``, it will turn off COW of the file. It's
878 only valid on btrfs, no effect on other file systems.
880 Btrfs has low performance when hosting a VM image file, even more
881 when the guest on the VM also using btrfs as file system. Turning
882 off COW is a way to mitigate this bad performance. Generally there
883 are two ways to turn off COW on btrfs:
885 - Disable it by mounting with nodatacow, then all newly created files
887 - For an empty file, add the NOCOW file attribute. That's what this
890 Note: this option is only valid to new or empty files. If there is
891 an existing file which is COW and has data blocks already, it
892 couldn't be changed to NOCOW by setting ``nocow=on``. One can
893 issue ``lsattr filename`` to check if the NOCOW flag is set or not
894 (Capital 'C' is NOCOW flag).
897 Filename where all guest data will be stored. If this option is used,
898 the qcow2 file will only contain the image's metadata.
900 Note: Data loss will occur if the given filename already exists when
901 using this option with ``qemu-img create`` since ``qemu-img`` will create
902 the data file anew, overwriting the file's original contents. To simply
903 update the reference to point to the given pre-existing file, use
907 If this option is set to ``on``, QEMU will always keep the external data
908 file consistent as a standalone read-only raw image.
910 It does this by forwarding all write accesses to the qcow2 file through to
911 the raw data file, including their offsets. Therefore, data that is visible
912 on the qcow2 node (i.e., to the guest) at some offset is visible at the same
913 offset in the raw data file. This results in a read-only raw image. Writes
914 that bypass the qcow2 metadata may corrupt the qcow2 metadata because the
915 out-of-band writes may result in the metadata falling out of sync with the
918 If this option is ``off``, QEMU will use the data file to store data in an
919 arbitrary manner. The file’s content will not make sense without the
920 accompanying qcow2 metadata. Where data is written will have no relation to
921 its offset as seen by the guest, and some writes (specifically zero writes)
922 may not be forwarded to the data file at all, but will only be handled by
923 modifying qcow2 metadata.
925 This option can only be enabled if ``data_file`` is set.
929 QEMU also supports various other image file formats for
930 compatibility with older QEMU versions or other hypervisors,
931 including VMDK, VDI, VHD (vpc), VHDX, qcow1 and QED. For a full list
932 of supported formats see ``qemu-img --help``. For a more detailed
933 description of these formats, see the QEMU block drivers reference
936 The main purpose of the block drivers for these formats is image
937 conversion. For running VMs, it is recommended to convert the disk
938 images to either raw or qcow2 in order to achieve good performance.