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 compare subcommand:
147 .. program:: qemu-img-compare
159 Strict mode - fail on different image size or sector allocation
161 Parameters to convert subcommand:
163 .. program:: qemu-img-convert
167 Skip the creation of the target volume
171 Number of parallel coroutines for the convert process
175 Allow out-of-order writes to the destination. This option improves performance,
176 but is only recommended for preallocated devices like host devices or other
181 Try to use copy offloading to move data from source image to target. This may
182 improve performance if the data is remote, such as with NFS or iSCSI backends,
183 but will not automatically sparsify zero sectors, and may result in a fully
184 allocated target image depending on the host support for getting allocation
187 .. option:: --salvage
189 Try to ignore I/O errors when reading. Unless in quiet mode (``-q``), errors
190 will still be printed. Areas that cannot be read from the source will be
191 treated as containing only zeroes.
193 .. option:: --target-is-zero
195 Assume that reading the destination image will always return
196 zeros. This parameter is mutually exclusive with a destination image
197 that has a backing file. It is required to also use the ``-n``
198 parameter to skip image creation.
200 Parameters to dd subcommand:
202 .. program:: qemu-img-dd
204 .. option:: bs=BLOCK_SIZE
206 Defines the block size
208 .. option:: count=BLOCKS
210 Sets the number of input blocks to copy
216 .. option:: of=OUTPUT
220 .. option:: skip=BLOCKS
222 Sets the number of input blocks to skip
224 Parameters to snapshot subcommand:
226 .. program:: qemu-img-snapshot
230 Is the name of the snapshot to create, apply or delete
234 Applies a snapshot (revert disk to saved state)
246 Lists all snapshots in the given image
250 .. program:: qemu-img-commands
252 .. option:: amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE] -o OPTIONS FILENAME
254 Amends the image format specific *OPTIONS* for the image file
255 *FILENAME*. Not all file formats support this operation.
257 .. 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
259 Run a simple sequential I/O benchmark on the specified image. If ``-w`` is
260 specified, a write test is performed, otherwise a read test is performed.
262 A total number of *COUNT* I/O requests is performed, each *BUFFER_SIZE*
263 bytes in size, and with *DEPTH* requests in parallel. The first request
264 starts at the position given by *OFFSET*, each following request increases
265 the current position by *STEP_SIZE*. If *STEP_SIZE* is not given,
266 *BUFFER_SIZE* is used for its value.
268 If *FLUSH_INTERVAL* is specified for a write test, the request queue is
269 drained and a flush is issued before new writes are made whenever the number of
270 remaining requests is a multiple of *FLUSH_INTERVAL*. If additionally
271 ``--no-drain`` is specified, a flush is issued without draining the request
274 if ``-i`` is specified, *AIO* option can be used to specify different
275 AIO backends: ``threads``, ``native`` or ``io_uring``.
277 If ``-n`` is specified, the native AIO backend is used if possible. On
278 Linux, this option only works if ``-t none`` or ``-t directsync`` is
281 For write tests, by default a buffer filled with zeros is written. This can be
282 overridden with a pattern byte specified by *PATTERN*.
284 .. 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
286 Perform one or more modifications of the persistent bitmap *BITMAP*
287 in the disk image *FILENAME*. The various modifications are:
289 ``--add`` to create *BITMAP*, enabled to record future edits.
291 ``--remove`` to remove *BITMAP*.
293 ``--clear`` to clear *BITMAP*.
295 ``--enable`` to change *BITMAP* to start recording future edits.
297 ``--disable`` to change *BITMAP* to stop recording future edits.
299 ``--merge`` to merge the contents of *SOURCE_BITMAP* into *BITMAP*.
301 Additional options include ``-g`` which sets a non-default
302 *GRANULARITY* for ``--add``, and ``-b`` and ``-F`` which select an
303 alternative source file for all *SOURCE* bitmaps used by
306 To see what bitmaps are present in an image, use ``qemu-img info``.
308 .. option:: check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT] [-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME
310 Perform a consistency check on the disk image *FILENAME*. The command can
311 output in the format *OFMT* which is either ``human`` or ``json``.
312 The JSON output is an object of QAPI type ``ImageCheck``.
314 If ``-r`` is specified, qemu-img tries to repair any inconsistencies found
315 during the check. ``-r leaks`` repairs only cluster leaks, whereas
316 ``-r all`` fixes all kinds of errors, with a higher risk of choosing the
317 wrong fix or hiding corruption that has already occurred.
319 Only the formats ``qcow2``, ``qed`` and ``vdi`` support
322 In case the image does not have any inconsistencies, check exits with ``0``.
323 Other exit codes indicate the kind of inconsistency found or if another error
324 occurred. The following table summarizes all exit codes of the check subcommand:
327 Check completed, the image is (now) consistent
329 Check not completed because of internal errors
331 Check completed, image is corrupted
333 Check completed, image has leaked clusters, but is not corrupted
335 Checks are not supported by the image format
337 If ``-r`` is specified, exit codes representing the image state refer to the
338 state after (the attempt at) repairing it. That is, a successful ``-r all``
339 will yield the exit code 0, independently of the image state before.
341 .. option:: commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b BASE] [-d] [-p] FILENAME
343 Commit the changes recorded in *FILENAME* in its base image or backing file.
344 If the backing file is smaller than the snapshot, then the backing file will be
345 resized to be the same size as the snapshot. If the snapshot is smaller than
346 the backing file, the backing file will not be truncated. If you want the
347 backing file to match the size of the smaller snapshot, you can safely truncate
348 it yourself once the commit operation successfully completes.
350 The image *FILENAME* is emptied after the operation has succeeded. If you do
351 not need *FILENAME* afterwards and intend to drop it, you may skip emptying
352 *FILENAME* by specifying the ``-d`` flag.
354 If the backing chain of the given image file *FILENAME* has more than one
355 layer, the backing file into which the changes will be committed may be
356 specified as *BASE* (which has to be part of *FILENAME*'s backing
357 chain). If *BASE* is not specified, the immediate backing file of the top
358 image (which is *FILENAME*) will be used. Note that after a commit operation
359 all images between *BASE* and the top image will be invalid and may return
360 garbage data when read. For this reason, ``-b`` implies ``-d`` (so that
361 the top image stays valid).
363 .. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 FILENAME2
365 Check if two images have the same content. You can compare images with
366 different format or settings.
368 The format is probed unless you specify it by ``-f`` (used for
369 *FILENAME1*) and/or ``-F`` (used for *FILENAME2*) option.
371 By default, images with different size are considered identical if the larger
372 image contains only unallocated and/or zeroed sectors in the area after the end
373 of the other image. In addition, if any sector is not allocated in one image
374 and contains only zero bytes in the second one, it is evaluated as equal. You
375 can use Strict mode by specifying the ``-s`` option. When compare runs in
376 Strict mode, it fails in case image size differs or a sector is allocated in
377 one image and is not allocated in the second one.
379 By default, compare prints out a result message. This message displays
380 information that both images are same or the position of the first different
381 byte. In addition, result message can report different image size in case
384 Compare exits with ``0`` in case the images are equal and with ``1``
385 in case the images differ. Other exit codes mean an error occurred during
386 execution and standard error output should contain an error message.
387 The following table sumarizes all exit codes of the compare subcommand:
394 Error on opening an image
396 Error on checking a sector allocation
398 Error on reading data
400 .. option:: convert [--object OBJECTDEF] [--image-opts] [--target-image-opts] [--target-is-zero] [-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
402 Convert the disk image *FILENAME* or a snapshot *SNAPSHOT_PARAM*
403 to disk image *OUTPUT_FILENAME* using format *OUTPUT_FMT*. It can
404 be optionally compressed (``-c`` option) or use any format specific
405 options like encryption (``-o`` option).
407 Only the formats ``qcow`` and ``qcow2`` support compression. The
408 compression is read-only. It means that if a compressed sector is
409 rewritten, then it is rewritten as uncompressed data.
411 Image conversion is also useful to get smaller image when using a
412 growable format such as ``qcow``: the empty sectors are detected and
413 suppressed from the destination image.
415 *SPARSE_SIZE* indicates the consecutive number of bytes (defaults to 4k)
416 that must contain only zeros for qemu-img to create a sparse image during
417 conversion. If *SPARSE_SIZE* is 0, the source will not be scanned for
418 unallocated or zero sectors, and the destination image will always be
421 You can use the *BACKING_FILE* option to force the output image to be
422 created as a copy on write image of the specified base image; the
423 *BACKING_FILE* should have the same content as the input's base image,
424 however the path, image format, etc may differ.
426 If a relative path name is given, the backing file is looked up relative to
427 the directory containing *OUTPUT_FILENAME*.
429 If the ``-n`` option is specified, the target volume creation will be
430 skipped. This is useful for formats such as ``rbd`` if the target
431 volume has already been created with site specific options that cannot
432 be supplied through qemu-img.
434 Out of order writes can be enabled with ``-W`` to improve performance.
435 This is only recommended for preallocated devices like host devices or other
436 raw block devices. Out of order write does not work in combination with
437 creating compressed images.
439 *NUM_COROUTINES* specifies how many coroutines work in parallel during
440 the convert process (defaults to 8).
442 .. option:: create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE] [-F BACKING_FMT] [-u] [-o OPTIONS] FILENAME [SIZE]
444 Create the new disk image *FILENAME* of size *SIZE* and format
445 *FMT*. Depending on the file format, you can add one or more *OPTIONS*
446 that enable additional features of this format.
448 If the option *BACKING_FILE* is specified, then the image will record
449 only the differences from *BACKING_FILE*. No size needs to be specified in
450 this case. *BACKING_FILE* will never be modified unless you use the
451 ``commit`` monitor command (or qemu-img commit).
453 If a relative path name is given, the backing file is looked up relative to
454 the directory containing *FILENAME*.
456 Note that a given backing file will be opened to check that it is valid. Use
457 the ``-u`` option to enable unsafe backing file mode, which means that the
458 image will be created even if the associated backing file cannot be opened. A
459 matching backing file must be created or additional options be used to make the
460 backing file specification valid when you want to use an image created this
463 The size can also be specified using the *SIZE* option with ``-o``,
464 it doesn't need to be specified separately in this case.
467 .. option:: dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE] [count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT
469 dd copies from *INPUT* file to *OUTPUT* file converting it from
470 *FMT* format to *OUTPUT_FMT* format.
472 The data is by default read and written using blocks of 512 bytes but can be
473 modified by specifying *BLOCK_SIZE*. If count=\ *BLOCKS* is specified
474 dd will stop reading input after reading *BLOCKS* input blocks.
476 The size syntax is similar to :manpage:`dd(1)`'s size syntax.
478 .. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [-U] FILENAME
480 Give information about the disk image *FILENAME*. Use it in
481 particular to know the size reserved on disk which can be different
482 from the displayed size. If VM snapshots are stored in the disk image,
483 they are displayed too.
485 If a disk image has a backing file chain, information about each disk image in
486 the chain can be recursively enumerated by using the option ``--backing-chain``.
488 For instance, if you have an image chain like:
492 base.qcow2 <- snap1.qcow2 <- snap2.qcow2
494 To enumerate information about each disk image in the above chain, starting from top to base, do:
498 qemu-img info --backing-chain snap2.qcow2
500 The command can output in the format *OFMT* which is either ``human`` or
501 ``json``. The JSON output is an object of QAPI type ``ImageInfo``; with
502 ``--backing-chain``, it is an array of ``ImageInfo`` objects.
504 ``--output=human`` reports the following information (for every image in the
514 The size of the guest disk
517 How much space the image file occupies on the host file system (may be
518 shown as 0 if this information is unavailable, e.g. because there is no
522 Cluster size of the image format, if applicable
525 Whether the image is encrypted (only present if so)
528 This is shown as ``no`` if the image is dirty and will have to be
529 auto-repaired the next time it is opened in qemu.
532 The backing file name, if present
534 *backing file format*
535 The format of the backing file, if the image enforces it
538 A list of all internal snapshots
540 *Format specific information*
541 Further information whose structure depends on the image format. This
542 section is a textual representation of the respective
543 ``ImageInfoSpecific*`` QAPI object (e.g. ``ImageInfoSpecificQCow2``
546 .. option:: map [--object OBJECTDEF] [--image-opts] [-f FMT] [--start-offset=OFFSET] [--max-length=LEN] [--output=OFMT] [-U] FILENAME
548 Dump the metadata of image *FILENAME* and its backing file chain.
549 In particular, this commands dumps the allocation state of every sector
550 of *FILENAME*, together with the topmost file that allocates it in
551 the backing file chain.
553 Two option formats are possible. The default format (``human``)
554 only dumps known-nonzero areas of the file. Known-zero parts of the
555 file are omitted altogether, and likewise for parts that are not allocated
556 throughout the chain. ``qemu-img`` output will identify a file
557 from where the data can be read, and the offset in the file. Each line
558 will include four fields, the first three of which are hexadecimal
559 numbers. For example the first line of:
563 Offset Length Mapped to File
564 0 0x20000 0x50000 /tmp/overlay.qcow2
565 0x100000 0x10000 0x95380000 /tmp/backing.qcow2
567 means that 0x20000 (131072) bytes starting at offset 0 in the image are
568 available in /tmp/overlay.qcow2 (opened in ``raw`` format) starting
569 at offset 0x50000 (327680). Data that is compressed, encrypted, or
570 otherwise not available in raw format will cause an error if ``human``
571 format is in use. Note that file names can include newlines, thus it is
572 not safe to parse this output format in scripts.
574 The alternative format ``json`` will return an array of dictionaries
575 in JSON format. It will include similar information in
576 the ``start``, ``length``, ``offset`` fields;
577 it will also include other more specific information:
579 - whether the sectors contain actual data or not (boolean field ``data``;
580 if false, the sectors are either unallocated or stored as optimized
582 - whether the data is known to read as zero (boolean field ``zero``);
583 - in order to make the output shorter, the target file is expressed as
584 a ``depth``; for example, a depth of 2 refers to the backing file
585 of the backing file of *FILENAME*.
587 In JSON format, the ``offset`` field is optional; it is absent in
588 cases where ``human`` format would omit the entry or exit with an error.
589 If ``data`` is false and the ``offset`` field is present, the
590 corresponding sectors in the file are not yet in use, but they are
593 For more information, consult ``include/block/block.h`` in QEMU's
596 .. option:: measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--object OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME]
598 Calculate the file size required for a new image. This information
599 can be used to size logical volumes or SAN LUNs appropriately for
600 the image that will be placed in them. The values reported are
601 guaranteed to be large enough to fit the image. The command can
602 output in the format *OFMT* which is either ``human`` or ``json``.
603 The JSON output is an object of QAPI type ``BlockMeasureInfo``.
605 If the size *N* is given then act as if creating a new empty image file
606 using ``qemu-img create``. If *FILENAME* is given then act as if
607 converting an existing image file using ``qemu-img convert``. The format
608 of the new file is given by *OUTPUT_FMT* while the format of an existing
609 file is given by *FMT*.
611 A snapshot in an existing image can be specified using *SNAPSHOT_PARAM*.
613 The following fields are reported:
617 required size: 524288
618 fully allocated size: 1074069504
620 The ``required size`` is the file size of the new image. It may be smaller
621 than the virtual disk size if the image format supports compact representation.
623 The ``fully allocated size`` is the file size of the new image once data has
624 been written to all sectors. This is the maximum size that the image file can
625 occupy with the exception of internal snapshots, dirty bitmaps, vmstate data,
626 and other advanced image format features.
628 .. option:: snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAPSHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME
630 List, apply, create or delete snapshots in image *FILENAME*.
632 .. option:: rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-p] [-u] -b BACKING_FILE [-F BACKING_FMT] FILENAME
634 Changes the backing file of an image. Only the formats ``qcow2`` and
635 ``qed`` support changing the backing file.
637 The backing file is changed to *BACKING_FILE* and (if the image format of
638 *FILENAME* supports this) the backing file format is changed to
639 *BACKING_FMT*. If *BACKING_FILE* is specified as "" (the empty
640 string), then the image is rebased onto no backing file (i.e. it will exist
641 independently of any backing file).
643 If a relative path name is given, the backing file is looked up relative to
644 the directory containing *FILENAME*.
646 *CACHE* specifies the cache mode to be used for *FILENAME*, whereas
647 *SRC_CACHE* specifies the cache mode for reading backing files.
649 There are two different modes in which ``rebase`` can operate:
652 This is the default mode and performs a real rebase operation. The
653 new backing file may differ from the old one and qemu-img rebase
654 will take care of keeping the guest-visible content of *FILENAME*
657 In order to achieve this, any clusters that differ between
658 *BACKING_FILE* and the old backing file of *FILENAME* are merged
659 into *FILENAME* before actually changing the backing file.
661 Note that the safe mode is an expensive operation, comparable to
662 converting an image. It only works if the old backing file still
666 qemu-img uses the unsafe mode if ``-u`` is specified. In this
667 mode, only the backing file name and format of *FILENAME* is changed
668 without any checks on the file contents. The user must take care of
669 specifying the correct new backing file, or the guest-visible
670 content of the image will be corrupted.
672 This mode is useful for renaming or moving the backing file to
673 somewhere else. It can be used without an accessible old backing
674 file, i.e. you can use it to fix an image whose backing file has
675 already been moved/renamed.
677 You can use ``rebase`` to perform a "diff" operation on two
678 disk images. This can be useful when you have copied or cloned
679 a guest, and you want to get back to a thin image on top of a
680 template or base image.
682 Say that ``base.img`` has been cloned as ``modified.img`` by
683 copying it, and that the ``modified.img`` guest has run so there
684 are now some changes compared to ``base.img``. To construct a thin
685 image called ``diff.qcow2`` that contains just the differences, do:
689 qemu-img create -f qcow2 -b modified.img diff.qcow2
690 qemu-img rebase -b base.img diff.qcow2
692 At this point, ``modified.img`` can be discarded, since
693 ``base.img + diff.qcow2`` contains the same information.
695 .. option:: resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--preallocation=PREALLOC] [-q] [--shrink] FILENAME [+ | -]SIZE
697 Change the disk image as if it had been created with *SIZE*.
699 Before using this command to shrink a disk image, you MUST use file system and
700 partitioning tools inside the VM to reduce allocated file systems and partition
701 sizes accordingly. Failure to do so will result in data loss!
703 When shrinking images, the ``--shrink`` option must be given. This informs
704 qemu-img that the user acknowledges all loss of data beyond the truncated
707 After using this command to grow a disk image, you must use file system and
708 partitioning tools inside the VM to actually begin using the new space on the
711 When growing an image, the ``--preallocation`` option may be used to specify
712 how the additional image area should be allocated on the host. See the format
713 description in the :ref:`notes` section which values are allowed. Using this
714 option may result in slightly more data being allocated than necessary.
721 Supported image file formats:
725 Raw disk image format (default). This format has the advantage of
726 being simple and easily exportable to all other emulators. If your
727 file system supports *holes* (for example in ext2 or ext3 on
728 Linux or NTFS on Windows), then only the written sectors will reserve
729 space. Use ``qemu-img info`` to know the real size used by the
730 image or ``ls -ls`` on Unix/Linux.
735 Preallocation mode (allowed values: ``off``, ``falloc``,
736 ``full``). ``falloc`` mode preallocates space for image by
737 calling ``posix_fallocate()``. ``full`` mode preallocates space
738 for image by writing data to underlying storage. This data may or
739 may not be zero, depending on the storage location.
743 QEMU image format, the most versatile format. Use it to have smaller
744 images (useful if your filesystem does not supports holes, for example
745 on Windows), optional AES encryption, zlib based compression and
746 support of multiple VM snapshots.
751 Determines the qcow2 version to use. ``compat=0.10`` uses the
752 traditional image format that can be read by any QEMU since 0.10.
753 ``compat=1.1`` enables image format extensions that only QEMU 1.1 and
754 newer understand (this is the default). Amongst others, this includes zero
755 clusters, which allow efficient copy-on-read for sparse images.
758 File name of a base image (see ``create`` subcommand)
761 Image format of the base image
764 If this option is set to ``on``, the image is encrypted with
767 The use of encryption in qcow and qcow2 images is considered to be
768 flawed by modern cryptography standards, suffering from a number
771 - The AES-CBC cipher is used with predictable initialization
772 vectors based on the sector number. This makes it vulnerable to
773 chosen plaintext attacks which can reveal the existence of
776 - The user passphrase is directly used as the encryption key. A
777 poorly chosen or short passphrase will compromise the security
780 - In the event of the passphrase being compromised there is no way
781 to change the passphrase to protect data in any qcow images. The
782 files must be cloned, using a different encryption passphrase in
783 the new file. The original file must then be securely erased
784 using a program like shred, though even this is ineffective with
785 many modern storage technologies.
787 - Initialization vectors used to encrypt sectors are based on the
788 guest virtual sector number, instead of the host physical
789 sector. When a disk image has multiple internal snapshots this
790 means that data in multiple physical sectors is encrypted with
791 the same initialization vector. With the CBC mode, this opens
792 the possibility of watermarking attacks if the attack can
793 collect multiple sectors encrypted with the same IV and some
794 predictable data. Having multiple qcow2 images with the same
795 passphrase also exposes this weakness since the passphrase is
796 directly used as the key.
798 Use of qcow / qcow2 encryption is thus strongly discouraged. Users are
799 recommended to use an alternative encryption technology such as the
800 Linux dm-crypt / LUKS system.
803 Changes the qcow2 cluster size (must be between 512 and
804 2M). Smaller cluster sizes can improve the image file size whereas
805 larger cluster sizes generally provide better performance.
808 Preallocation mode (allowed values: ``off``, ``metadata``,
809 ``falloc``, ``full``). An image with preallocated metadata is
810 initially larger but can improve performance when the image needs
811 to grow. ``falloc`` and ``full`` preallocations are like the same
812 options of ``raw`` format, but sets up metadata also.
815 If this option is set to ``on``, reference count updates are
816 postponed with the goal of avoiding metadata I/O and improving
817 performance. This is particularly interesting with
818 ``cache=writethrough`` which doesn't batch metadata
819 updates. The tradeoff is that after a host crash, the reference
820 count tables must be rebuilt, i.e. on the next open an (automatic)
821 ``qemu-img check -r all`` is required, which may take some time.
823 This option can only be enabled if ``compat=1.1`` is specified.
826 If this option is set to ``on``, it will turn off COW of the file. It's
827 only valid on btrfs, no effect on other file systems.
829 Btrfs has low performance when hosting a VM image file, even more
830 when the guest on the VM also using btrfs as file system. Turning
831 off COW is a way to mitigate this bad performance. Generally there
832 are two ways to turn off COW on btrfs:
834 - Disable it by mounting with nodatacow, then all newly created files
836 - For an empty file, add the NOCOW file attribute. That's what this
839 Note: this option is only valid to new or empty files. If there is
840 an existing file which is COW and has data blocks already, it
841 couldn't be changed to NOCOW by setting ``nocow=on``. One can
842 issue ``lsattr filename`` to check if the NOCOW flag is set or not
843 (Capital 'C' is NOCOW flag).
847 QEMU also supports various other image file formats for
848 compatibility with older QEMU versions or other hypervisors,
849 including VMDK, VDI, VHD (vpc), VHDX, qcow1 and QED. For a full list
850 of supported formats see ``qemu-img --help``. For a more detailed
851 description of these formats, see the QEMU block drivers reference
854 The main purpose of the block drivers for these formats is image
855 conversion. For running VMs, it is recommended to convert the disk
856 images to either raw or qcow2 in order to achieve good performance.