1 QEMU disk image utility
2 =======================
7 **qemu-img** [*standard options*] *command* [*command options*]
12 qemu-img allows you to create, convert and modify images offline. It can handle
13 all image formats supported by QEMU.
15 **Warning:** Never use qemu-img to modify images in use by a running virtual
16 machine or any other process; this may destroy the image. Also, be aware that
17 querying an image that is being modified by another process may encounter
27 .. option:: -h, --help
29 Display this help and exit
31 .. option:: -V, --version
33 Display version information and exit
35 .. option:: -T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE]
37 .. include:: qemu-option-trace.rst.inc
39 The following commands are supported:
41 .. hxtool-doc:: qemu-img-cmds.hx
45 *FILENAME* is a disk image filename.
47 *FMT* is the disk image format. It is guessed automatically in most
48 cases. See below for a description of the supported disk formats.
50 *SIZE* is the disk image size in bytes. Optional suffixes ``k`` or
51 ``K`` (kilobyte, 1024) ``M`` (megabyte, 1024k) and ``G`` (gigabyte,
52 1024M) and T (terabyte, 1024G) are supported. ``b`` is ignored.
54 *OUTPUT_FILENAME* is the destination disk image filename.
56 *OUTPUT_FMT* is the destination format.
58 *OPTIONS* is a comma separated list of format specific options in a
59 name=value format. Use ``-o ?`` for an overview of the options supported
60 by the used format or see the format descriptions below for details.
62 *SNAPSHOT_PARAM* is param used for internal snapshot, format is
63 'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'.
66 Note the use of a new 'program'; otherwise Sphinx complains about
67 the -h option appearing both in the above option list and this one.
69 .. program:: qemu-img-common-opts
71 .. option:: --object OBJECTDEF
73 is a QEMU user creatable object definition. See the :manpage:`qemu(1)`
74 manual page for a description of the object properties. The most common
75 object type is a ``secret``, which is used to supply passwords and/or
78 .. option:: --image-opts
80 Indicates that the source *FILENAME* parameter is to be interpreted as a
81 full option string, not a plain filename. This parameter is mutually
82 exclusive with the *-f* parameter.
84 .. option:: --target-image-opts
86 Indicates that the OUTPUT_FILENAME parameter(s) are to be interpreted as
87 a full option string, not a plain filename. This parameter is mutually
88 exclusive with the *-O* parameters. It is currently required to also use
89 the *-n* parameter to skip image creation. This restriction may be relaxed
92 .. option:: --force-share (-U)
94 If specified, ``qemu-img`` will open the image in shared mode, allowing
95 other QEMU processes to open it in write mode. For example, this can be used to
96 get the image information (with 'info' subcommand) when the image is used by a
97 running guest. Note that this could produce inconsistent results because of
98 concurrent metadata changes, etc. This option is only allowed when opening
99 images in read-only mode.
101 .. option:: --backing-chain
103 Will enumerate information about backing files in a disk image chain. Refer
104 below for further description.
108 Indicates that target image must be compressed (qcow format only).
112 With or without a command, shows help and lists the supported formats.
116 Display progress bar (compare, convert and rebase commands only).
117 If the *-p* option is not used for a command that supports it, the
118 progress is reported when the process receives a ``SIGUSR1`` or
123 Quiet mode - do not print any output (except errors). There's no progress bar
124 in case both *-q* and *-p* options are used.
128 Indicates the consecutive number of bytes that must contain only zeros
129 for qemu-img to create a sparse image during conversion. This value is rounded
130 down to the nearest 512 bytes. You may use the common size suffixes like
135 Specifies the cache mode that should be used with the (destination) file. See
136 the documentation of the emulator's ``-drive cache=...`` option for allowed
139 .. option:: -T SRC_CACHE
141 Specifies the cache mode that should be used with the source file(s). See
142 the documentation of the emulator's ``-drive cache=...`` option for allowed
145 Parameters to snapshot subcommand:
147 .. program:: qemu-img-snapshot
151 Is the name of the snapshot to create, apply or delete
155 Applies a snapshot (revert disk to saved state)
167 Lists all snapshots in the given image
169 Parameters to compare subcommand:
171 .. program:: qemu-img-compare
183 Strict mode - fail on different image size or sector allocation
185 Parameters to convert subcommand:
187 .. program:: qemu-img-convert
191 Skip the creation of the target volume
195 Number of parallel coroutines for the convert process
199 Allow out-of-order writes to the destination. This option improves performance,
200 but is only recommended for preallocated devices like host devices or other
205 Try to use copy offloading to move data from source image to target. This may
206 improve performance if the data is remote, such as with NFS or iSCSI backends,
207 but will not automatically sparsify zero sectors, and may result in a fully
208 allocated target image depending on the host support for getting allocation
211 .. option:: --salvage
213 Try to ignore I/O errors when reading. Unless in quiet mode (``-q``), errors
214 will still be printed. Areas that cannot be read from the source will be
215 treated as containing only zeroes.
217 Parameters to dd subcommand:
219 .. program:: qemu-img-dd
221 .. option:: bs=BLOCK_SIZE
223 Defines the block size
225 .. option:: count=BLOCKS
227 Sets the number of input blocks to copy
233 .. option:: of=OUTPUT
237 .. option:: skip=BLOCKS
239 Sets the number of input blocks to skip
243 .. program:: qemu-img-commands
245 .. option:: amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE] -o OPTIONS FILENAME
247 Amends the image format specific *OPTIONS* for the image file
248 *FILENAME*. Not all file formats support this operation.
250 .. option:: bench [-c COUNT] [-d DEPTH] [-f FMT] [--flush-interval=FLUSH_INTERVAL] [-n] [-i AIO] [--no-drain] [-o OFFSET] [--pattern=PATTERN] [-q] [-s BUFFER_SIZE] [-S STEP_SIZE] [-t CACHE] [-w] [-U] FILENAME
252 Run a simple sequential I/O benchmark on the specified image. If ``-w`` is
253 specified, a write test is performed, otherwise a read test is performed.
255 A total number of *COUNT* I/O requests is performed, each *BUFFER_SIZE*
256 bytes in size, and with *DEPTH* requests in parallel. The first request
257 starts at the position given by *OFFSET*, each following request increases
258 the current position by *STEP_SIZE*. If *STEP_SIZE* is not given,
259 *BUFFER_SIZE* is used for its value.
261 If *FLUSH_INTERVAL* is specified for a write test, the request queue is
262 drained and a flush is issued before new writes are made whenever the number of
263 remaining requests is a multiple of *FLUSH_INTERVAL*. If additionally
264 ``--no-drain`` is specified, a flush is issued without draining the request
267 If ``-n`` is specified, the native AIO backend is used if possible. On
268 Linux, this option only works if ``-t none`` or ``-t directsync`` is
271 if ``-i`` is specified, *AIO* option can be used to specify different
272 AIO backends: ``threads``, ``native`` or ``io_uring``.
274 For write tests, by default a buffer filled with zeros is written. This can be
275 overridden with a pattern byte specified by *PATTERN*.
277 .. option:: check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT] [-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME
279 Perform a consistency check on the disk image *FILENAME*. The command can
280 output in the format *OFMT* which is either ``human`` or ``json``.
281 The JSON output is an object of QAPI type ``ImageCheck``.
283 If ``-r`` is specified, qemu-img tries to repair any inconsistencies found
284 during the check. ``-r leaks`` repairs only cluster leaks, whereas
285 ``-r all`` fixes all kinds of errors, with a higher risk of choosing the
286 wrong fix or hiding corruption that has already occurred.
288 Only the formats ``qcow2``, ``qed`` and ``vdi`` support
291 In case the image does not have any inconsistencies, check exits with ``0``.
292 Other exit codes indicate the kind of inconsistency found or if another error
293 occurred. The following table summarizes all exit codes of the check subcommand:
296 Check completed, the image is (now) consistent
298 Check not completed because of internal errors
300 Check completed, image is corrupted
302 Check completed, image has leaked clusters, but is not corrupted
304 Checks are not supported by the image format
306 If ``-r`` is specified, exit codes representing the image state refer to the
307 state after (the attempt at) repairing it. That is, a successful ``-r all``
308 will yield the exit code 0, independently of the image state before.
310 .. option:: commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b BASE] [-d] [-p] FILENAME
312 Commit the changes recorded in *FILENAME* in its base image or backing file.
313 If the backing file is smaller than the snapshot, then the backing file will be
314 resized to be the same size as the snapshot. If the snapshot is smaller than
315 the backing file, the backing file will not be truncated. If you want the
316 backing file to match the size of the smaller snapshot, you can safely truncate
317 it yourself once the commit operation successfully completes.
319 The image *FILENAME* is emptied after the operation has succeeded. If you do
320 not need *FILENAME* afterwards and intend to drop it, you may skip emptying
321 *FILENAME* by specifying the ``-d`` flag.
323 If the backing chain of the given image file *FILENAME* has more than one
324 layer, the backing file into which the changes will be committed may be
325 specified as *BASE* (which has to be part of *FILENAME*'s backing
326 chain). If *BASE* is not specified, the immediate backing file of the top
327 image (which is *FILENAME*) will be used. Note that after a commit operation
328 all images between *BASE* and the top image will be invalid and may return
329 garbage data when read. For this reason, ``-b`` implies ``-d`` (so that
330 the top image stays valid).
332 .. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 FILENAME2
334 Check if two images have the same content. You can compare images with
335 different format or settings.
337 The format is probed unless you specify it by ``-f`` (used for
338 *FILENAME1*) and/or ``-F`` (used for *FILENAME2*) option.
340 By default, images with different size are considered identical if the larger
341 image contains only unallocated and/or zeroed sectors in the area after the end
342 of the other image. In addition, if any sector is not allocated in one image
343 and contains only zero bytes in the second one, it is evaluated as equal. You
344 can use Strict mode by specifying the ``-s`` option. When compare runs in
345 Strict mode, it fails in case image size differs or a sector is allocated in
346 one image and is not allocated in the second one.
348 By default, compare prints out a result message. This message displays
349 information that both images are same or the position of the first different
350 byte. In addition, result message can report different image size in case
353 Compare exits with ``0`` in case the images are equal and with ``1``
354 in case the images differ. Other exit codes mean an error occurred during
355 execution and standard error output should contain an error message.
356 The following table sumarizes all exit codes of the compare subcommand:
363 Error on opening an image
365 Error on checking a sector allocation
367 Error on reading data
369 .. option:: convert [--object OBJECTDEF] [--image-opts] [--target-image-opts] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-O OUTPUT_FMT] [-B BACKING_FILE] [-o OPTIONS] [-l SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-m NUM_COROUTINES] [-W] FILENAME [FILENAME2 [...]] OUTPUT_FILENAME
371 Convert the disk image *FILENAME* or a snapshot *SNAPSHOT_PARAM*
372 to disk image *OUTPUT_FILENAME* using format *OUTPUT_FMT*. It can
373 be optionally compressed (``-c`` option) or use any format specific
374 options like encryption (``-o`` option).
376 Only the formats ``qcow`` and ``qcow2`` support compression. The
377 compression is read-only. It means that if a compressed sector is
378 rewritten, then it is rewritten as uncompressed data.
380 Image conversion is also useful to get smaller image when using a
381 growable format such as ``qcow``: the empty sectors are detected and
382 suppressed from the destination image.
384 *SPARSE_SIZE* indicates the consecutive number of bytes (defaults to 4k)
385 that must contain only zeros for qemu-img to create a sparse image during
386 conversion. If *SPARSE_SIZE* is 0, the source will not be scanned for
387 unallocated or zero sectors, and the destination image will always be
390 You can use the *BACKING_FILE* option to force the output image to be
391 created as a copy on write image of the specified base image; the
392 *BACKING_FILE* should have the same content as the input's base image,
393 however the path, image format, etc may differ.
395 If a relative path name is given, the backing file is looked up relative to
396 the directory containing *OUTPUT_FILENAME*.
398 If the ``-n`` option is specified, the target volume creation will be
399 skipped. This is useful for formats such as ``rbd`` if the target
400 volume has already been created with site specific options that cannot
401 be supplied through qemu-img.
403 Out of order writes can be enabled with ``-W`` to improve performance.
404 This is only recommended for preallocated devices like host devices or other
405 raw block devices. Out of order write does not work in combination with
406 creating compressed images.
408 *NUM_COROUTINES* specifies how many coroutines work in parallel during
409 the convert process (defaults to 8).
411 .. option:: create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE] [-F BACKING_FMT] [-u] [-o OPTIONS] FILENAME [SIZE]
413 Create the new disk image *FILENAME* of size *SIZE* and format
414 *FMT*. Depending on the file format, you can add one or more *OPTIONS*
415 that enable additional features of this format.
417 If the option *BACKING_FILE* is specified, then the image will record
418 only the differences from *BACKING_FILE*. No size needs to be specified in
419 this case. *BACKING_FILE* will never be modified unless you use the
420 ``commit`` monitor command (or qemu-img commit).
422 If a relative path name is given, the backing file is looked up relative to
423 the directory containing *FILENAME*.
425 Note that a given backing file will be opened to check that it is valid. Use
426 the ``-u`` option to enable unsafe backing file mode, which means that the
427 image will be created even if the associated backing file cannot be opened. A
428 matching backing file must be created or additional options be used to make the
429 backing file specification valid when you want to use an image created this
432 The size can also be specified using the *SIZE* option with ``-o``,
433 it doesn't need to be specified separately in this case.
436 .. option:: dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE] [count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT
438 dd copies from *INPUT* file to *OUTPUT* file converting it from
439 *FMT* format to *OUTPUT_FMT* format.
441 The data is by default read and written using blocks of 512 bytes but can be
442 modified by specifying *BLOCK_SIZE*. If count=\ *BLOCKS* is specified
443 dd will stop reading input after reading *BLOCKS* input blocks.
445 The size syntax is similar to :manpage:`dd(1)`'s size syntax.
447 .. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [-U] FILENAME
449 Give information about the disk image *FILENAME*. Use it in
450 particular to know the size reserved on disk which can be different
451 from the displayed size. If VM snapshots are stored in the disk image,
452 they are displayed too.
454 If a disk image has a backing file chain, information about each disk image in
455 the chain can be recursively enumerated by using the option ``--backing-chain``.
457 For instance, if you have an image chain like:
461 base.qcow2 <- snap1.qcow2 <- snap2.qcow2
463 To enumerate information about each disk image in the above chain, starting from top to base, do:
467 qemu-img info --backing-chain snap2.qcow2
469 The command can output in the format *OFMT* which is either ``human`` or
470 ``json``. The JSON output is an object of QAPI type ``ImageInfo``; with
471 ``--backing-chain``, it is an array of ``ImageInfo`` objects.
473 ``--output=human`` reports the following information (for every image in the
483 The size of the guest disk
486 How much space the image file occupies on the host file system (may be
487 shown as 0 if this information is unavailable, e.g. because there is no
491 Cluster size of the image format, if applicable
494 Whether the image is encrypted (only present if so)
497 This is shown as ``no`` if the image is dirty and will have to be
498 auto-repaired the next time it is opened in qemu.
501 The backing file name, if present
503 *backing file format*
504 The format of the backing file, if the image enforces it
507 A list of all internal snapshots
509 *Format specific information*
510 Further information whose structure depends on the image format. This
511 section is a textual representation of the respective
512 ``ImageInfoSpecific*`` QAPI object (e.g. ``ImageInfoSpecificQCow2``
515 .. option:: map [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [-U] FILENAME
517 Dump the metadata of image *FILENAME* and its backing file chain.
518 In particular, this commands dumps the allocation state of every sector
519 of *FILENAME*, together with the topmost file that allocates it in
520 the backing file chain.
522 Two option formats are possible. The default format (``human``)
523 only dumps known-nonzero areas of the file. Known-zero parts of the
524 file are omitted altogether, and likewise for parts that are not allocated
525 throughout the chain. ``qemu-img`` output will identify a file
526 from where the data can be read, and the offset in the file. Each line
527 will include four fields, the first three of which are hexadecimal
528 numbers. For example the first line of:
532 Offset Length Mapped to File
533 0 0x20000 0x50000 /tmp/overlay.qcow2
534 0x100000 0x10000 0x95380000 /tmp/backing.qcow2
536 means that 0x20000 (131072) bytes starting at offset 0 in the image are
537 available in /tmp/overlay.qcow2 (opened in ``raw`` format) starting
538 at offset 0x50000 (327680). Data that is compressed, encrypted, or
539 otherwise not available in raw format will cause an error if ``human``
540 format is in use. Note that file names can include newlines, thus it is
541 not safe to parse this output format in scripts.
543 The alternative format ``json`` will return an array of dictionaries
544 in JSON format. It will include similar information in
545 the ``start``, ``length``, ``offset`` fields;
546 it will also include other more specific information:
548 - whether the sectors contain actual data or not (boolean field ``data``;
549 if false, the sectors are either unallocated or stored as optimized
551 - whether the data is known to read as zero (boolean field ``zero``);
552 - in order to make the output shorter, the target file is expressed as
553 a ``depth``; for example, a depth of 2 refers to the backing file
554 of the backing file of *FILENAME*.
556 In JSON format, the ``offset`` field is optional; it is absent in
557 cases where ``human`` format would omit the entry or exit with an error.
558 If ``data`` is false and the ``offset`` field is present, the
559 corresponding sectors in the file are not yet in use, but they are
562 For more information, consult ``include/block/block.h`` in QEMU's
565 .. option:: measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--object OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME]
567 Calculate the file size required for a new image. This information
568 can be used to size logical volumes or SAN LUNs appropriately for
569 the image that will be placed in them. The values reported are
570 guaranteed to be large enough to fit the image. The command can
571 output in the format *OFMT* which is either ``human`` or ``json``.
572 The JSON output is an object of QAPI type ``BlockMeasureInfo``.
574 If the size *N* is given then act as if creating a new empty image file
575 using ``qemu-img create``. If *FILENAME* is given then act as if
576 converting an existing image file using ``qemu-img convert``. The format
577 of the new file is given by *OUTPUT_FMT* while the format of an existing
578 file is given by *FMT*.
580 A snapshot in an existing image can be specified using *SNAPSHOT_PARAM*.
582 The following fields are reported:
586 required size: 524288
587 fully allocated size: 1074069504
589 The ``required size`` is the file size of the new image. It may be smaller
590 than the virtual disk size if the image format supports compact representation.
592 The ``fully allocated size`` is the file size of the new image once data has
593 been written to all sectors. This is the maximum size that the image file can
594 occupy with the exception of internal snapshots, dirty bitmaps, vmstate data,
595 and other advanced image format features.
597 .. option:: snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAPSHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME
599 List, apply, create or delete snapshots in image *FILENAME*.
601 .. option:: rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-p] [-u] -b BACKING_FILE [-F BACKING_FMT] FILENAME
603 Changes the backing file of an image. Only the formats ``qcow2`` and
604 ``qed`` support changing the backing file.
606 The backing file is changed to *BACKING_FILE* and (if the image format of
607 *FILENAME* supports this) the backing file format is changed to
608 *BACKING_FMT*. If *BACKING_FILE* is specified as "" (the empty
609 string), then the image is rebased onto no backing file (i.e. it will exist
610 independently of any backing file).
612 If a relative path name is given, the backing file is looked up relative to
613 the directory containing *FILENAME*.
615 *CACHE* specifies the cache mode to be used for *FILENAME*, whereas
616 *SRC_CACHE* specifies the cache mode for reading backing files.
618 There are two different modes in which ``rebase`` can operate:
621 This is the default mode and performs a real rebase operation. The
622 new backing file may differ from the old one and qemu-img rebase
623 will take care of keeping the guest-visible content of *FILENAME*
626 In order to achieve this, any clusters that differ between
627 *BACKING_FILE* and the old backing file of *FILENAME* are merged
628 into *FILENAME* before actually changing the backing file.
630 Note that the safe mode is an expensive operation, comparable to
631 converting an image. It only works if the old backing file still
635 qemu-img uses the unsafe mode if ``-u`` is specified. In this
636 mode, only the backing file name and format of *FILENAME* is changed
637 without any checks on the file contents. The user must take care of
638 specifying the correct new backing file, or the guest-visible
639 content of the image will be corrupted.
641 This mode is useful for renaming or moving the backing file to
642 somewhere else. It can be used without an accessible old backing
643 file, i.e. you can use it to fix an image whose backing file has
644 already been moved/renamed.
646 You can use ``rebase`` to perform a "diff" operation on two
647 disk images. This can be useful when you have copied or cloned
648 a guest, and you want to get back to a thin image on top of a
649 template or base image.
651 Say that ``base.img`` has been cloned as ``modified.img`` by
652 copying it, and that the ``modified.img`` guest has run so there
653 are now some changes compared to ``base.img``. To construct a thin
654 image called ``diff.qcow2`` that contains just the differences, do:
658 qemu-img create -f qcow2 -b modified.img diff.qcow2
659 qemu-img rebase -b base.img diff.qcow2
661 At this point, ``modified.img`` can be discarded, since
662 ``base.img + diff.qcow2`` contains the same information.
664 .. option:: resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--preallocation=PREALLOC] [-q] [--shrink] FILENAME [+ | -]SIZE
666 Change the disk image as if it had been created with *SIZE*.
668 Before using this command to shrink a disk image, you MUST use file system and
669 partitioning tools inside the VM to reduce allocated file systems and partition
670 sizes accordingly. Failure to do so will result in data loss!
672 When shrinking images, the ``--shrink`` option must be given. This informs
673 qemu-img that the user acknowledges all loss of data beyond the truncated
676 After using this command to grow a disk image, you must use file system and
677 partitioning tools inside the VM to actually begin using the new space on the
680 When growing an image, the ``--preallocation`` option may be used to specify
681 how the additional image area should be allocated on the host. See the format
682 description in the :ref:`notes` section which values are allowed. Using this
683 option may result in slightly more data being allocated than necessary.
690 Supported image file formats:
694 Raw disk image format (default). This format has the advantage of
695 being simple and easily exportable to all other emulators. If your
696 file system supports *holes* (for example in ext2 or ext3 on
697 Linux or NTFS on Windows), then only the written sectors will reserve
698 space. Use ``qemu-img info`` to know the real size used by the
699 image or ``ls -ls`` on Unix/Linux.
704 Preallocation mode (allowed values: ``off``, ``falloc``,
705 ``full``). ``falloc`` mode preallocates space for image by
706 calling ``posix_fallocate()``. ``full`` mode preallocates space
707 for image by writing data to underlying storage. This data may or
708 may not be zero, depending on the storage location.
712 QEMU image format, the most versatile format. Use it to have smaller
713 images (useful if your filesystem does not supports holes, for example
714 on Windows), optional AES encryption, zlib based compression and
715 support of multiple VM snapshots.
720 Determines the qcow2 version to use. ``compat=0.10`` uses the
721 traditional image format that can be read by any QEMU since 0.10.
722 ``compat=1.1`` enables image format extensions that only QEMU 1.1 and
723 newer understand (this is the default). Amongst others, this includes zero
724 clusters, which allow efficient copy-on-read for sparse images.
727 File name of a base image (see ``create`` subcommand)
730 Image format of the base image
733 If this option is set to ``on``, the image is encrypted with
736 The use of encryption in qcow and qcow2 images is considered to be
737 flawed by modern cryptography standards, suffering from a number
740 - The AES-CBC cipher is used with predictable initialization
741 vectors based on the sector number. This makes it vulnerable to
742 chosen plaintext attacks which can reveal the existence of
745 - The user passphrase is directly used as the encryption key. A
746 poorly chosen or short passphrase will compromise the security
749 - In the event of the passphrase being compromised there is no way
750 to change the passphrase to protect data in any qcow images. The
751 files must be cloned, using a different encryption passphrase in
752 the new file. The original file must then be securely erased
753 using a program like shred, though even this is ineffective with
754 many modern storage technologies.
756 - Initialization vectors used to encrypt sectors are based on the
757 guest virtual sector number, instead of the host physical
758 sector. When a disk image has multiple internal snapshots this
759 means that data in multiple physical sectors is encrypted with
760 the same initialization vector. With the CBC mode, this opens
761 the possibility of watermarking attacks if the attack can
762 collect multiple sectors encrypted with the same IV and some
763 predictable data. Having multiple qcow2 images with the same
764 passphrase also exposes this weakness since the passphrase is
765 directly used as the key.
767 Use of qcow / qcow2 encryption is thus strongly discouraged. Users are
768 recommended to use an alternative encryption technology such as the
769 Linux dm-crypt / LUKS system.
772 Changes the qcow2 cluster size (must be between 512 and
773 2M). Smaller cluster sizes can improve the image file size whereas
774 larger cluster sizes generally provide better performance.
777 Preallocation mode (allowed values: ``off``, ``metadata``,
778 ``falloc``, ``full``). An image with preallocated metadata is
779 initially larger but can improve performance when the image needs
780 to grow. ``falloc`` and ``full`` preallocations are like the same
781 options of ``raw`` format, but sets up metadata also.
784 If this option is set to ``on``, reference count updates are
785 postponed with the goal of avoiding metadata I/O and improving
786 performance. This is particularly interesting with
787 ``cache=writethrough`` which doesn't batch metadata
788 updates. The tradeoff is that after a host crash, the reference
789 count tables must be rebuilt, i.e. on the next open an (automatic)
790 ``qemu-img check -r all`` is required, which may take some time.
792 This option can only be enabled if ``compat=1.1`` is specified.
795 If this option is set to ``on``, it will turn off COW of the file. It's
796 only valid on btrfs, no effect on other file systems.
798 Btrfs has low performance when hosting a VM image file, even more
799 when the guest on the VM also using btrfs as file system. Turning
800 off COW is a way to mitigate this bad performance. Generally there
801 are two ways to turn off COW on btrfs:
803 - Disable it by mounting with nodatacow, then all newly created files
805 - For an empty file, add the NOCOW file attribute. That's what this
808 Note: this option is only valid to new or empty files. If there is
809 an existing file which is COW and has data blocks already, it
810 couldn't be changed to NOCOW by setting ``nocow=on``. One can
811 issue ``lsattr filename`` to check if the NOCOW flag is set or not
812 (Capital 'C' is NOCOW flag).
816 QEMU also supports various other image file formats for
817 compatibility with older QEMU versions or other hypervisors,
818 including VMDK, VDI, VHD (vpc), VHDX, qcow1 and QED. For a full list
819 of supported formats see ``qemu-img --help``. For a more detailed
820 description of these formats, see the QEMU block drivers reference
823 The main purpose of the block drivers for these formats is image
824 conversion. For running VMs, it is recommended to convert the disk
825 images to either raw or qcow2 in order to achieve good performance.