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] [-o OP‐
77 TIONS] [-l SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-r RATE_LIMIT] [-m
78 NUM_COROUTINES] [-W] [--salvage] FILENAME [FILENAME2 [...]] OUT‐
79 PUT_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 ? for an overview of the options supported by
123 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 and vdi support consistency checks.
343 In case the image does not have any inconsistencies, check exits
345 with 0. Other exit codes indicate the kind of inconsistency
346 found or if another error occurred. The following table summa‐
347 rizes all exit codes of the check subcommand:
348 0 Check completed, the image is (now) consistent
350 1 Check not completed because of internal errors
352 2 Check completed, image is corrupted
354 3 Check completed, image has leaked clusters, but is not
356 corrupted
357 63 Checks are not supported by the image format
359 If -r is specified, exit codes representing the image state re‐
361 fer to the state after (the attempt at) repairing it. That is, a
362 successful -r all will yield the exit code 0, independently of
363 the image state before.
364 commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b
366 BASE] [-r RATE_LIMIT] [-d] [-p] FILENAME
367 Commit the changes recorded in FILENAME in its base image or
368 backing file. If the backing file is smaller than the snapshot,
369 then the backing file will be resized to be the same size as the
370 snapshot. If the snapshot is smaller than the backing file, the
371 backing file will not be truncated. If you want the backing
372 file to match the size of the smaller snapshot, you can safely
373 truncate it yourself once the commit operation successfully com‐
374 pletes.
375 The image FILENAME is emptied after the operation has succeeded.
377 If you do not need FILENAME afterwards and intend to drop it,
378 you may skip emptying FILENAME by specifying the -d flag.
379 If the backing chain of the given image file FILENAME has more
381 than one layer, the backing file into which the changes will be
382 committed may be specified as BASE (which has to be part of
383 FILENAME's backing chain). If BASE is not specified, the immedi‐
384 ate backing file of the top image (which is FILENAME) will be
385 used. Note that after a commit operation all images between BASE
386 and the top image will be invalid and may return garbage data
387 when read. For this reason, -b implies -d (so that the top image
388 stays valid).
389 The rate limit for the commit process is specified by -r.
391 compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T
393 SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 FILENAME2
394 Check if two images have the same content. You can compare im‐
395 ages with different format or settings.
396 The format is probed unless you specify it by -f (used for FILE‐
398 NAME1) and/or -F (used for FILENAME2) option.
399 By default, images with different size are considered identical
401 if the larger image contains only unallocated and/or zeroed sec‐
402 tors in the area after the end of the other image. In addition,
403 if any sector is not allocated in one image and contains only
404 zero bytes in the second one, it is evaluated as equal. You can
405 use Strict mode by specifying the -s option. When compare runs
406 in Strict mode, it fails in case image size differs or a sector
407 is allocated in one image and is not allocated in the second
408 one.
409 By default, compare prints out a result message. This message
411 displays information that both images are same or the position
412 of the first different byte. In addition, result message can re‐
413 port different image size in case Strict mode is used.
414 Compare exits with 0 in case the images are equal and with 1 in
416 case the images differ. Other exit codes mean an error occurred
417 during execution and standard error output should contain an er‐
418 ror message. The following table sumarizes all exit codes of
419 the compare subcommand:
420 0 Images are identical
422 1 Images differ
424 2 Error on opening an image
426 3 Error on checking a sector allocation
428 4 Error on reading data
430 convert [--object OBJECTDEF] [--image-opts] [--target-image-opts]
432 [--target-is-zero] [--bitmaps] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT]
433 [-t CACHE] [-T SRC_CACHE] [-O OUTPUT_FMT] [-B BACKING_FILE] [-o OP‐
434 TIONS] [-l SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-r RATE_LIMIT] [-m
435 NUM_COROUTINES] [-W] FILENAME [FILENAME2 [...]] OUTPUT_FILENAME
436 Convert the disk image FILENAME or a snapshot SNAPSHOT_PARAM to
437 disk image OUTPUT_FILENAME using format OUTPUT_FMT. It can be
438 optionally compressed (-c option) or use any format specific op‐
439 tions like encryption (-o option).
440 Only the formats qcow and qcow2 support compression. The com‐
442 pression is read-only. It means that if a compressed sector is
443 rewritten, then it is rewritten as uncompressed data.
444 Image conversion is also useful to get smaller image when using
446 a growable format such as qcow: the empty sectors are detected
447 and suppressed from the destination image.
448 SPARSE_SIZE indicates the consecutive number of bytes (defaults
450 to 4k) that must contain only zeros for qemu-img to create a
451 sparse image during conversion. If SPARSE_SIZE is 0, the source
452 will not be scanned for unallocated or zero sectors, and the
453 destination image will always be fully allocated.
454 You can use the BACKING_FILE option to force the output image to
456 be created as a copy on write image of the specified base image;
457 the BACKING_FILE should have the same content as the input's
458 base image, however the path, image format, etc may differ.
459 If a relative path name is given, the backing file is looked up
461 relative to the directory containing OUTPUT_FILENAME.
462 If the -n option is specified, the target volume creation will
464 be skipped. This is useful for formats such as rbd if the target
465 volume has already been created with site specific options that
466 cannot be supplied through qemu-img.
467 Out of order writes can be enabled with -W to improve perfor‐
469 mance. This is only recommended for preallocated devices like
470 host devices or other raw block devices. Out of order write does
471 not work in combination with creating compressed images.
472 NUM_COROUTINES specifies how many coroutines work in parallel
474 during the convert process (defaults to 8).
475 create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE] [-F BACK‐
477 ING_FMT] [-u] [-o OPTIONS] FILENAME [SIZE]
478 Create the new disk image FILENAME of size SIZE and format FMT.
479 Depending on the file format, you can add one or more OPTIONS
480 that enable additional features of this format.
481 If the option BACKING_FILE is specified, then the image will
483 record only the differences from BACKING_FILE. No size needs to
484 be specified in this case. BACKING_FILE will never be modified
485 unless you use the commit monitor command (or qemu-img commit).
486 If a relative path name is given, the backing file is looked up
488 relative to the directory containing FILENAME.
489 Note that a given backing file will be opened to check that it
491 is valid. Use the -u option to enable unsafe backing file mode,
492 which means that the image will be created even if the associ‐
493 ated backing file cannot be opened. A matching backing file must
494 be created or additional options be used to make the backing
495 file specification valid when you want to use an image created
496 this way.
497 The size can also be specified using the SIZE option with -o, it
499 doesn't need to be specified separately in this case.
500 dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE]
502 [count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT
503 dd copies from INPUT file to OUTPUT file converting it from FMT
504 format to OUTPUT_FMT format.
505 The data is by default read and written using blocks of 512
507 bytes but can be modified by specifying BLOCK_SIZE. If
508 count=BLOCKS is specified dd will stop reading input after read‐
509 ing BLOCKS input blocks.
510 The size syntax is similar to dd(1)'s size syntax.
512 info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT]
514 [--backing-chain] [-U] FILENAME
515 Give information about the disk image FILENAME. Use it in par‐
516 ticular to know the size reserved on disk which can be different
517 from the displayed size. If VM snapshots are stored in the disk
518 image, they are displayed too.
519 If a disk image has a backing file chain, information about each
521 disk image in the chain can be recursively enumerated by using
522 the option --backing-chain.
523 For instance, if you have an image chain like:
525 base.qcow2 <- snap1.qcow2 <- snap2.qcow2
527 To enumerate information about each disk image in the above
529 chain, starting from top to base, do:
530 qemu-img info --backing-chain snap2.qcow2
532 The command can output in the format OFMT which is either human
534 or json. The JSON output is an object of QAPI type ImageInfo;
535 with --backing-chain, it is an array of ImageInfo objects.
536 --output=human reports the following information (for every im‐
538 age in the chain):
539 image The image file name
541 file format
543 The image format
544 virtual size
546 The size of the guest disk
547 disk size
549 How much space the image file occupies on the host file
550 system (may be shown as 0 if this information is unavail‐
551 able, e.g. because there is no file system)
552 cluster_size
554 Cluster size of the image format, if applicable
555 encrypted
557 Whether the image is encrypted (only present if so)
558 cleanly shut down
560 This is shown as no if the image is dirty and will have
561 to be auto-repaired the next time it is opened in qemu.
562 backing file
564 The backing file name, if present
565 backing file format
567 The format of the backing file, if the image enforces it
568 Snapshot list
570 A list of all internal snapshots
571 Format specific information
573 Further information whose structure depends on the image
574 format. This section is a textual representation of the
575 respective ImageInfoSpecific* QAPI object (e.g. ImageIn‐
576 foSpecificQCow2 for qcow2 images).
577 map [--object OBJECTDEF] [--image-opts] [-f FMT] [--start-offset=OFF‐
579 SET] [--max-length=LEN] [--output=OFMT] [-U] FILENAME
580 Dump the metadata of image FILENAME and its backing file chain.
581 In particular, this commands dumps the allocation state of every
582 sector of FILENAME, together with the topmost file that allo‐
583 cates it in the backing file chain.
584 Two option formats are possible. The default format (human)
586 only dumps known-nonzero areas of the file. Known-zero parts of
587 the file are omitted altogether, and likewise for parts that are
588 not allocated throughout the chain. qemu-img output will iden‐
589 tify a file from where the data can be read, and the offset in
590 the file. Each line will include four fields, the first three
591 of which are hexadecimal numbers. For example the first line
592 of:
593 Offset Length Mapped to File
595 0 0x20000 0x50000 /tmp/overlay.qcow2
596 0x100000 0x10000 0x95380000 /tmp/backing.qcow2
597 means that 0x20000 (131072) bytes starting at offset 0 in the
599 image are available in /tmp/overlay.qcow2 (opened in raw format)
600 starting at offset 0x50000 (327680). Data that is compressed,
601 encrypted, or otherwise not available in raw format will cause
602 an error if human format is in use. Note that file names can
603 include newlines, thus it is not safe to parse this output for‐
604 mat in scripts.
605 The alternative format json will return an array of dictionaries
607 in JSON format. It will include similar information in the
608 start, length, offset fields; it will also include other more
609 specific information:
610 • whether the sectors contain actual data or not (boolean field
612 data; if false, the sectors are either unallocated or stored
613 as optimized all-zero clusters);
614 • whether the data is known to read as zero (boolean field
616 zero);
617 • in order to make the output shorter, the target file is ex‐
619 pressed as a depth; for example, a depth of 2 refers to the
620 backing file of the backing file of FILENAME.
621 In JSON format, the offset field is optional; it is absent in
623 cases where human format would omit the entry or exit with an
624 error. If data is false and the offset field is present, the
625 corresponding sectors in the file are not yet in use, but they
626 are preallocated.
627 For more information, consult include/block/block.h in QEMU's
629 source code.
630 measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--ob‐
632 ject OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME]
633 Calculate the file size required for a new image. This informa‐
634 tion can be used to size logical volumes or SAN LUNs appropri‐
635 ately for the image that will be placed in them. The values re‐
636 ported are guaranteed to be large enough to fit the image. The
637 command can output in the format OFMT which is either human or
638 json. The JSON output is an object of QAPI type BlockMeasure‐
639 Info.
640 If the size N is given then act as if creating a new empty image
642 file using qemu-img create. If FILENAME is given then act as if
643 converting an existing image file using qemu-img convert. The
644 format of the new file is given by OUTPUT_FMT while the format
645 of an existing file is given by FMT.
646 A snapshot in an existing image can be specified using SNAP‐
648 SHOT_PARAM.
649 The following fields are reported:
651 required size: 524288
653 fully allocated size: 1074069504
654 bitmaps size: 0
655 The required size is the file size of the new image. It may be
657 smaller than the virtual disk size if the image format supports
658 compact representation.
659 The fully allocated size is the file size of the new image once
661 data has been written to all sectors. This is the maximum size
662 that the image file can occupy with the exception of internal
663 snapshots, dirty bitmaps, vmstate data, and other advanced image
664 format features.
665 The bitmaps size is the additional size required in order to
667 copy bitmaps from a source image in addition to the guest-visi‐
668 ble data; the line is omitted if either source or destination
669 lacks bitmap support, or 0 if bitmaps are supported but there is
670 nothing to copy.
671 snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAP‐
673 SHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME
674 List, apply, create or delete snapshots in image FILENAME.
675 rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t
677 CACHE] [-T SRC_CACHE] [-p] [-u] -b BACKING_FILE [-F BACKING_FMT] FILE‐
678 NAME
679 Changes the backing file of an image. Only the formats qcow2 and
680 qed support changing the backing file.
681 The backing file is changed to BACKING_FILE and (if the image
683 format of FILENAME supports this) the backing file format is
684 changed to BACKING_FMT. If BACKING_FILE is specified as "" (the
685 empty string), then the image is rebased onto no backing file
686 (i.e. it will exist independently of any backing file).
687 If a relative path name is given, the backing file is looked up
689 relative to the directory containing FILENAME.
690 CACHE specifies the cache mode to be used for FILENAME, whereas
692 SRC_CACHE specifies the cache mode for reading backing files.
693 There are two different modes in which rebase can operate:
695 Safe mode
697 This is the default mode and performs a real rebase oper‐
698 ation. The new backing file may differ from the old one
699 and qemu-img rebase will take care of keeping the
700 guest-visible content of FILENAME unchanged.
701 In order to achieve this, any clusters that differ be‐
703 tween BACKING_FILE and the old backing file of FILENAME
704 are merged into FILENAME before actually changing the
705 backing file.
706 Note that the safe mode is an expensive operation, compa‐
708 rable to converting an image. It only works if the old
709 backing file still exists.
710 Unsafe mode
712 qemu-img uses the unsafe mode if -u is specified. In this
713 mode, only the backing file name and format of FILENAME
714 is changed without any checks on the file contents. The
715 user must take care of specifying the correct new backing
716 file, or the guest-visible content of the image will be
717 corrupted.
718 This mode is useful for renaming or moving the backing
720 file to somewhere else. It can be used without an acces‐
721 sible old backing file, i.e. you can use it to fix an im‐
722 age whose backing file has already been moved/renamed.
723 You can use rebase to perform a "diff" operation on two disk im‐
725 ages. This can be useful when you have copied or cloned a
726 guest, and you want to get back to a thin image on top of a tem‐
727 plate or base image.
728 Say that base.img has been cloned as modified.img by copying it,
730 and that the modified.img guest has run so there are now some
731 changes compared to base.img. To construct a thin image called
732 diff.qcow2 that contains just the differences, do:
733 qemu-img create -f qcow2 -b modified.img diff.qcow2
735 qemu-img rebase -b base.img diff.qcow2
736 At this point, modified.img can be discarded, since base.img +
738 diff.qcow2 contains the same information.
739 resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--prealloca‐
741 tion=PREALLOC] [-q] [--shrink] FILENAME [+ | -]SIZE
742 Change the disk image as if it had been created with SIZE.
743 Before using this command to shrink a disk image, you MUST use
745 file system and partitioning tools inside the VM to reduce allo‐
746 cated file systems and partition sizes accordingly. Failure to
747 do so will result in data loss!
748 When shrinking images, the --shrink option must be given. This
750 informs qemu-img that the user acknowledges all loss of data be‐
751 yond the truncated image's end.
752 After using this command to grow a disk image, you must use file
754 system and partitioning tools inside the VM to actually begin
755 using the new space on the device.
756 When growing an image, the --preallocation option may be used to
758 specify how the additional image area should be allocated on the
759 host. See the format description in the Notes section which
760 values are allowed. Using this option may result in slightly
761 more data being allocated than necessary.
762
764 Supported image file formats:
765 raw
767 Raw disk image format (default). This format has the advantage of
768 being simple and easily exportable to all other emulators. If your
769 file system supports holes (for example in ext2 or ext3 on Linux or
770 NTFS on Windows), then only the written sectors will reserve space.
771 Use qemu-img info to know the real size used by the image or ls -ls
772 on Unix/Linux.
773 Supported options:
775 preallocation
777 Preallocation mode (allowed values: off, falloc, full). fal‐
778 loc mode preallocates space for image by calling posix_fallo‐
779 cate(). full mode preallocates space for image by writing
780 data to underlying storage. This data may or may not be
781 zero, depending on the storage location.
782 qcow2
784 QEMU image format, the most versatile format. Use it to have smaller
785 images (useful if your filesystem does not supports holes, for exam‐
786 ple on Windows), optional AES encryption, zlib based compression and
787 support of multiple VM snapshots.
788 Supported options:
790 compat Determines the qcow2 version to use. compat=0.10 uses the
792 traditional image format that can be read by any QEMU since
793 0.10. compat=1.1 enables image format extensions that only
794 QEMU 1.1 and newer understand (this is the default). Amongst
795 others, this includes zero clusters, which allow efficient
796 copy-on-read for sparse images.
797 backing_file
799 File name of a base image (see create subcommand)
800 backing_fmt
802 Image format of the base image
803 encryption
805 If this option is set to on, the image is encrypted with
806 128-bit AES-CBC.
807 The use of encryption in qcow and qcow2 images is considered
809 to be flawed by modern cryptography standards, suffering from
810 a number of design problems:
811 • The AES-CBC cipher is used with predictable initialization
813 vectors based on the sector number. This makes it vulnera‐
814 ble to chosen plaintext attacks which can reveal the exis‐
815 tence of encrypted data.
816 • The user passphrase is directly used as the encryption key.
818 A poorly chosen or short passphrase will compromise the se‐
819 curity of the encryption.
820 • In the event of the passphrase being compromised there is
822 no way to change the passphrase to protect data in any qcow
823 images. The files must be cloned, using a different encryp‐
824 tion passphrase in the new file. The original file must
825 then be securely erased using a program like shred, though
826 even this is ineffective with many modern storage technolo‐
827 gies.
828 • Initialization vectors used to encrypt sectors are based on
830 the guest virtual sector number, instead of the host physi‐
831 cal sector. When a disk image has multiple internal snap‐
832 shots this means that data in multiple physical sectors is
833 encrypted with the same initialization vector. With the CBC
834 mode, this opens the possibility of watermarking attacks if
835 the attack can collect multiple sectors encrypted with the
836 same IV and some predictable data. Having multiple qcow2
837 images with the same passphrase also exposes this weakness
838 since the passphrase is directly used as the key.
839 Use of qcow / qcow2 encryption is thus strongly discouraged.
841 Users are recommended to use an alternative encryption tech‐
842 nology such as the Linux dm-crypt / LUKS system.
843 cluster_size
845 Changes the qcow2 cluster size (must be between 512 and 2M).
846 Smaller cluster sizes can improve the image file size whereas
847 larger cluster sizes generally provide better performance.
848 preallocation
850 Preallocation mode (allowed values: off, metadata, falloc,
851 full). An image with preallocated metadata is initially
852 larger but can improve performance when the image needs to
853 grow. falloc and full preallocations are like the same op‐
854 tions of raw format, but sets up metadata also.
855 lazy_refcounts
857 If this option is set to on, reference count updates are
858 postponed with the goal of avoiding metadata I/O and improv‐
859 ing performance. This is particularly interesting with
860 cache=writethrough which doesn't batch metadata updates. The
861 tradeoff is that after a host crash, the reference count ta‐
862 bles must be rebuilt, i.e. on the next open an (automatic)
863 qemu-img check -r all is required, which may take some time.
864 This option can only be enabled if compat=1.1 is specified.
866 nocow If this option is set to on, it will turn off COW of the
868 file. It's only valid on btrfs, no effect on other file sys‐
869 tems.
870 Btrfs has low performance when hosting a VM image file, even
872 more when the guest on the VM also using btrfs as file sys‐
873 tem. Turning off COW is a way to mitigate this bad perfor‐
874 mance. Generally there are two ways to turn off COW on btrfs:
875 • Disable it by mounting with nodatacow, then all newly cre‐
877 ated files will be NOCOW
878 • For an empty file, add the NOCOW file attribute. That's
880 what this option does.
881 Note: this option is only valid to new or empty files. If
883 there is an existing file which is COW and has data blocks
884 already, it couldn't be changed to NOCOW by setting nocow=on.
885 One can issue lsattr filename to check if the NOCOW flag is
886 set or not (Capital 'C' is NOCOW flag).
887 Other
889 QEMU also supports various other image file formats for compatibil‐
890 ity with older QEMU versions or other hypervisors, including VMDK,
891 VDI, VHD (vpc), VHDX, qcow1 and QED. For a full list of supported
892 formats see qemu-img --help. For a more detailed description of
893 these formats, see the QEMU block drivers reference documentation.
894 The main purpose of the block drivers for these formats is image
896 conversion. For running VMs, it is recommended to convert the disk
897 images to either raw or qcow2 in order to achieve good performance.
898
900 Fabrice Bellard
901
903 2021, The QEMU Project Developers
9045.2.0 May 19, 2021 QEMU-IMG(1)