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
165 .. option:: --bitmaps
167 Additionally copy all persistent bitmaps from the top layer of the source
171 Skip the creation of the target volume
175 Number of parallel coroutines for the convert process
179 Allow out-of-order writes to the destination. This option improves performance,
180 but is only recommended for preallocated devices like host devices or other
185 Try to use copy offloading to move data from source image to target. This may
186 improve performance if the data is remote, such as with NFS or iSCSI backends,
187 but will not automatically sparsify zero sectors, and may result in a fully
188 allocated target image depending on the host support for getting allocation
191 .. option:: --salvage
193 Try to ignore I/O errors when reading. Unless in quiet mode (``-q``), errors
194 will still be printed. Areas that cannot be read from the source will be
195 treated as containing only zeroes.
197 .. option:: --target-is-zero
199 Assume that reading the destination image will always return
200 zeros. This parameter is mutually exclusive with a destination image
201 that has a backing file. It is required to also use the ``-n``
202 parameter to skip image creation.
204 Parameters to dd subcommand:
206 .. program:: qemu-img-dd
208 .. option:: bs=BLOCK_SIZE
210 Defines the block size
212 .. option:: count=BLOCKS
214 Sets the number of input blocks to copy
220 .. option:: of=OUTPUT
224 .. option:: skip=BLOCKS
226 Sets the number of input blocks to skip
228 Parameters to snapshot subcommand:
230 .. program:: qemu-img-snapshot
234 Is the name of the snapshot to create, apply or delete
238 Applies a snapshot (revert disk to saved state)
250 Lists all snapshots in the given image
254 .. program:: qemu-img-commands
256 .. option:: amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE] [--force] -o OPTIONS FILENAME
258 Amends the image format specific *OPTIONS* for the image file
259 *FILENAME*. Not all file formats support this operation.
261 The set of options that can be amended are dependent on the image
262 format, but note that amending the backing chain relationship should
263 instead be performed with ``qemu-img rebase``.
265 --force allows some unsafe operations. Currently for -f luks, it allows to
266 erase the last encryption key, and to overwrite an active encryption key.
268 .. 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
270 Run a simple sequential I/O benchmark on the specified image. If ``-w`` is
271 specified, a write test is performed, otherwise a read test is performed.
273 A total number of *COUNT* I/O requests is performed, each *BUFFER_SIZE*
274 bytes in size, and with *DEPTH* requests in parallel. The first request
275 starts at the position given by *OFFSET*, each following request increases
276 the current position by *STEP_SIZE*. If *STEP_SIZE* is not given,
277 *BUFFER_SIZE* is used for its value.
279 If *FLUSH_INTERVAL* is specified for a write test, the request queue is
280 drained and a flush is issued before new writes are made whenever the number of
281 remaining requests is a multiple of *FLUSH_INTERVAL*. If additionally
282 ``--no-drain`` is specified, a flush is issued without draining the request
285 if ``-i`` is specified, *AIO* option can be used to specify different
286 AIO backends: ``threads``, ``native`` or ``io_uring``.
288 If ``-n`` is specified, the native AIO backend is used if possible. On
289 Linux, this option only works if ``-t none`` or ``-t directsync`` is
292 For write tests, by default a buffer filled with zeros is written. This can be
293 overridden with a pattern byte specified by *PATTERN*.
295 .. 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
297 Perform one or more modifications of the persistent bitmap *BITMAP*
298 in the disk image *FILENAME*. The various modifications are:
300 ``--add`` to create *BITMAP*, enabled to record future edits.
302 ``--remove`` to remove *BITMAP*.
304 ``--clear`` to clear *BITMAP*.
306 ``--enable`` to change *BITMAP* to start recording future edits.
308 ``--disable`` to change *BITMAP* to stop recording future edits.
310 ``--merge`` to merge the contents of the *SOURCE* bitmap into *BITMAP*.
312 Additional options include ``-g`` which sets a non-default
313 *GRANULARITY* for ``--add``, and ``-b`` and ``-F`` which select an
314 alternative source file for all *SOURCE* bitmaps used by
317 To see what bitmaps are present in an image, use ``qemu-img info``.
319 .. option:: check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT] [-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME
321 Perform a consistency check on the disk image *FILENAME*. The command can
322 output in the format *OFMT* which is either ``human`` or ``json``.
323 The JSON output is an object of QAPI type ``ImageCheck``.
325 If ``-r`` is specified, qemu-img tries to repair any inconsistencies found
326 during the check. ``-r leaks`` repairs only cluster leaks, whereas
327 ``-r all`` fixes all kinds of errors, with a higher risk of choosing the
328 wrong fix or hiding corruption that has already occurred.
330 Only the formats ``qcow2``, ``qed`` and ``vdi`` support
333 In case the image does not have any inconsistencies, check exits with ``0``.
334 Other exit codes indicate the kind of inconsistency found or if another error
335 occurred. The following table summarizes all exit codes of the check subcommand:
338 Check completed, the image is (now) consistent
340 Check not completed because of internal errors
342 Check completed, image is corrupted
344 Check completed, image has leaked clusters, but is not corrupted
346 Checks are not supported by the image format
348 If ``-r`` is specified, exit codes representing the image state refer to the
349 state after (the attempt at) repairing it. That is, a successful ``-r all``
350 will yield the exit code 0, independently of the image state before.
352 .. option:: commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b BASE] [-d] [-p] FILENAME
354 Commit the changes recorded in *FILENAME* in its base image or backing file.
355 If the backing file is smaller than the snapshot, then the backing file will be
356 resized to be the same size as the snapshot. If the snapshot is smaller than
357 the backing file, the backing file will not be truncated. If you want the
358 backing file to match the size of the smaller snapshot, you can safely truncate
359 it yourself once the commit operation successfully completes.
361 The image *FILENAME* is emptied after the operation has succeeded. If you do
362 not need *FILENAME* afterwards and intend to drop it, you may skip emptying
363 *FILENAME* by specifying the ``-d`` flag.
365 If the backing chain of the given image file *FILENAME* has more than one
366 layer, the backing file into which the changes will be committed may be
367 specified as *BASE* (which has to be part of *FILENAME*'s backing
368 chain). If *BASE* is not specified, the immediate backing file of the top
369 image (which is *FILENAME*) will be used. Note that after a commit operation
370 all images between *BASE* and the top image will be invalid and may return
371 garbage data when read. For this reason, ``-b`` implies ``-d`` (so that
372 the top image stays valid).
374 .. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 FILENAME2
376 Check if two images have the same content. You can compare images with
377 different format or settings.
379 The format is probed unless you specify it by ``-f`` (used for
380 *FILENAME1*) and/or ``-F`` (used for *FILENAME2*) option.
382 By default, images with different size are considered identical if the larger
383 image contains only unallocated and/or zeroed sectors in the area after the end
384 of the other image. In addition, if any sector is not allocated in one image
385 and contains only zero bytes in the second one, it is evaluated as equal. You
386 can use Strict mode by specifying the ``-s`` option. When compare runs in
387 Strict mode, it fails in case image size differs or a sector is allocated in
388 one image and is not allocated in the second one.
390 By default, compare prints out a result message. This message displays
391 information that both images are same or the position of the first different
392 byte. In addition, result message can report different image size in case
395 Compare exits with ``0`` in case the images are equal and with ``1``
396 in case the images differ. Other exit codes mean an error occurred during
397 execution and standard error output should contain an error message.
398 The following table sumarizes all exit codes of the compare subcommand:
405 Error on opening an image
407 Error on checking a sector allocation
409 Error on reading data
411 .. option:: convert [--object OBJECTDEF] [--image-opts] [--target-image-opts] [--target-is-zero] [--bitmaps] [-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
413 Convert the disk image *FILENAME* or a snapshot *SNAPSHOT_PARAM*
414 to disk image *OUTPUT_FILENAME* using format *OUTPUT_FMT*. It can
415 be optionally compressed (``-c`` option) or use any format specific
416 options like encryption (``-o`` option).
418 Only the formats ``qcow`` and ``qcow2`` support compression. The
419 compression is read-only. It means that if a compressed sector is
420 rewritten, then it is rewritten as uncompressed data.
422 Image conversion is also useful to get smaller image when using a
423 growable format such as ``qcow``: the empty sectors are detected and
424 suppressed from the destination image.
426 *SPARSE_SIZE* indicates the consecutive number of bytes (defaults to 4k)
427 that must contain only zeros for qemu-img to create a sparse image during
428 conversion. If *SPARSE_SIZE* is 0, the source will not be scanned for
429 unallocated or zero sectors, and the destination image will always be
432 You can use the *BACKING_FILE* option to force the output image to be
433 created as a copy on write image of the specified base image; the
434 *BACKING_FILE* should have the same content as the input's base image,
435 however the path, image format, etc may differ.
437 If a relative path name is given, the backing file is looked up relative to
438 the directory containing *OUTPUT_FILENAME*.
440 If the ``-n`` option is specified, the target volume creation will be
441 skipped. This is useful for formats such as ``rbd`` if the target
442 volume has already been created with site specific options that cannot
443 be supplied through qemu-img.
445 Out of order writes can be enabled with ``-W`` to improve performance.
446 This is only recommended for preallocated devices like host devices or other
447 raw block devices. Out of order write does not work in combination with
448 creating compressed images.
450 *NUM_COROUTINES* specifies how many coroutines work in parallel during
451 the convert process (defaults to 8).
453 .. option:: create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE] [-F BACKING_FMT] [-u] [-o OPTIONS] FILENAME [SIZE]
455 Create the new disk image *FILENAME* of size *SIZE* and format
456 *FMT*. Depending on the file format, you can add one or more *OPTIONS*
457 that enable additional features of this format.
459 If the option *BACKING_FILE* is specified, then the image will record
460 only the differences from *BACKING_FILE*. No size needs to be specified in
461 this case. *BACKING_FILE* will never be modified unless you use the
462 ``commit`` monitor command (or qemu-img commit).
464 If a relative path name is given, the backing file is looked up relative to
465 the directory containing *FILENAME*.
467 Note that a given backing file will be opened to check that it is valid. Use
468 the ``-u`` option to enable unsafe backing file mode, which means that the
469 image will be created even if the associated backing file cannot be opened. A
470 matching backing file must be created or additional options be used to make the
471 backing file specification valid when you want to use an image created this
474 The size can also be specified using the *SIZE* option with ``-o``,
475 it doesn't need to be specified separately in this case.
478 .. option:: dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE] [count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT
480 dd copies from *INPUT* file to *OUTPUT* file converting it from
481 *FMT* format to *OUTPUT_FMT* format.
483 The data is by default read and written using blocks of 512 bytes but can be
484 modified by specifying *BLOCK_SIZE*. If count=\ *BLOCKS* is specified
485 dd will stop reading input after reading *BLOCKS* input blocks.
487 The size syntax is similar to :manpage:`dd(1)`'s size syntax.
489 .. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [-U] FILENAME
491 Give information about the disk image *FILENAME*. Use it in
492 particular to know the size reserved on disk which can be different
493 from the displayed size. If VM snapshots are stored in the disk image,
494 they are displayed too.
496 If a disk image has a backing file chain, information about each disk image in
497 the chain can be recursively enumerated by using the option ``--backing-chain``.
499 For instance, if you have an image chain like:
503 base.qcow2 <- snap1.qcow2 <- snap2.qcow2
505 To enumerate information about each disk image in the above chain, starting from top to base, do:
509 qemu-img info --backing-chain snap2.qcow2
511 The command can output in the format *OFMT* which is either ``human`` or
512 ``json``. The JSON output is an object of QAPI type ``ImageInfo``; with
513 ``--backing-chain``, it is an array of ``ImageInfo`` objects.
515 ``--output=human`` reports the following information (for every image in the
525 The size of the guest disk
528 How much space the image file occupies on the host file system (may be
529 shown as 0 if this information is unavailable, e.g. because there is no
533 Cluster size of the image format, if applicable
536 Whether the image is encrypted (only present if so)
539 This is shown as ``no`` if the image is dirty and will have to be
540 auto-repaired the next time it is opened in qemu.
543 The backing file name, if present
545 *backing file format*
546 The format of the backing file, if the image enforces it
549 A list of all internal snapshots
551 *Format specific information*
552 Further information whose structure depends on the image format. This
553 section is a textual representation of the respective
554 ``ImageInfoSpecific*`` QAPI object (e.g. ``ImageInfoSpecificQCow2``
557 .. option:: map [--object OBJECTDEF] [--image-opts] [-f FMT] [--start-offset=OFFSET] [--max-length=LEN] [--output=OFMT] [-U] FILENAME
559 Dump the metadata of image *FILENAME* and its backing file chain.
560 In particular, this commands dumps the allocation state of every sector
561 of *FILENAME*, together with the topmost file that allocates it in
562 the backing file chain.
564 Two option formats are possible. The default format (``human``)
565 only dumps known-nonzero areas of the file. Known-zero parts of the
566 file are omitted altogether, and likewise for parts that are not allocated
567 throughout the chain. ``qemu-img`` output will identify a file
568 from where the data can be read, and the offset in the file. Each line
569 will include four fields, the first three of which are hexadecimal
570 numbers. For example the first line of:
574 Offset Length Mapped to File
575 0 0x20000 0x50000 /tmp/overlay.qcow2
576 0x100000 0x10000 0x95380000 /tmp/backing.qcow2
578 means that 0x20000 (131072) bytes starting at offset 0 in the image are
579 available in /tmp/overlay.qcow2 (opened in ``raw`` format) starting
580 at offset 0x50000 (327680). Data that is compressed, encrypted, or
581 otherwise not available in raw format will cause an error if ``human``
582 format is in use. Note that file names can include newlines, thus it is
583 not safe to parse this output format in scripts.
585 The alternative format ``json`` will return an array of dictionaries
586 in JSON format. It will include similar information in
587 the ``start``, ``length``, ``offset`` fields;
588 it will also include other more specific information:
590 - whether the sectors contain actual data or not (boolean field ``data``;
591 if false, the sectors are either unallocated or stored as optimized
593 - whether the data is known to read as zero (boolean field ``zero``);
594 - in order to make the output shorter, the target file is expressed as
595 a ``depth``; for example, a depth of 2 refers to the backing file
596 of the backing file of *FILENAME*.
598 In JSON format, the ``offset`` field is optional; it is absent in
599 cases where ``human`` format would omit the entry or exit with an error.
600 If ``data`` is false and the ``offset`` field is present, the
601 corresponding sectors in the file are not yet in use, but they are
604 For more information, consult ``include/block/block.h`` in QEMU's
607 .. option:: measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--object OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME]
609 Calculate the file size required for a new image. This information
610 can be used to size logical volumes or SAN LUNs appropriately for
611 the image that will be placed in them. The values reported are
612 guaranteed to be large enough to fit the image. The command can
613 output in the format *OFMT* which is either ``human`` or ``json``.
614 The JSON output is an object of QAPI type ``BlockMeasureInfo``.
616 If the size *N* is given then act as if creating a new empty image file
617 using ``qemu-img create``. If *FILENAME* is given then act as if
618 converting an existing image file using ``qemu-img convert``. The format
619 of the new file is given by *OUTPUT_FMT* while the format of an existing
620 file is given by *FMT*.
622 A snapshot in an existing image can be specified using *SNAPSHOT_PARAM*.
624 The following fields are reported:
628 required size: 524288
629 fully allocated size: 1074069504
632 The ``required size`` is the file size of the new image. It may be smaller
633 than the virtual disk size if the image format supports compact representation.
635 The ``fully allocated size`` is the file size of the new image once data has
636 been written to all sectors. This is the maximum size that the image file can
637 occupy with the exception of internal snapshots, dirty bitmaps, vmstate data,
638 and other advanced image format features.
640 The ``bitmaps size`` is the additional size required in order to
641 copy bitmaps from a source image in addition to the guest-visible
642 data; the line is omitted if either source or destination lacks
643 bitmap support, or 0 if bitmaps are supported but there is nothing
646 .. option:: snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAPSHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME
648 List, apply, create or delete snapshots in image *FILENAME*.
650 .. option:: rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-p] [-u] -b BACKING_FILE [-F BACKING_FMT] FILENAME
652 Changes the backing file of an image. Only the formats ``qcow2`` and
653 ``qed`` support changing the backing file.
655 The backing file is changed to *BACKING_FILE* and (if the image format of
656 *FILENAME* supports this) the backing file format is changed to
657 *BACKING_FMT*. If *BACKING_FILE* is specified as "" (the empty
658 string), then the image is rebased onto no backing file (i.e. it will exist
659 independently of any backing file).
661 If a relative path name is given, the backing file is looked up relative to
662 the directory containing *FILENAME*.
664 *CACHE* specifies the cache mode to be used for *FILENAME*, whereas
665 *SRC_CACHE* specifies the cache mode for reading backing files.
667 There are two different modes in which ``rebase`` can operate:
670 This is the default mode and performs a real rebase operation. The
671 new backing file may differ from the old one and qemu-img rebase
672 will take care of keeping the guest-visible content of *FILENAME*
675 In order to achieve this, any clusters that differ between
676 *BACKING_FILE* and the old backing file of *FILENAME* are merged
677 into *FILENAME* before actually changing the backing file.
679 Note that the safe mode is an expensive operation, comparable to
680 converting an image. It only works if the old backing file still
684 qemu-img uses the unsafe mode if ``-u`` is specified. In this
685 mode, only the backing file name and format of *FILENAME* is changed
686 without any checks on the file contents. The user must take care of
687 specifying the correct new backing file, or the guest-visible
688 content of the image will be corrupted.
690 This mode is useful for renaming or moving the backing file to
691 somewhere else. It can be used without an accessible old backing
692 file, i.e. you can use it to fix an image whose backing file has
693 already been moved/renamed.
695 You can use ``rebase`` to perform a "diff" operation on two
696 disk images. This can be useful when you have copied or cloned
697 a guest, and you want to get back to a thin image on top of a
698 template or base image.
700 Say that ``base.img`` has been cloned as ``modified.img`` by
701 copying it, and that the ``modified.img`` guest has run so there
702 are now some changes compared to ``base.img``. To construct a thin
703 image called ``diff.qcow2`` that contains just the differences, do:
707 qemu-img create -f qcow2 -b modified.img diff.qcow2
708 qemu-img rebase -b base.img diff.qcow2
710 At this point, ``modified.img`` can be discarded, since
711 ``base.img + diff.qcow2`` contains the same information.
713 .. option:: resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--preallocation=PREALLOC] [-q] [--shrink] FILENAME [+ | -]SIZE
715 Change the disk image as if it had been created with *SIZE*.
717 Before using this command to shrink a disk image, you MUST use file system and
718 partitioning tools inside the VM to reduce allocated file systems and partition
719 sizes accordingly. Failure to do so will result in data loss!
721 When shrinking images, the ``--shrink`` option must be given. This informs
722 qemu-img that the user acknowledges all loss of data beyond the truncated
725 After using this command to grow a disk image, you must use file system and
726 partitioning tools inside the VM to actually begin using the new space on the
729 When growing an image, the ``--preallocation`` option may be used to specify
730 how the additional image area should be allocated on the host. See the format
731 description in the :ref:`notes` section which values are allowed. Using this
732 option may result in slightly more data being allocated than necessary.
739 Supported image file formats:
743 Raw disk image format (default). This format has the advantage of
744 being simple and easily exportable to all other emulators. If your
745 file system supports *holes* (for example in ext2 or ext3 on
746 Linux or NTFS on Windows), then only the written sectors will reserve
747 space. Use ``qemu-img info`` to know the real size used by the
748 image or ``ls -ls`` on Unix/Linux.
753 Preallocation mode (allowed values: ``off``, ``falloc``,
754 ``full``). ``falloc`` mode preallocates space for image by
755 calling ``posix_fallocate()``. ``full`` mode preallocates space
756 for image by writing data to underlying storage. This data may or
757 may not be zero, depending on the storage location.
761 QEMU image format, the most versatile format. Use it to have smaller
762 images (useful if your filesystem does not supports holes, for example
763 on Windows), optional AES encryption, zlib based compression and
764 support of multiple VM snapshots.
769 Determines the qcow2 version to use. ``compat=0.10`` uses the
770 traditional image format that can be read by any QEMU since 0.10.
771 ``compat=1.1`` enables image format extensions that only QEMU 1.1 and
772 newer understand (this is the default). Amongst others, this includes zero
773 clusters, which allow efficient copy-on-read for sparse images.
776 File name of a base image (see ``create`` subcommand)
779 Image format of the base image
782 If this option is set to ``on``, the image is encrypted with
785 The use of encryption in qcow and qcow2 images is considered to be
786 flawed by modern cryptography standards, suffering from a number
789 - The AES-CBC cipher is used with predictable initialization
790 vectors based on the sector number. This makes it vulnerable to
791 chosen plaintext attacks which can reveal the existence of
794 - The user passphrase is directly used as the encryption key. A
795 poorly chosen or short passphrase will compromise the security
798 - In the event of the passphrase being compromised there is no way
799 to change the passphrase to protect data in any qcow images. The
800 files must be cloned, using a different encryption passphrase in
801 the new file. The original file must then be securely erased
802 using a program like shred, though even this is ineffective with
803 many modern storage technologies.
805 - Initialization vectors used to encrypt sectors are based on the
806 guest virtual sector number, instead of the host physical
807 sector. When a disk image has multiple internal snapshots this
808 means that data in multiple physical sectors is encrypted with
809 the same initialization vector. With the CBC mode, this opens
810 the possibility of watermarking attacks if the attack can
811 collect multiple sectors encrypted with the same IV and some
812 predictable data. Having multiple qcow2 images with the same
813 passphrase also exposes this weakness since the passphrase is
814 directly used as the key.
816 Use of qcow / qcow2 encryption is thus strongly discouraged. Users are
817 recommended to use an alternative encryption technology such as the
818 Linux dm-crypt / LUKS system.
821 Changes the qcow2 cluster size (must be between 512 and
822 2M). Smaller cluster sizes can improve the image file size whereas
823 larger cluster sizes generally provide better performance.
826 Preallocation mode (allowed values: ``off``, ``metadata``,
827 ``falloc``, ``full``). An image with preallocated metadata is
828 initially larger but can improve performance when the image needs
829 to grow. ``falloc`` and ``full`` preallocations are like the same
830 options of ``raw`` format, but sets up metadata also.
833 If this option is set to ``on``, reference count updates are
834 postponed with the goal of avoiding metadata I/O and improving
835 performance. This is particularly interesting with
836 ``cache=writethrough`` which doesn't batch metadata
837 updates. The tradeoff is that after a host crash, the reference
838 count tables must be rebuilt, i.e. on the next open an (automatic)
839 ``qemu-img check -r all`` is required, which may take some time.
841 This option can only be enabled if ``compat=1.1`` is specified.
844 If this option is set to ``on``, it will turn off COW of the file. It's
845 only valid on btrfs, no effect on other file systems.
847 Btrfs has low performance when hosting a VM image file, even more
848 when the guest on the VM also using btrfs as file system. Turning
849 off COW is a way to mitigate this bad performance. Generally there
850 are two ways to turn off COW on btrfs:
852 - Disable it by mounting with nodatacow, then all newly created files
854 - For an empty file, add the NOCOW file attribute. That's what this
857 Note: this option is only valid to new or empty files. If there is
858 an existing file which is COW and has data blocks already, it
859 couldn't be changed to NOCOW by setting ``nocow=on``. One can
860 issue ``lsattr filename`` to check if the NOCOW flag is set or not
861 (Capital 'C' is NOCOW flag).
865 QEMU also supports various other image file formats for
866 compatibility with older QEMU versions or other hypervisors,
867 including VMDK, VDI, VHD (vpc), VHDX, qcow1 and QED. For a full list
868 of supported formats see ``qemu-img --help``. For a more detailed
869 description of these formats, see the QEMU block drivers reference
872 The main purpose of the block drivers for these formats is image
873 conversion. For running VMs, it is recommended to convert the disk
874 images to either raw or qcow2 in order to achieve good performance.
projects / mirror_qemu.git / blob