xref: /openbmc/qemu/docs/tools/qemu-img.rst (revision eabfeb0c)
1QEMU disk image utility
2=======================
3
4Synopsis
5--------
6
7**qemu-img** [*standard options*] *command* [*command options*]
8
9Description
10-----------
11
12qemu-img allows you to create, convert and modify images offline. It can handle
13all image formats supported by QEMU.
14
15**Warning:** Never use qemu-img to modify images in use by a running virtual
16machine or any other process; this may destroy the image. Also, be aware that
17querying an image that is being modified by another process may encounter
18inconsistent state.
19
20Options
21-------
22
23.. program:: qemu-img
24
25Standard options:
26
27.. option:: -h, --help
28
29  Display this help and exit
30
31.. option:: -V, --version
32
33  Display version information and exit
34
35.. option:: -T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE]
36
37  .. include:: ../qemu-option-trace.rst.inc
38
39The following commands are supported:
40
41.. hxtool-doc:: qemu-img-cmds.hx
42
43Command parameters:
44
45*FILENAME* is a disk image filename.
46
47*FMT* is the disk image format. It is guessed automatically in most
48cases. See below for a description of the supported disk formats.
49
50*SIZE* is the disk image size in bytes. Optional suffixes ``k`` or
51``K`` (kilobyte, 1024) ``M`` (megabyte, 1024k) and ``G`` (gigabyte,
521024M) and T (terabyte, 1024G) are supported.  ``b`` is ignored.
53
54*OUTPUT_FILENAME* is the destination disk image filename.
55
56*OUTPUT_FMT* is the destination format.
57
58*OPTIONS* is a comma separated list of format specific options in a
59name=value format. Use ``-o ?`` for an overview of the options supported
60by the used format or see the format descriptions below for details.
61
62*SNAPSHOT_PARAM* is param used for internal snapshot, format is
63'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'.
64
65..
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.
68
69.. program:: qemu-img-common-opts
70
71.. option:: --object OBJECTDEF
72
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
76  encryption keys.
77
78.. option:: --image-opts
79
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.
83
84.. option:: --target-image-opts
85
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
90  in a future release.
91
92.. option:: --force-share (-U)
93
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.
100
101.. option:: --backing-chain
102
103  Will enumerate information about backing files in a disk image chain. Refer
104  below for further description.
105
106.. option:: -c
107
108  Indicates that target image must be compressed (qcow format only).
109
110.. option:: -h
111
112  With or without a command, shows help and lists the supported formats.
113
114.. option:: -p
115
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
119  ``SIGINFO`` signal.
120
121.. option:: -q
122
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.
125
126.. option:: -S SIZE
127
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
131  ``k`` for kilobytes.
132
133.. option:: -t CACHE
134
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
137  values.
138
139.. option:: -T SRC_CACHE
140
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
143  values.
144
145Parameters to compare subcommand:
146
147.. program:: qemu-img-compare
148
149.. option:: -f
150
151  First image format
152
153.. option:: -F
154
155  Second image format
156
157.. option:: -s
158
159  Strict mode - fail on different image size or sector allocation
160
161Parameters to convert subcommand:
162
163.. program:: qemu-img-convert
164
165.. option:: --bitmaps
166
167  Additionally copy all persistent bitmaps from the top layer of the source
168
169.. option:: -n
170
171  Skip the creation of the target volume
172
173.. option:: -m
174
175  Number of parallel coroutines for the convert process
176
177.. option:: -W
178
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
181  raw block devices.
182
183.. option:: -C
184
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
189  information.
190
191.. option:: --salvage
192
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.
196
197.. option:: --target-is-zero
198
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.
203
204Parameters to dd subcommand:
205
206.. program:: qemu-img-dd
207
208.. option:: bs=BLOCK_SIZE
209
210  Defines the block size
211
212.. option:: count=BLOCKS
213
214  Sets the number of input blocks to copy
215
216.. option:: if=INPUT
217
218  Sets the input file
219
220.. option:: of=OUTPUT
221
222  Sets the output file
223
224.. option:: skip=BLOCKS
225
226  Sets the number of input blocks to skip
227
228Parameters to snapshot subcommand:
229
230.. program:: qemu-img-snapshot
231
232.. option:: snapshot
233
234  Is the name of the snapshot to create, apply or delete
235
236.. option:: -a
237
238  Applies a snapshot (revert disk to saved state)
239
240.. option:: -c
241
242  Creates a snapshot
243
244.. option:: -d
245
246  Deletes a snapshot
247
248.. option:: -l
249
250  Lists all snapshots in the given image
251
252Command description:
253
254.. program:: qemu-img-commands
255
256.. option:: amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE] [--force] -o OPTIONS FILENAME
257
258  Amends the image format specific *OPTIONS* for the image file
259  *FILENAME*. Not all file formats support this operation.
260
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``.
264
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.
267
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
269
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.
272
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.
278
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
283  queue first.
284
285  if ``-i`` is specified, *AIO* option can be used to specify different
286  AIO backends: ``threads``, ``native`` or ``io_uring``.
287
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
290  specified as well.
291
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*.
294
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
296
297  Perform one or more modifications of the persistent bitmap *BITMAP*
298  in the disk image *FILENAME*.  The various modifications are:
299
300  ``--add`` to create *BITMAP*, enabled to record future edits.
301
302  ``--remove`` to remove *BITMAP*.
303
304  ``--clear`` to clear *BITMAP*.
305
306  ``--enable`` to change *BITMAP* to start recording future edits.
307
308  ``--disable`` to change *BITMAP* to stop recording future edits.
309
310  ``--merge`` to merge the contents of the *SOURCE* bitmap into *BITMAP*.
311
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
315  ``--merge``.
316
317  To see what bitmaps are present in an image, use ``qemu-img info``.
318
319.. option:: check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT] [-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME
320
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``.
324
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.
329
330  Only the formats ``qcow2``, ``qed`` and ``vdi`` support
331  consistency checks.
332
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:
336
337  0
338    Check completed, the image is (now) consistent
339  1
340    Check not completed because of internal errors
341  2
342    Check completed, image is corrupted
343  3
344    Check completed, image has leaked clusters, but is not corrupted
345  63
346    Checks are not supported by the image format
347
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.
351
352.. option:: commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b BASE] [-d] [-p] FILENAME
353
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.
360
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.
364
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).
373
374.. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 FILENAME2
375
376  Check if two images have the same content. You can compare images with
377  different format or settings.
378
379  The format is probed unless you specify it by ``-f`` (used for
380  *FILENAME1*) and/or ``-F`` (used for *FILENAME2*) option.
381
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.
389
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
393  Strict mode is used.
394
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:
399
400  0
401    Images are identical
402  1
403    Images differ
404  2
405    Error on opening an image
406  3
407    Error on checking a sector allocation
408  4
409    Error on reading data
410
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
412
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).
417
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.
421
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.
425
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
430  fully allocated.
431
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.
436
437  If a relative path name is given, the backing file is looked up relative to
438  the directory containing *OUTPUT_FILENAME*.
439
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.
444
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.
449
450  *NUM_COROUTINES* specifies how many coroutines work in parallel during
451  the convert process (defaults to 8).
452
453.. option:: create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE] [-F BACKING_FMT] [-u] [-o OPTIONS] FILENAME [SIZE]
454
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.
458
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).
463
464  If a relative path name is given, the backing file is looked up relative to
465  the directory containing *FILENAME*.
466
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
472  way.
473
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.
476
477
478.. option:: dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE] [count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT
479
480  dd copies from *INPUT* file to *OUTPUT* file converting it from
481  *FMT* format to *OUTPUT_FMT* format.
482
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.
486
487  The size syntax is similar to :manpage:`dd(1)`'s size syntax.
488
489.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [-U] FILENAME
490
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.
495
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``.
498
499  For instance, if you have an image chain like:
500
501  ::
502
503    base.qcow2 <- snap1.qcow2 <- snap2.qcow2
504
505  To enumerate information about each disk image in the above chain, starting from top to base, do:
506
507  ::
508
509    qemu-img info --backing-chain snap2.qcow2
510
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.
514
515  ``--output=human`` reports the following information (for every image in the
516  chain):
517
518  *image*
519    The image file name
520
521  *file format*
522    The image format
523
524  *virtual size*
525    The size of the guest disk
526
527  *disk size*
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
530    file system)
531
532  *cluster_size*
533    Cluster size of the image format, if applicable
534
535  *encrypted*
536    Whether the image is encrypted (only present if so)
537
538  *cleanly shut down*
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.
541
542  *backing file*
543    The backing file name, if present
544
545  *backing file format*
546    The format of the backing file, if the image enforces it
547
548  *Snapshot list*
549    A list of all internal snapshots
550
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``
555    for qcow2 images).
556
557.. option:: map [--object OBJECTDEF] [--image-opts] [-f FMT] [--start-offset=OFFSET] [--max-length=LEN] [--output=OFMT] [-U] FILENAME
558
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.
563
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:
571
572  ::
573
574    Offset          Length          Mapped to       File
575    0               0x20000         0x50000         /tmp/overlay.qcow2
576    0x100000        0x10000         0x95380000      /tmp/backing.qcow2
577
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.
584
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:
589
590  - whether the sectors contain actual data or not (boolean field ``data``;
591    if false, the sectors are either unallocated or stored as optimized
592    all-zero clusters);
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*.
597
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
602  preallocated.
603
604  For more information, consult ``include/block/block.h`` in QEMU's
605  source code.
606
607.. option:: measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--object OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME]
608
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``.
615
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*.
621
622  A snapshot in an existing image can be specified using *SNAPSHOT_PARAM*.
623
624  The following fields are reported:
625
626  ::
627
628    required size: 524288
629    fully allocated size: 1074069504
630    bitmaps size: 0
631
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.
634
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.
639
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
644  to copy.
645
646.. option:: snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAPSHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME
647
648  List, apply, create or delete snapshots in image *FILENAME*.
649
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
651
652  Changes the backing file of an image. Only the formats ``qcow2`` and
653  ``qed`` support changing the backing file.
654
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).
660
661  If a relative path name is given, the backing file is looked up relative to
662  the directory containing *FILENAME*.
663
664  *CACHE* specifies the cache mode to be used for *FILENAME*, whereas
665  *SRC_CACHE* specifies the cache mode for reading backing files.
666
667  There are two different modes in which ``rebase`` can operate:
668
669  Safe mode
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*
673    unchanged.
674
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.
678
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
681    exists.
682
683  Unsafe mode
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.
689
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.
694
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.
699
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:
704
705  ::
706
707    qemu-img create -f qcow2 -b modified.img diff.qcow2
708    qemu-img rebase -b base.img diff.qcow2
709
710  At this point, ``modified.img`` can be discarded, since
711  ``base.img + diff.qcow2`` contains the same information.
712
713.. option:: resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--preallocation=PREALLOC] [-q] [--shrink] FILENAME [+ | -]SIZE
714
715  Change the disk image as if it had been created with *SIZE*.
716
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!
720
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
723  image's end.
724
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
727  device.
728
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.
733
734.. _notes:
735
736Notes
737-----
738
739Supported image file formats:
740
741``raw``
742
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.
749
750  Supported options:
751
752  ``preallocation``
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.
758
759``qcow2``
760
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.
765
766  Supported options:
767
768  ``compat``
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.
774
775  ``backing_file``
776    File name of a base image (see ``create`` subcommand)
777
778  ``backing_fmt``
779    Image format of the base image
780
781  ``encryption``
782    If this option is set to ``on``, the image is encrypted with
783    128-bit AES-CBC.
784
785    The use of encryption in qcow and qcow2 images is considered to be
786    flawed by modern cryptography standards, suffering from a number
787    of design problems:
788
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
792      encrypted data.
793
794    - The user passphrase is directly used as the encryption key. A
795      poorly chosen or short passphrase will compromise the security
796      of the encryption.
797
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.
804
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.
815
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.
819
820  ``cluster_size``
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.
824
825  ``preallocation``
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.
831
832  ``lazy_refcounts``
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.
840
841    This option can only be enabled if ``compat=1.1`` is specified.
842
843  ``nocow``
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.
846
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:
851
852    - Disable it by mounting with nodatacow, then all newly created files
853      will be NOCOW
854    - For an empty file, add the NOCOW file attribute. That's what this
855      option does.
856
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).
862
863``Other``
864
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
870  documentation.
871
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.
875