1QEMU-IMG(1) QEMU QEMU-IMG(1)
2
6 qemu-img - QEMU disk image utility
7
9 qemu-img [standard options] command [command options]
10
12 qemu-img allows you to create, convert and modify images offline. It
13 can handle all image formats supported by QEMU.
14 Warning: Never use qemu-img to modify images in use by a running vir‐
16 tual machine or any other process; this may destroy the image. Also, be
17 aware that querying an image that is being modified by another process
18 may encounter inconsistent state.
19
21 Standard options:
22 -h, --help
24 Display this help and exit
25 -V, --version
27 Display version information and exit
28 -T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE]
30 Specify tracing options.
31 [enable=]PATTERN
33 Immediately enable events matching PATTERN (either event name
34 or a globbing pattern). This option is only available if
35 QEMU has been compiled with the simple, log or ftrace tracing
36 backend. To specify multiple events or patterns, specify the
37 -trace option multiple times.
38 Use -trace help to print a list of names of trace points.
40 events=FILE
42 Immediately enable events listed in FILE. The file must con‐
43 tain one event name (as listed in the trace-events-all file)
44 per line; globbing patterns are accepted too. This option is
45 only available if QEMU has been compiled with the simple, log
46 or ftrace tracing backend.
47 file=FILE
49 Log output traces to FILE. This option is only available if
50 QEMU has been compiled with the simple tracing backend.
51 The following commands are supported:
53 amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE]
55 [--force] -o OPTIONS FILENAME
56 bench [-c COUNT] [-d DEPTH] [-f FMT] [--flush-interval=FLUSH_INTERVAL]
58 [-i AIO] [-n] [--no-drain] [-o OFFSET] [--pattern=PATTERN] [-q] [-s
59 BUFFER_SIZE] [-S STEP_SIZE] [-t CACHE] [-w] [-U] FILENAME
60 bitmap (--merge SOURCE | --add | --remove | --clear | --enable | --dis‐
62 able)... [-b SOURCE_FILE [-F SOURCE_FMT]] [-g GRANULARITY] [--object
63 OBJECTDEF] [--image-opts | -f FMT] FILENAME BITMAP
64 check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT]
66 [-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME
67 commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b
69 BASE] [-r RATE_LIMIT] [-d] [-p] FILENAME
70 compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T
72 SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 FILENAME2
73 convert [--object OBJECTDEF] [--image-opts] [--target-image-opts]
75 [--target-is-zero] [--bitmaps] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT]
76 [-t CACHE] [-T SRC_CACHE] [-O OUTPUT_FMT] [-B BACKING_FILE [-F BACK‐
77 ING_FMT]] [-o OPTIONS] [-l SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-r
78 RATE_LIMIT] [-m NUM_COROUTINES] [-W] [--salvage] FILENAME [FILENAME2
79 [...]] OUTPUT_FILENAME
80 create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE [-F BACK‐
82 ING_FMT]] [-u] [-o OPTIONS] FILENAME [SIZE]
83 dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE]
85 [count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT
86 info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT]
88 [--backing-chain] [-U] FILENAME
89 map [--object OBJECTDEF] [--image-opts] [-f FMT] [--start-offset=OFF‐
91 SET] [--max-length=LEN] [--output=OFMT] [-U] FILENAME
92 measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--ob‐
94 ject OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME]
95 snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAP‐
97 SHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME
98 rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t
100 CACHE] [-T SRC_CACHE] [-p] [-u] -b BACKING_FILE [-F BACKING_FMT] FILE‐
101 NAME
102 resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--prealloca‐
104 tion=PREALLOC] [-q] [--shrink] FILENAME [+ | -]SIZE
105 Command parameters:
107 FILENAME is a disk image filename.
109 FMT is the disk image format. It is guessed automatically in most
111 cases. See below for a description of the supported disk formats.
112 SIZE is the disk image size in bytes. Optional suffixes k or K (kilo‐
114 byte, 1024) M (megabyte, 1024k) and G (gigabyte, 1024M) and T (ter‐
115 abyte, 1024G) are supported. b is ignored.
116 OUTPUT_FILENAME is the destination disk image filename.
118 OUTPUT_FMT is the destination format.
120 OPTIONS is a comma separated list of format specific options in a
122 name=value format. Use -o help for an overview of the options supported
123 by the used format or see the format descriptions below for details.
124 SNAPSHOT_PARAM is param used for internal snapshot, format is 'snap‐
126 shot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'.
127 --object OBJECTDEF
129 is a QEMU user creatable object definition. See the qemu(1) man‐
130 ual page for a description of the object properties. The most
131 common object type is a secret, which is used to supply pass‐
132 words and/or encryption keys.
133 --image-opts
135 Indicates that the source FILENAME parameter is to be inter‐
136 preted as a full option string, not a plain filename. This pa‐
137 rameter is mutually exclusive with the -f parameter.
138 --target-image-opts
140 Indicates that the OUTPUT_FILENAME parameter(s) are to be inter‐
141 preted as a full option string, not a plain filename. This pa‐
142 rameter is mutually exclusive with the -O parameters. It is cur‐
143 rently required to also use the -n parameter to skip image cre‐
144 ation. This restriction may be relaxed in a future release.
145 --force-share (-U)
147 If specified, qemu-img will open the image in shared mode, al‐
148 lowing other QEMU processes to open it in write mode. For exam‐
149 ple, this can be used to get the image information (with 'info'
150 subcommand) when the image is used by a running guest. Note
151 that this could produce inconsistent results because of concur‐
152 rent metadata changes, etc. This option is only allowed when
153 opening images in read-only mode.
154 --backing-chain
156 Will enumerate information about backing files in a disk image
157 chain. Refer below for further description.
158 -c Indicates that target image must be compressed (qcow format
160 only).
161 -h With or without a command, shows help and lists the supported
163 formats.
164 -p Display progress bar (compare, convert and rebase commands
166 only). If the -p option is not used for a command that supports
167 it, the progress is reported when the process receives a SIGUSR1
168 or SIGINFO signal.
169 -q Quiet mode - do not print any output (except errors). There's no
171 progress bar in case both -q and -p options are used.
172 -S SIZE
174 Indicates the consecutive number of bytes that must contain only
175 zeros for qemu-img to create a sparse image during conversion.
176 This value is rounded down to the nearest 512 bytes. You may use
177 the common size suffixes like k for kilobytes.
178 -t CACHE
180 Specifies the cache mode that should be used with the (destina‐
181 tion) file. See the documentation of the emulator's -drive
182 cache=... option for allowed values.
183 -T SRC_CACHE
185 Specifies the cache mode that should be used with the source
186 file(s). See the documentation of the emulator's -drive
187 cache=... option for allowed values.
188 Parameters to compare subcommand:
190 -f First image format
192 -F Second image format
194 -s Strict mode - fail on different image size or sector allocation
196 Parameters to convert subcommand:
198 --bitmaps
200 Additionally copy all persistent bitmaps from the top layer of
201 the source
202 -n Skip the creation of the target volume
204 -m Number of parallel coroutines for the convert process
206 -W Allow out-of-order writes to the destination. This option im‐
208 proves performance, but is only recommended for preallocated de‐
209 vices like host devices or other raw block devices.
210 -C Try to use copy offloading to move data from source image to
212 target. This may improve performance if the data is remote, such
213 as with NFS or iSCSI backends, but will not automatically spar‐
214 sify zero sectors, and may result in a fully allocated target
215 image depending on the host support for getting allocation in‐
216 formation.
217 -r Rate limit for the convert process
219 --salvage
221 Try to ignore I/O errors when reading. Unless in quiet mode
222 (-q), errors will still be printed. Areas that cannot be read
223 from the source will be treated as containing only zeroes.
224 --target-is-zero
226 Assume that reading the destination image will always return ze‐
227 ros. This parameter is mutually exclusive with a destination im‐
228 age that has a backing file. It is required to also use the -n
229 parameter to skip image creation.
230 Parameters to dd subcommand:
232 bs=BLOCK_SIZE
234 Defines the block size
235 count=BLOCKS
237 Sets the number of input blocks to copy
238 if=INPUT
240 Sets the input file
241 of=OUTPUT
243 Sets the output file
244 skip=BLOCKS
246 Sets the number of input blocks to skip
247 Parameters to snapshot subcommand:
249 snapshot
251 Is the name of the snapshot to create, apply or delete
252 -a Applies a snapshot (revert disk to saved state)
254 -c Creates a snapshot
256 -d Deletes a snapshot
258 -l Lists all snapshots in the given image
260 Command description:
262 amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE]
264 [--force] -o OPTIONS FILENAME
265 Amends the image format specific OPTIONS for the image file
266 FILENAME. Not all file formats support this operation.
267 The set of options that can be amended are dependent on the im‐
269 age format, but note that amending the backing chain relation‐
270 ship should instead be performed with qemu-img rebase.
271 --force allows some unsafe operations. Currently for -f luks, it
273 allows to erase the last encryption key, and to overwrite an ac‐
274 tive encryption key.
275 bench [-c COUNT] [-d DEPTH] [-f FMT] [--flush-interval=FLUSH_INTERVAL]
277 [-i AIO] [-n] [--no-drain] [-o OFFSET] [--pattern=PATTERN] [-q] [-s
278 BUFFER_SIZE] [-S STEP_SIZE] [-t CACHE] [-w] [-U] FILENAME
279 Run a simple sequential I/O benchmark on the specified image. If
280 -w is specified, a write test is performed, otherwise a read
281 test is performed.
282 A total number of COUNT I/O requests is performed, each BUF‐
284 FER_SIZE bytes in size, and with DEPTH requests in parallel. The
285 first request starts at the position given by OFFSET, each fol‐
286 lowing request increases the current position by STEP_SIZE. If
287 STEP_SIZE is not given, BUFFER_SIZE is used for its value.
288 If FLUSH_INTERVAL is specified for a write test, the request
290 queue is drained and a flush is issued before new writes are
291 made whenever the number of remaining requests is a multiple of
292 FLUSH_INTERVAL. If additionally --no-drain is specified, a flush
293 is issued without draining the request queue first.
294 if -i is specified, AIO option can be used to specify different
296 AIO backends: threads, native or io_uring.
297 If -n is specified, the native AIO backend is used if possible.
299 On Linux, this option only works if -t none or -t directsync is
300 specified as well.
301 For write tests, by default a buffer filled with zeros is writ‐
303 ten. This can be overridden with a pattern byte specified by
304 PATTERN.
305 bitmap (--merge SOURCE | --add | --remove | --clear | --enable | --dis‐
307 able)... [-b SOURCE_FILE [-F SOURCE_FMT]] [-g GRANULARITY] [--object
308 OBJECTDEF] [--image-opts | -f FMT] FILENAME BITMAP
309 Perform one or more modifications of the persistent bitmap BIT‐
310 MAP in the disk image FILENAME. The various modifications are:
311 --add to create BITMAP, enabled to record future edits.
313 --remove to remove BITMAP.
315 --clear to clear BITMAP.
317 --enable to change BITMAP to start recording future edits.
319 --disable to change BITMAP to stop recording future edits.
321 --merge to merge the contents of the SOURCE bitmap into BITMAP.
323 Additional options include -g which sets a non-default GRANULAR‐
325 ITY for --add, and -b and -F which select an alternative source
326 file for all SOURCE bitmaps used by --merge.
327 To see what bitmaps are present in an image, use qemu-img info.
329 check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT]
331 [-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME
332 Perform a consistency check on the disk image FILENAME. The com‐
333 mand can output in the format OFMT which is either human or
334 json. The JSON output is an object of QAPI type ImageCheck.
335 If -r is specified, qemu-img tries to repair any inconsistencies
337 found during the check. -r leaks repairs only cluster leaks,
338 whereas -r all fixes all kinds of errors, with a higher risk of
339 choosing the wrong fix or hiding corruption that has already oc‐
340 curred.
341 Only the formats qcow2, qed, parallels, vhdx, vmdk and vdi sup‐
343 port consistency checks.
344 In case the image does not have any inconsistencies, check exits
346 with 0. Other exit codes indicate the kind of inconsistency
347 found or if another error occurred. The following table summa‐
348 rizes all exit codes of the check subcommand:
349 0 Check completed, the image is (now) consistent
351 1 Check not completed because of internal errors
353 2 Check completed, image is corrupted
355 3 Check completed, image has leaked clusters, but is not
357 corrupted
358 63 Checks are not supported by the image format
360 If -r is specified, exit codes representing the image state re‐
362 fer to the state after (the attempt at) repairing it. That is, a
363 successful -r all will yield the exit code 0, independently of
364 the image state before.
365 commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b
367 BASE] [-r RATE_LIMIT] [-d] [-p] FILENAME
368 Commit the changes recorded in FILENAME in its base image or
369 backing file. If the backing file is smaller than the snapshot,
370 then the backing file will be resized to be the same size as the
371 snapshot. If the snapshot is smaller than the backing file, the
372 backing file will not be truncated. If you want the backing
373 file to match the size of the smaller snapshot, you can safely
374 truncate it yourself once the commit operation successfully com‐
375 pletes.
376 The image FILENAME is emptied after the operation has succeeded.
378 If you do not need FILENAME afterwards and intend to drop it,
379 you may skip emptying FILENAME by specifying the -d flag.
380 If the backing chain of the given image file FILENAME has more
382 than one layer, the backing file into which the changes will be
383 committed may be specified as BASE (which has to be part of
384 FILENAME's backing chain). If BASE is not specified, the immedi‐
385 ate backing file of the top image (which is FILENAME) will be
386 used. Note that after a commit operation all images between BASE
387 and the top image will be invalid and may return garbage data
388 when read. For this reason, -b implies -d (so that the top image
389 stays valid).
390 The rate limit for the commit process is specified by -r.
392 compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T
394 SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 FILENAME2
395 Check if two images have the same content. You can compare im‐
396 ages with different format or settings.
397 The format is probed unless you specify it by -f (used for FILE‐
399 NAME1) and/or -F (used for FILENAME2) option.
400 By default, images with different size are considered identical
402 if the larger image contains only unallocated and/or zeroed sec‐
403 tors in the area after the end of the other image. In addition,
404 if any sector is not allocated in one image and contains only
405 zero bytes in the second one, it is evaluated as equal. You can
406 use Strict mode by specifying the -s option. When compare runs
407 in Strict mode, it fails in case image size differs or a sector
408 is allocated in one image and is not allocated in the second
409 one.
410 By default, compare prints out a result message. This message
412 displays information that both images are same or the position
413 of the first different byte. In addition, result message can re‐
414 port different image size in case Strict mode is used.
415 Compare exits with 0 in case the images are equal and with 1 in
417 case the images differ. Other exit codes mean an error occurred
418 during execution and standard error output should contain an er‐
419 ror message. The following table sumarizes all exit codes of
420 the compare subcommand:
421 0 Images are identical (or requested help was printed)
423 1 Images differ
425 2 Error on opening an image
427 3 Error on checking a sector allocation
429 4 Error on reading data
431 convert [--object OBJECTDEF] [--image-opts] [--target-image-opts]
433 [--target-is-zero] [--bitmaps [--skip-broken-bitmaps]] [-U] [-C] [-c]
434 [-p] [-q] [-n] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-O OUTPUT_FMT] [-B
435 BACKING_FILE [-F BACKING_FMT]] [-o OPTIONS] [-l SNAPSHOT_PARAM] [-S
436 SPARSE_SIZE] [-r RATE_LIMIT] [-m NUM_COROUTINES] [-W] FILENAME [FILE‐
437 NAME2 [...]] OUTPUT_FILENAME
438 Convert the disk image FILENAME or a snapshot SNAPSHOT_PARAM to
439 disk image OUTPUT_FILENAME using format OUTPUT_FMT. It can be
440 optionally compressed (-c option) or use any format specific op‐
441 tions like encryption (-o option).
442 Only the formats qcow and qcow2 support compression. The com‐
444 pression is read-only. It means that if a compressed sector is
445 rewritten, then it is rewritten as uncompressed data.
446 Image conversion is also useful to get smaller image when using
448 a growable format such as qcow: the empty sectors are detected
449 and suppressed from the destination image.
450 SPARSE_SIZE indicates the consecutive number of bytes (defaults
452 to 4k) that must contain only zeros for qemu-img to create a
453 sparse image during conversion. If SPARSE_SIZE is 0, the source
454 will not be scanned for unallocated or zero sectors, and the
455 destination image will always be fully allocated.
456 You can use the BACKING_FILE option to force the output image to
458 be created as a copy on write image of the specified base image;
459 the BACKING_FILE should have the same content as the input's
460 base image, however the path, image format (as given by BACK‐
461 ING_FMT), etc may differ.
462 If a relative path name is given, the backing file is looked up
464 relative to the directory containing OUTPUT_FILENAME.
465 If the -n option is specified, the target volume creation will
467 be skipped. This is useful for formats such as rbd if the target
468 volume has already been created with site specific options that
469 cannot be supplied through qemu-img.
470 Out of order writes can be enabled with -W to improve perfor‐
472 mance. This is only recommended for preallocated devices like
473 host devices or other raw block devices. Out of order write does
474 not work in combination with creating compressed images.
475 NUM_COROUTINES specifies how many coroutines work in parallel
477 during the convert process (defaults to 8).
478 Use of --bitmaps requests that any persistent bitmaps present in
480 the original are also copied to the destination. If any bitmap
481 is inconsistent in the source, the conversion will fail unless
482 --skip-broken-bitmaps is also specified to copy only the consis‐
483 tent bitmaps.
484 create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE [-F BACK‐
486 ING_FMT]] [-u] [-o OPTIONS] FILENAME [SIZE]
487 Create the new disk image FILENAME of size SIZE and format FMT.
488 Depending on the file format, you can add one or more OPTIONS
489 that enable additional features of this format.
490 If the option BACKING_FILE is specified, then the image will
492 record only the differences from BACKING_FILE. No size needs to
493 be specified in this case. BACKING_FILE will never be modified
494 unless you use the commit monitor command (or qemu-img commit).
495 If a relative path name is given, the backing file is looked up
497 relative to the directory containing FILENAME.
498 Note that a given backing file will be opened to check that it
500 is valid. Use the -u option to enable unsafe backing file mode,
501 which means that the image will be created even if the associ‐
502 ated backing file cannot be opened. A matching backing file must
503 be created or additional options be used to make the backing
504 file specification valid when you want to use an image created
505 this way.
506 The size can also be specified using the SIZE option with -o, it
508 doesn't need to be specified separately in this case.
509 dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE]
511 [count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT
512 dd copies from INPUT file to OUTPUT file converting it from FMT
513 format to OUTPUT_FMT format.
514 The data is by default read and written using blocks of 512
516 bytes but can be modified by specifying BLOCK_SIZE. If
517 count=BLOCKS is specified dd will stop reading input after read‐
518 ing BLOCKS input blocks.
519 The size syntax is similar to dd(1)'s size syntax.
521 info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT]
523 [--backing-chain] [-U] FILENAME
524 Give information about the disk image FILENAME. Use it in par‐
525 ticular to know the size reserved on disk which can be different
526 from the displayed size. If VM snapshots are stored in the disk
527 image, they are displayed too.
528 If a disk image has a backing file chain, information about each
530 disk image in the chain can be recursively enumerated by using
531 the option --backing-chain.
532 For instance, if you have an image chain like:
534 base.qcow2 <- snap1.qcow2 <- snap2.qcow2
536 To enumerate information about each disk image in the above
538 chain, starting from top to base, do:
539 qemu-img info --backing-chain snap2.qcow2
541 The command can output in the format OFMT which is either human
543 or json. The JSON output is an object of QAPI type ImageInfo;
544 with --backing-chain, it is an array of ImageInfo objects.
545 --output=human reports the following information (for every im‐
547 age in the chain):
548 image The image file name
550 file format
552 The image format
553 virtual size
555 The size of the guest disk
556 disk size
558 How much space the image file occupies on the host file
559 system (may be shown as 0 if this information is unavail‐
560 able, e.g. because there is no file system)
561 cluster_size
563 Cluster size of the image format, if applicable
564 encrypted
566 Whether the image is encrypted (only present if so)
567 cleanly shut down
569 This is shown as no if the image is dirty and will have
570 to be auto-repaired the next time it is opened in qemu.
571 backing file
573 The backing file name, if present
574 backing file format
576 The format of the backing file, if the image enforces it
577 Snapshot list
579 A list of all internal snapshots
580 Format specific information
582 Further information whose structure depends on the image
583 format. This section is a textual representation of the
584 respective ImageInfoSpecific* QAPI object (e.g. ImageIn‐
585 foSpecificQCow2 for qcow2 images).
586 map [--object OBJECTDEF] [--image-opts] [-f FMT] [--start-offset=OFF‐
588 SET] [--max-length=LEN] [--output=OFMT] [-U] FILENAME
589 Dump the metadata of image FILENAME and its backing file chain.
590 In particular, this commands dumps the allocation state of every
591 sector of FILENAME, together with the topmost file that allo‐
592 cates it in the backing file chain.
593 Two option formats are possible. The default format (human)
595 only dumps known-nonzero areas of the file. Known-zero parts of
596 the file are omitted altogether, and likewise for parts that are
597 not allocated throughout the chain. qemu-img output will iden‐
598 tify a file from where the data can be read, and the offset in
599 the file. Each line will include four fields, the first three
600 of which are hexadecimal numbers. For example the first line
601 of:
602 Offset Length Mapped to File
604 0 0x20000 0x50000 /tmp/overlay.qcow2
605 0x100000 0x10000 0x95380000 /tmp/backing.qcow2
606 means that 0x20000 (131072) bytes starting at offset 0 in the
608 image are available in /tmp/overlay.qcow2 (opened in raw format)
609 starting at offset 0x50000 (327680). Data that is compressed,
610 encrypted, or otherwise not available in raw format will cause
611 an error if human format is in use. Note that file names can
612 include newlines, thus it is not safe to parse this output for‐
613 mat in scripts.
614 The alternative format json will return an array of dictionaries
616 in JSON format. It will include similar information in the
617 start, length, offset fields; it will also include other more
618 specific information:
619 • boolean field data: true if the sectors contain actual data,
621 false if the sectors are either unallocated or stored as opti‐
622 mized all-zero clusters
623 • boolean field zero: true if the data is known to read as zero
625 • boolean field present: true if the data belongs to the backing
627 chain, false if rebasing the backing chain onto a deeper file
628 would pick up data from the deeper file;
629 • integer field depth: the depth within the backing chain at
631 which the data was resolved; for example, a depth of 2 refers
632 to the backing file of the backing file of FILENAME.
633 In JSON format, the offset field is optional; it is absent in
635 cases where human format would omit the entry or exit with an
636 error. If data is false and the offset field is present, the
637 corresponding sectors in the file are not yet in use, but they
638 are preallocated.
639 For more information, consult include/block/block.h in QEMU's
641 source code.
642 measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--ob‐
644 ject OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME]
645 Calculate the file size required for a new image. This informa‐
646 tion can be used to size logical volumes or SAN LUNs appropri‐
647 ately for the image that will be placed in them. The values re‐
648 ported are guaranteed to be large enough to fit the image. The
649 command can output in the format OFMT which is either human or
650 json. The JSON output is an object of QAPI type BlockMeasure‐
651 Info.
652 If the size N is given then act as if creating a new empty image
654 file using qemu-img create. If FILENAME is given then act as if
655 converting an existing image file using qemu-img convert. The
656 format of the new file is given by OUTPUT_FMT while the format
657 of an existing file is given by FMT.
658 A snapshot in an existing image can be specified using SNAP‐
660 SHOT_PARAM.
661 The following fields are reported:
663 required size: 524288
665 fully allocated size: 1074069504
666 bitmaps size: 0
667 The required size is the file size of the new image. It may be
669 smaller than the virtual disk size if the image format supports
670 compact representation.
671 The fully allocated size is the file size of the new image once
673 data has been written to all sectors. This is the maximum size
674 that the image file can occupy with the exception of internal
675 snapshots, dirty bitmaps, vmstate data, and other advanced image
676 format features.
677 The bitmaps size is the additional size required in order to
679 copy bitmaps from a source image in addition to the guest-visi‐
680 ble data; the line is omitted if either source or destination
681 lacks bitmap support, or 0 if bitmaps are supported but there is
682 nothing to copy.
683 snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAP‐
685 SHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME
686 List, apply, create or delete snapshots in image FILENAME.
687 rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t
689 CACHE] [-T SRC_CACHE] [-p] [-u] -b BACKING_FILE [-F BACKING_FMT] FILE‐
690 NAME
691 Changes the backing file of an image. Only the formats qcow2 and
692 qed support changing the backing file.
693 The backing file is changed to BACKING_FILE and (if the image
695 format of FILENAME supports this) the backing file format is
696 changed to BACKING_FMT. If BACKING_FILE is specified as "" (the
697 empty string), then the image is rebased onto no backing file
698 (i.e. it will exist independently of any backing file).
699 If a relative path name is given, the backing file is looked up
701 relative to the directory containing FILENAME.
702 CACHE specifies the cache mode to be used for FILENAME, whereas
704 SRC_CACHE specifies the cache mode for reading backing files.
705 There are two different modes in which rebase can operate:
707 Safe mode
709 This is the default mode and performs a real rebase oper‐
710 ation. The new backing file may differ from the old one
711 and qemu-img rebase will take care of keeping the
712 guest-visible content of FILENAME unchanged.
713 In order to achieve this, any clusters that differ be‐
715 tween BACKING_FILE and the old backing file of FILENAME
716 are merged into FILENAME before actually changing the
717 backing file.
718 Note that the safe mode is an expensive operation, compa‐
720 rable to converting an image. It only works if the old
721 backing file still exists.
722 Unsafe mode
724 qemu-img uses the unsafe mode if -u is specified. In this
725 mode, only the backing file name and format of FILENAME
726 is changed without any checks on the file contents. The
727 user must take care of specifying the correct new backing
728 file, or the guest-visible content of the image will be
729 corrupted.
730 This mode is useful for renaming or moving the backing
732 file to somewhere else. It can be used without an acces‐
733 sible old backing file, i.e. you can use it to fix an im‐
734 age whose backing file has already been moved/renamed.
735 You can use rebase to perform a "diff" operation on two disk im‐
737 ages. This can be useful when you have copied or cloned a
738 guest, and you want to get back to a thin image on top of a tem‐
739 plate or base image.
740 Say that base.img has been cloned as modified.img by copying it,
742 and that the modified.img guest has run so there are now some
743 changes compared to base.img. To construct a thin image called
744 diff.qcow2 that contains just the differences, do:
745 qemu-img create -f qcow2 -b modified.img diff.qcow2
747 qemu-img rebase -b base.img diff.qcow2
748 At this point, modified.img can be discarded, since base.img +
750 diff.qcow2 contains the same information.
751 resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--prealloca‐
753 tion=PREALLOC] [-q] [--shrink] FILENAME [+ | -]SIZE
754 Change the disk image as if it had been created with SIZE.
755 Before using this command to shrink a disk image, you MUST use
757 file system and partitioning tools inside the VM to reduce allo‐
758 cated file systems and partition sizes accordingly. Failure to
759 do so will result in data loss!
760 When shrinking images, the --shrink option must be given. This
762 informs qemu-img that the user acknowledges all loss of data be‐
763 yond the truncated image's end.
764 After using this command to grow a disk image, you must use file
766 system and partitioning tools inside the VM to actually begin
767 using the new space on the device.
768 When growing an image, the --preallocation option may be used to
770 specify how the additional image area should be allocated on the
771 host. See the format description in the Notes section which
772 values are allowed. Using this option may result in slightly
773 more data being allocated than necessary.
774
776 Supported image file formats:
777 raw
779 Raw disk image format (default). This format has the advantage of
780 being simple and easily exportable to all other emulators. If your
781 file system supports holes (for example in ext2 or ext3 on Linux or
782 NTFS on Windows), then only the written sectors will reserve space.
783 Use qemu-img info to know the real size used by the image or ls -ls
784 on Unix/Linux.
785 Supported options:
787 preallocation
789 Preallocation mode (allowed values: off, falloc, full). fal‐
790 loc mode preallocates space for image by calling posix_fallo‐
791 cate(). full mode preallocates space for image by writing
792 data to underlying storage. This data may or may not be
793 zero, depending on the storage location.
794 qcow2
796 QEMU image format, the most versatile format. Use it to have smaller
797 images (useful if your filesystem does not supports holes, for exam‐
798 ple on Windows), optional AES encryption, zlib based compression and
799 support of multiple VM snapshots.
800 Supported options:
802 compat Determines the qcow2 version to use. compat=0.10 uses the
804 traditional image format that can be read by any QEMU since
805 0.10. compat=1.1 enables image format extensions that only
806 QEMU 1.1 and newer understand (this is the default). Amongst
807 others, this includes zero clusters, which allow efficient
808 copy-on-read for sparse images.
809 backing_file
811 File name of a base image (see create subcommand)
812 backing_fmt
814 Image format of the base image
815 encryption
817 If this option is set to on, the image is encrypted with
818 128-bit AES-CBC.
819 The use of encryption in qcow and qcow2 images is considered
821 to be flawed by modern cryptography standards, suffering from
822 a number of design problems:
823 • The AES-CBC cipher is used with predictable initialization
825 vectors based on the sector number. This makes it vulnera‐
826 ble to chosen plaintext attacks which can reveal the exis‐
827 tence of encrypted data.
828 • The user passphrase is directly used as the encryption key.
830 A poorly chosen or short passphrase will compromise the se‐
831 curity of the encryption.
832 • In the event of the passphrase being compromised there is
834 no way to change the passphrase to protect data in any qcow
835 images. The files must be cloned, using a different encryp‐
836 tion passphrase in the new file. The original file must
837 then be securely erased using a program like shred, though
838 even this is ineffective with many modern storage technolo‐
839 gies.
840 • Initialization vectors used to encrypt sectors are based on
842 the guest virtual sector number, instead of the host physi‐
843 cal sector. When a disk image has multiple internal snap‐
844 shots this means that data in multiple physical sectors is
845 encrypted with the same initialization vector. With the CBC
846 mode, this opens the possibility of watermarking attacks if
847 the attack can collect multiple sectors encrypted with the
848 same IV and some predictable data. Having multiple qcow2
849 images with the same passphrase also exposes this weakness
850 since the passphrase is directly used as the key.
851 Use of qcow / qcow2 encryption is thus strongly discouraged.
853 Users are recommended to use an alternative encryption tech‐
854 nology such as the Linux dm-crypt / LUKS system.
855 cluster_size
857 Changes the qcow2 cluster size (must be between 512 and 2M).
858 Smaller cluster sizes can improve the image file size whereas
859 larger cluster sizes generally provide better performance.
860 preallocation
862 Preallocation mode (allowed values: off, metadata, falloc,
863 full). An image with preallocated metadata is initially
864 larger but can improve performance when the image needs to
865 grow. falloc and full preallocations are like the same op‐
866 tions of raw format, but sets up metadata also.
867 lazy_refcounts
869 If this option is set to on, reference count updates are
870 postponed with the goal of avoiding metadata I/O and improv‐
871 ing performance. This is particularly interesting with
872 cache=writethrough which doesn't batch metadata updates. The
873 tradeoff is that after a host crash, the reference count ta‐
874 bles must be rebuilt, i.e. on the next open an (automatic)
875 qemu-img check -r all is required, which may take some time.
876 This option can only be enabled if compat=1.1 is specified.
878 nocow If this option is set to on, it will turn off COW of the
880 file. It's only valid on btrfs, no effect on other file sys‐
881 tems.
882 Btrfs has low performance when hosting a VM image file, even
884 more when the guest on the VM also using btrfs as file sys‐
885 tem. Turning off COW is a way to mitigate this bad perfor‐
886 mance. Generally there are two ways to turn off COW on btrfs:
887 • Disable it by mounting with nodatacow, then all newly cre‐
889 ated files will be NOCOW
890 • For an empty file, add the NOCOW file attribute. That's
892 what this option does.
893 Note: this option is only valid to new or empty files. If
895 there is an existing file which is COW and has data blocks
896 already, it couldn't be changed to NOCOW by setting nocow=on.
897 One can issue lsattr filename to check if the NOCOW flag is
898 set or not (Capital 'C' is NOCOW flag).
899 data_file
901 Filename where all guest data will be stored. If this option
902 is used, the qcow2 file will only contain the image's meta‐
903 data.
904 Note: Data loss will occur if the given filename already ex‐
906 ists when using this option with qemu-img create since
907 qemu-img will create the data file anew, overwriting the
908 file's original contents. To simply update the reference to
909 point to the given pre-existing file, use qemu-img amend.
910 data_file_raw
912 If this option is set to on, QEMU will always keep the exter‐
913 nal data file consistent as a standalone read-only raw image.
914 It does this by forwarding all write accesses to the qcow2
916 file through to the raw data file, including their offsets.
917 Therefore, data that is visible on the qcow2 node (i.e., to
918 the guest) at some offset is visible at the same offset in
919 the raw data file. This results in a read-only raw image.
920 Writes that bypass the qcow2 metadata may corrupt the qcow2
921 metadata because the out-of-band writes may result in the
922 metadata falling out of sync with the raw image.
923 If this option is off, QEMU will use the data file to store
925 data in an arbitrary manner. The file’s content will not make
926 sense without the accompanying qcow2 metadata. Where data is
927 written will have no relation to its offset as seen by the
928 guest, and some writes (specifically zero writes) may not be
929 forwarded to the data file at all, but will only be handled
930 by modifying qcow2 metadata.
931 This option can only be enabled if data_file is set.
933 Other
935 QEMU also supports various other image file formats for compatibil‐
936 ity with older QEMU versions or other hypervisors, including VMDK,
937 VDI, VHD (vpc), VHDX, qcow1 and QED. For a full list of supported
938 formats see qemu-img --help. For a more detailed description of
939 these formats, see the QEMU block drivers reference documentation.
940 The main purpose of the block drivers for these formats is image
942 conversion. For running VMs, it is recommended to convert the disk
943 images to either raw or qcow2 in order to achieve good performance.
944
946 Fabrice Bellard
947
949 2023, The QEMU Project Developers
9508.1.3 Nov 28, 2023 QEMU-IMG(1)