1======================= 2QEMU disk image utility 3======================= 4 5Synopsis 6-------- 7 8**qemu-img** [*standard options*] *command* [*command options*] 9 10Description 11----------- 12 13qemu-img allows you to create, convert and modify images offline. It can handle 14all image formats supported by QEMU. 15 16**Warning:** Never use qemu-img to modify images in use by a running virtual 17machine or any other process; this may destroy the image. Also, be aware that 18querying an image that is being modified by another process may encounter 19inconsistent state. 20 21Options 22------- 23 24.. program:: qemu-img 25 26Standard options: 27 28.. option:: -h, --help 29 30 Display this help and exit 31 32.. option:: -V, --version 33 34 Display version information and exit 35 36.. option:: -T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE] 37 38 .. include:: ../qemu-option-trace.rst.inc 39 40The following commands are supported: 41 42.. hxtool-doc:: qemu-img-cmds.hx 43 44Command parameters: 45 46*FILENAME* is a disk image filename. 47 48*FMT* is the disk image format. It is guessed automatically in most 49cases. See below for a description of the supported disk formats. 50 51*SIZE* is the disk image size in bytes. Optional suffixes ``k`` or 52``K`` (kilobyte, 1024) ``M`` (megabyte, 1024k) and ``G`` (gigabyte, 531024M) and T (terabyte, 1024G) are supported. ``b`` is ignored. 54 55*OUTPUT_FILENAME* is the destination disk image filename. 56 57*OUTPUT_FMT* is the destination format. 58 59*OPTIONS* is a comma separated list of format specific options in a 60name=value format. Use ``-o ?`` for an overview of the options supported 61by the used format or see the format descriptions below for details. 62 63*SNAPSHOT_PARAM* is param used for internal snapshot, format is 64'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'. 65 66.. 67 Note the use of a new 'program'; otherwise Sphinx complains about 68 the -h option appearing both in the above option list and this one. 69 70.. program:: qemu-img-common-opts 71 72.. option:: --object OBJECTDEF 73 74 is a QEMU user creatable object definition. See the :manpage:`qemu(1)` 75 manual page for a description of the object properties. The most common 76 object type is a ``secret``, which is used to supply passwords and/or 77 encryption keys. 78 79.. option:: --image-opts 80 81 Indicates that the source *FILENAME* parameter is to be interpreted as a 82 full option string, not a plain filename. This parameter is mutually 83 exclusive with the *-f* parameter. 84 85.. option:: --target-image-opts 86 87 Indicates that the OUTPUT_FILENAME parameter(s) are to be interpreted as 88 a full option string, not a plain filename. This parameter is mutually 89 exclusive with the *-O* parameters. It is currently required to also use 90 the *-n* parameter to skip image creation. This restriction may be relaxed 91 in a future release. 92 93.. option:: --force-share (-U) 94 95 If specified, ``qemu-img`` will open the image in shared mode, allowing 96 other QEMU processes to open it in write mode. For example, this can be used to 97 get the image information (with 'info' subcommand) when the image is used by a 98 running guest. Note that this could produce inconsistent results because of 99 concurrent metadata changes, etc. This option is only allowed when opening 100 images in read-only mode. 101 102.. option:: --backing-chain 103 104 Will enumerate information about backing files in a disk image chain. Refer 105 below for further description. 106 107.. option:: -c 108 109 Indicates that target image must be compressed (qcow format only). 110 111.. option:: -h 112 113 With or without a command, shows help and lists the supported formats. 114 115.. option:: -p 116 117 Display progress bar (compare, convert and rebase commands only). 118 If the *-p* option is not used for a command that supports it, the 119 progress is reported when the process receives a ``SIGUSR1`` or 120 ``SIGINFO`` signal. 121 122.. option:: -q 123 124 Quiet mode - do not print any output (except errors). There's no progress bar 125 in case both *-q* and *-p* options are used. 126 127.. option:: -S SIZE 128 129 Indicates the consecutive number of bytes that must contain only zeros 130 for ``qemu-img`` to create a sparse image during conversion. This value is 131 rounded down to the nearest 512 bytes. You may use the common size suffixes 132 like ``k`` for kilobytes. 133 134.. option:: -t CACHE 135 136 Specifies the cache mode that should be used with the (destination) file. See 137 the documentation of the emulator's ``-drive cache=...`` option for allowed 138 values. 139 140.. option:: -T SRC_CACHE 141 142 Specifies the cache mode that should be used with the source file(s). See 143 the documentation of the emulator's ``-drive cache=...`` option for allowed 144 values. 145 146Parameters to compare subcommand: 147 148.. program:: qemu-img-compare 149 150.. option:: -f 151 152 First image format 153 154.. option:: -F 155 156 Second image format 157 158.. option:: -s 159 160 Strict mode - fail on different image size or sector allocation 161 162Parameters to convert subcommand: 163 164.. program:: qemu-img-convert 165 166.. option:: --bitmaps 167 168 Additionally copy all persistent bitmaps from the top layer of the source 169 170.. option:: -n 171 172 Skip the creation of the target volume 173 174.. option:: -m 175 176 Number of parallel coroutines for the convert process 177 178.. option:: -W 179 180 Allow out-of-order writes to the destination. This option improves performance, 181 but is only recommended for preallocated devices like host devices or other 182 raw block devices. 183 184.. option:: -C 185 186 Try to use copy offloading to move data from source image to target. This may 187 improve performance if the data is remote, such as with NFS or iSCSI backends, 188 but will not automatically sparsify zero sectors, and may result in a fully 189 allocated target image depending on the host support for getting allocation 190 information. 191 192.. option:: -r 193 194 Rate limit for the convert process 195 196.. option:: --salvage 197 198 Try to ignore I/O errors when reading. Unless in quiet mode (``-q``), errors 199 will still be printed. Areas that cannot be read from the source will be 200 treated as containing only zeroes. 201 202.. option:: --target-is-zero 203 204 Assume that reading the destination image will always return 205 zeros. This parameter is mutually exclusive with a destination image 206 that has a backing file. It is required to also use the ``-n`` 207 parameter to skip image creation. 208 209Parameters to dd subcommand: 210 211.. program:: qemu-img-dd 212 213.. option:: bs=BLOCK_SIZE 214 215 Defines the block size 216 217.. option:: count=BLOCKS 218 219 Sets the number of input blocks to copy 220 221.. option:: if=INPUT 222 223 Sets the input file 224 225.. option:: of=OUTPUT 226 227 Sets the output file 228 229.. option:: skip=BLOCKS 230 231 Sets the number of input blocks to skip 232 233Parameters to snapshot subcommand: 234 235.. program:: qemu-img-snapshot 236 237.. option:: snapshot 238 239 Is the name of the snapshot to create, apply or delete 240 241.. option:: -a 242 243 Applies a snapshot (revert disk to saved state) 244 245.. option:: -c 246 247 Creates a snapshot 248 249.. option:: -d 250 251 Deletes a snapshot 252 253.. option:: -l 254 255 Lists all snapshots in the given image 256 257Command description: 258 259.. program:: qemu-img-commands 260 261.. option:: amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE] [--force] -o OPTIONS FILENAME 262 263 Amends the image format specific *OPTIONS* for the image file 264 *FILENAME*. Not all file formats support this operation. 265 266 The set of options that can be amended are dependent on the image 267 format, but note that amending the backing chain relationship should 268 instead be performed with ``qemu-img rebase``. 269 270 --force allows some unsafe operations. Currently for -f luks, it allows to 271 erase the last encryption key, and to overwrite an active encryption key. 272 273.. 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 274 275 Run a simple sequential I/O benchmark on the specified image. If ``-w`` is 276 specified, a write test is performed, otherwise a read test is performed. 277 278 A total number of *COUNT* I/O requests is performed, each *BUFFER_SIZE* 279 bytes in size, and with *DEPTH* requests in parallel. The first request 280 starts at the position given by *OFFSET*, each following request increases 281 the current position by *STEP_SIZE*. If *STEP_SIZE* is not given, 282 *BUFFER_SIZE* is used for its value. 283 284 If *FLUSH_INTERVAL* is specified for a write test, the request queue is 285 drained and a flush is issued before new writes are made whenever the number of 286 remaining requests is a multiple of *FLUSH_INTERVAL*. If additionally 287 ``--no-drain`` is specified, a flush is issued without draining the request 288 queue first. 289 290 if ``-i`` is specified, *AIO* option can be used to specify different 291 AIO backends: ``threads``, ``native`` or ``io_uring``. 292 293 If ``-n`` is specified, the native AIO backend is used if possible. On 294 Linux, this option only works if ``-t none`` or ``-t directsync`` is 295 specified as well. 296 297 For write tests, by default a buffer filled with zeros is written. This can be 298 overridden with a pattern byte specified by *PATTERN*. 299 300.. 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 301 302 Perform one or more modifications of the persistent bitmap *BITMAP* 303 in the disk image *FILENAME*. The various modifications are: 304 305 ``--add`` to create *BITMAP*, enabled to record future edits. 306 307 ``--remove`` to remove *BITMAP*. 308 309 ``--clear`` to clear *BITMAP*. 310 311 ``--enable`` to change *BITMAP* to start recording future edits. 312 313 ``--disable`` to change *BITMAP* to stop recording future edits. 314 315 ``--merge`` to merge the contents of the *SOURCE* bitmap into *BITMAP*. 316 317 Additional options include ``-g`` which sets a non-default 318 *GRANULARITY* for ``--add``, and ``-b`` and ``-F`` which select an 319 alternative source file for all *SOURCE* bitmaps used by 320 ``--merge``. 321 322 To see what bitmaps are present in an image, use ``qemu-img info``. 323 324.. option:: check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT] [-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME 325 326 Perform a consistency check on the disk image *FILENAME*. The command can 327 output in the format *OFMT* which is either ``human`` or ``json``. 328 The JSON output is an object of QAPI type ``ImageCheck``. 329 330 If ``-r`` is specified, qemu-img tries to repair any inconsistencies found 331 during the check. ``-r leaks`` repairs only cluster leaks, whereas 332 ``-r all`` fixes all kinds of errors, with a higher risk of choosing the 333 wrong fix or hiding corruption that has already occurred. 334 335 Only the formats ``qcow2``, ``qed``, ``parallels``, ``vhdx``, ``vmdk`` and 336 ``vdi`` support consistency checks. 337 338 In case the image does not have any inconsistencies, check exits with ``0``. 339 Other exit codes indicate the kind of inconsistency found or if another error 340 occurred. The following table summarizes all exit codes of the check subcommand: 341 342 0 343 Check completed, the image is (now) consistent 344 1 345 Check not completed because of internal errors 346 2 347 Check completed, image is corrupted 348 3 349 Check completed, image has leaked clusters, but is not corrupted 350 63 351 Checks are not supported by the image format 352 353 If ``-r`` is specified, exit codes representing the image state refer to the 354 state after (the attempt at) repairing it. That is, a successful ``-r all`` 355 will yield the exit code 0, independently of the image state before. 356 357.. option:: commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b BASE] [-r RATE_LIMIT] [-d] [-p] FILENAME 358 359 Commit the changes recorded in *FILENAME* in its base image or backing file. 360 If the backing file is smaller than the snapshot, then the backing file will be 361 resized to be the same size as the snapshot. If the snapshot is smaller than 362 the backing file, the backing file will not be truncated. If you want the 363 backing file to match the size of the smaller snapshot, you can safely truncate 364 it yourself once the commit operation successfully completes. 365 366 The image *FILENAME* is emptied after the operation has succeeded. If you do 367 not need *FILENAME* afterwards and intend to drop it, you may skip emptying 368 *FILENAME* by specifying the ``-d`` flag. 369 370 If the backing chain of the given image file *FILENAME* has more than one 371 layer, the backing file into which the changes will be committed may be 372 specified as *BASE* (which has to be part of *FILENAME*'s backing 373 chain). If *BASE* is not specified, the immediate backing file of the top 374 image (which is *FILENAME*) will be used. Note that after a commit operation 375 all images between *BASE* and the top image will be invalid and may return 376 garbage data when read. For this reason, ``-b`` implies ``-d`` (so that 377 the top image stays valid). 378 379 The rate limit for the commit process is specified by ``-r``. 380 381.. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 FILENAME2 382 383 Check if two images have the same content. You can compare images with 384 different format or settings. 385 386 The format is probed unless you specify it by ``-f`` (used for 387 *FILENAME1*) and/or ``-F`` (used for *FILENAME2*) option. 388 389 By default, images with different size are considered identical if the larger 390 image contains only unallocated and/or zeroed sectors in the area after the end 391 of the other image. In addition, if any sector is not allocated in one image 392 and contains only zero bytes in the second one, it is evaluated as equal. You 393 can use Strict mode by specifying the ``-s`` option. When compare runs in 394 Strict mode, it fails in case image size differs or a sector is allocated in 395 one image and is not allocated in the second one. 396 397 By default, compare prints out a result message. This message displays 398 information that both images are same or the position of the first different 399 byte. In addition, result message can report different image size in case 400 Strict mode is used. 401 402 Compare exits with ``0`` in case the images are equal and with ``1`` 403 in case the images differ. Other exit codes mean an error occurred during 404 execution and standard error output should contain an error message. 405 The following table sumarizes all exit codes of the compare subcommand: 406 407 0 408 Images are identical (or requested help was printed) 409 1 410 Images differ 411 2 412 Error on opening an image 413 3 414 Error on checking a sector allocation 415 4 416 Error on reading data 417 418.. option:: convert [--object OBJECTDEF] [--image-opts] [--target-image-opts] [--target-is-zero] [--bitmaps [--skip-broken-bitmaps]] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-O OUTPUT_FMT] [-B BACKING_FILE [-F BACKING_FMT]] [-o OPTIONS] [-l SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-r RATE_LIMIT] [-m NUM_COROUTINES] [-W] FILENAME [FILENAME2 [...]] OUTPUT_FILENAME 419 420 Convert the disk image *FILENAME* or a snapshot *SNAPSHOT_PARAM* 421 to disk image *OUTPUT_FILENAME* using format *OUTPUT_FMT*. It can 422 be optionally compressed (``-c`` option) or use any format specific 423 options like encryption (``-o`` option). 424 425 Only the formats ``qcow`` and ``qcow2`` support compression. The 426 compression is read-only. It means that if a compressed sector is 427 rewritten, then it is rewritten as uncompressed data. 428 429 Image conversion is also useful to get smaller image when using a 430 growable format such as ``qcow``: the empty sectors are detected and 431 suppressed from the destination image. 432 433 *SPARSE_SIZE* indicates the consecutive number of bytes (defaults to 4k) 434 that must contain only zeros for ``qemu-img`` to create a sparse image during 435 conversion. If *SPARSE_SIZE* is 0, the source will not be scanned for 436 unallocated or zero sectors, and the destination image will always be 437 fully allocated. 438 439 You can use the *BACKING_FILE* option to force the output image to be 440 created as a copy on write image of the specified base image; the 441 *BACKING_FILE* should have the same content as the input's base image, 442 however the path, image format (as given by *BACKING_FMT*), etc may differ. 443 444 If a relative path name is given, the backing file is looked up relative to 445 the directory containing *OUTPUT_FILENAME*. 446 447 If the ``-n`` option is specified, the target volume creation will be 448 skipped. This is useful for formats such as ``rbd`` if the target 449 volume has already been created with site specific options that cannot 450 be supplied through ``qemu-img``. 451 452 Out of order writes can be enabled with ``-W`` to improve performance. 453 This is only recommended for preallocated devices like host devices or other 454 raw block devices. Out of order write does not work in combination with 455 creating compressed images. 456 457 *NUM_COROUTINES* specifies how many coroutines work in parallel during 458 the convert process (defaults to 8). 459 460 Use of ``--bitmaps`` requests that any persistent bitmaps present in 461 the original are also copied to the destination. If any bitmap is 462 inconsistent in the source, the conversion will fail unless 463 ``--skip-broken-bitmaps`` is also specified to copy only the 464 consistent bitmaps. 465 466.. option:: create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE [-F BACKING_FMT]] [-u] [-o OPTIONS] FILENAME [SIZE] 467 468 Create the new disk image *FILENAME* of size *SIZE* and format 469 *FMT*. Depending on the file format, you can add one or more *OPTIONS* 470 that enable additional features of this format. 471 472 If the option *BACKING_FILE* is specified, then the image will record 473 only the differences from *BACKING_FILE*. No size needs to be specified in 474 this case. *BACKING_FILE* will never be modified unless you use the 475 ``commit`` monitor command (or ``qemu-img commit``). 476 477 If a relative path name is given, the backing file is looked up relative to 478 the directory containing *FILENAME*. 479 480 Note that a given backing file will be opened to check that it is valid. Use 481 the ``-u`` option to enable unsafe backing file mode, which means that the 482 image will be created even if the associated backing file cannot be opened. A 483 matching backing file must be created or additional options be used to make the 484 backing file specification valid when you want to use an image created this 485 way. 486 487 The size can also be specified using the *SIZE* option with ``-o``, 488 it doesn't need to be specified separately in this case. 489 490 491.. option:: dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE] [count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT 492 493 dd copies from *INPUT* file to *OUTPUT* file converting it from 494 *FMT* format to *OUTPUT_FMT* format. 495 496 The data is by default read and written using blocks of 512 bytes but can be 497 modified by specifying *BLOCK_SIZE*. If count=\ *BLOCKS* is specified 498 dd will stop reading input after reading *BLOCKS* input blocks. 499 500 The size syntax is similar to :manpage:`dd(1)`'s size syntax. 501 502.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [-U] FILENAME 503 504 Give information about the disk image *FILENAME*. Use it in 505 particular to know the size reserved on disk which can be different 506 from the displayed size. If VM snapshots are stored in the disk image, 507 they are displayed too. 508 509 If a disk image has a backing file chain, information about each disk image in 510 the chain can be recursively enumerated by using the option ``--backing-chain``. 511 512 For instance, if you have an image chain like: 513 514 :: 515 516 base.qcow2 <- snap1.qcow2 <- snap2.qcow2 517 518 To enumerate information about each disk image in the above chain, starting from top to base, do: 519 520 :: 521 522 qemu-img info --backing-chain snap2.qcow2 523 524 The command can output in the format *OFMT* which is either ``human`` or 525 ``json``. The JSON output is an object of QAPI type ``ImageInfo``; with 526 ``--backing-chain``, it is an array of ``ImageInfo`` objects. 527 528 ``--output=human`` reports the following information (for every image in the 529 chain): 530 531 *image* 532 The image file name 533 534 *file format* 535 The image format 536 537 *virtual size* 538 The size of the guest disk 539 540 *disk size* 541 How much space the image file occupies on the host file system (may be 542 shown as 0 if this information is unavailable, e.g. because there is no 543 file system) 544 545 *cluster_size* 546 Cluster size of the image format, if applicable 547 548 *encrypted* 549 Whether the image is encrypted (only present if so) 550 551 *cleanly shut down* 552 This is shown as ``no`` if the image is dirty and will have to be 553 auto-repaired the next time it is opened in qemu. 554 555 *backing file* 556 The backing file name, if present 557 558 *backing file format* 559 The format of the backing file, if the image enforces it 560 561 *Snapshot list* 562 A list of all internal snapshots 563 564 *Format specific information* 565 Further information whose structure depends on the image format. This 566 section is a textual representation of the respective 567 ``ImageInfoSpecific*`` QAPI object (e.g. ``ImageInfoSpecificQCow2`` 568 for qcow2 images). 569 570.. option:: map [--object OBJECTDEF] [--image-opts] [-f FMT] [--start-offset=OFFSET] [--max-length=LEN] [--output=OFMT] [-U] FILENAME 571 572 Dump the metadata of image *FILENAME* and its backing file chain. 573 In particular, this commands dumps the allocation state of every sector 574 of *FILENAME*, together with the topmost file that allocates it in 575 the backing file chain. 576 577 Two option formats are possible. The default format (``human``) 578 only dumps known-nonzero areas of the file. Known-zero parts of the 579 file are omitted altogether, and likewise for parts that are not allocated 580 throughout the chain. ``qemu-img`` output will identify a file 581 from where the data can be read, and the offset in the file. Each line 582 will include four fields, the first three of which are hexadecimal 583 numbers. For example the first line of: 584 585 :: 586 587 Offset Length Mapped to File 588 0 0x20000 0x50000 /tmp/overlay.qcow2 589 0x100000 0x10000 0x95380000 /tmp/backing.qcow2 590 591 means that 0x20000 (131072) bytes starting at offset 0 in the image are 592 available in /tmp/overlay.qcow2 (opened in ``raw`` format) starting 593 at offset 0x50000 (327680). Data that is compressed, encrypted, or 594 otherwise not available in raw format will cause an error if ``human`` 595 format is in use. Note that file names can include newlines, thus it is 596 not safe to parse this output format in scripts. 597 598 The alternative format ``json`` will return an array of dictionaries 599 in JSON format. It will include similar information in 600 the ``start``, ``length``, ``offset`` fields; 601 it will also include other more specific information: 602 603 - boolean field ``data``: true if the sectors contain actual data, 604 false if the sectors are either unallocated or stored as optimized 605 all-zero clusters 606 - boolean field ``zero``: true if the data is known to read as zero 607 - boolean field ``present``: true if the data belongs to the backing 608 chain, false if rebasing the backing chain onto a deeper file 609 would pick up data from the deeper file; 610 - integer field ``depth``: the depth within the backing chain at 611 which the data was resolved; for example, a depth of 2 refers to 612 the backing file of the backing file of *FILENAME*. 613 614 In JSON format, the ``offset`` field is optional; it is absent in 615 cases where ``human`` format would omit the entry or exit with an error. 616 If ``data`` is false and the ``offset`` field is present, the 617 corresponding sectors in the file are not yet in use, but they are 618 preallocated. 619 620 For more information, consult ``include/block/block.h`` in QEMU's 621 source code. 622 623.. option:: measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--object OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME] 624 625 Calculate the file size required for a new image. This information 626 can be used to size logical volumes or SAN LUNs appropriately for 627 the image that will be placed in them. The values reported are 628 guaranteed to be large enough to fit the image. The command can 629 output in the format *OFMT* which is either ``human`` or ``json``. 630 The JSON output is an object of QAPI type ``BlockMeasureInfo``. 631 632 If the size *N* is given then act as if creating a new empty image file 633 using ``qemu-img create``. If *FILENAME* is given then act as if 634 converting an existing image file using ``qemu-img convert``. The format 635 of the new file is given by *OUTPUT_FMT* while the format of an existing 636 file is given by *FMT*. 637 638 A snapshot in an existing image can be specified using *SNAPSHOT_PARAM*. 639 640 The following fields are reported: 641 642 :: 643 644 required size: 524288 645 fully allocated size: 1074069504 646 bitmaps size: 0 647 648 The ``required size`` is the file size of the new image. It may be smaller 649 than the virtual disk size if the image format supports compact representation. 650 651 The ``fully allocated size`` is the file size of the new image once data has 652 been written to all sectors. This is the maximum size that the image file can 653 occupy with the exception of internal snapshots, dirty bitmaps, vmstate data, 654 and other advanced image format features. 655 656 The ``bitmaps size`` is the additional size required in order to 657 copy bitmaps from a source image in addition to the guest-visible 658 data; the line is omitted if either source or destination lacks 659 bitmap support, or 0 if bitmaps are supported but there is nothing 660 to copy. 661 662.. option:: snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAPSHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME 663 664 List, apply, create or delete snapshots in image *FILENAME*. 665 666.. option:: rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-p] [-u] -b BACKING_FILE [-F BACKING_FMT] FILENAME 667 668 Changes the backing file of an image. Only the formats ``qcow2`` and 669 ``qed`` support changing the backing file. 670 671 The backing file is changed to *BACKING_FILE* and (if the image format of 672 *FILENAME* supports this) the backing file format is changed to 673 *BACKING_FMT*. If *BACKING_FILE* is specified as "" (the empty 674 string), then the image is rebased onto no backing file (i.e. it will exist 675 independently of any backing file). 676 677 If a relative path name is given, the backing file is looked up relative to 678 the directory containing *FILENAME*. 679 680 *CACHE* specifies the cache mode to be used for *FILENAME*, whereas 681 *SRC_CACHE* specifies the cache mode for reading backing files. 682 683 There are two different modes in which ``rebase`` can operate: 684 685 Safe mode 686 This is the default mode and performs a real rebase operation. The 687 new backing file may differ from the old one and ``qemu-img rebase`` 688 will take care of keeping the guest-visible content of *FILENAME* 689 unchanged. 690 691 In order to achieve this, any clusters that differ between 692 *BACKING_FILE* and the old backing file of *FILENAME* are merged 693 into *FILENAME* before actually changing the backing file. 694 695 Note that the safe mode is an expensive operation, comparable to 696 converting an image. It only works if the old backing file still 697 exists. 698 699 Unsafe mode 700 ``qemu-img`` uses the unsafe mode if ``-u`` is specified. In this 701 mode, only the backing file name and format of *FILENAME* is changed 702 without any checks on the file contents. The user must take care of 703 specifying the correct new backing file, or the guest-visible 704 content of the image will be corrupted. 705 706 This mode is useful for renaming or moving the backing file to 707 somewhere else. It can be used without an accessible old backing 708 file, i.e. you can use it to fix an image whose backing file has 709 already been moved/renamed. 710 711 You can use ``rebase`` to perform a "diff" operation on two 712 disk images. This can be useful when you have copied or cloned 713 a guest, and you want to get back to a thin image on top of a 714 template or base image. 715 716 Say that ``base.img`` has been cloned as ``modified.img`` by 717 copying it, and that the ``modified.img`` guest has run so there 718 are now some changes compared to ``base.img``. To construct a thin 719 image called ``diff.qcow2`` that contains just the differences, do: 720 721 :: 722 723 qemu-img create -f qcow2 -b modified.img diff.qcow2 724 qemu-img rebase -b base.img diff.qcow2 725 726 At this point, ``modified.img`` can be discarded, since 727 ``base.img + diff.qcow2`` contains the same information. 728 729.. option:: resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--preallocation=PREALLOC] [-q] [--shrink] FILENAME [+ | -]SIZE 730 731 Change the disk image as if it had been created with *SIZE*. 732 733 Before using this command to shrink a disk image, you MUST use file system and 734 partitioning tools inside the VM to reduce allocated file systems and partition 735 sizes accordingly. Failure to do so will result in data loss! 736 737 When shrinking images, the ``--shrink`` option must be given. This informs 738 ``qemu-img`` that the user acknowledges all loss of data beyond the truncated 739 image's end. 740 741 After using this command to grow a disk image, you must use file system and 742 partitioning tools inside the VM to actually begin using the new space on the 743 device. 744 745 When growing an image, the ``--preallocation`` option may be used to specify 746 how the additional image area should be allocated on the host. See the format 747 description in the :ref:`notes` section which values are allowed. Using this 748 option may result in slightly more data being allocated than necessary. 749 750.. _notes: 751 752Notes 753----- 754 755Supported image file formats: 756 757``raw`` 758 759 Raw disk image format (default). This format has the advantage of 760 being simple and easily exportable to all other emulators. If your 761 file system supports *holes* (for example in ext2 or ext3 on 762 Linux or NTFS on Windows), then only the written sectors will reserve 763 space. Use ``qemu-img info`` to know the real size used by the 764 image or ``ls -ls`` on Unix/Linux. 765 766 Supported options: 767 768 ``preallocation`` 769 Preallocation mode (allowed values: ``off``, ``falloc``, 770 ``full``). ``falloc`` mode preallocates space for image by 771 calling ``posix_fallocate()``. ``full`` mode preallocates space 772 for image by writing data to underlying storage. This data may or 773 may not be zero, depending on the storage location. 774 775``qcow2`` 776 777 QEMU image format, the most versatile format. Use it to have smaller 778 images (useful if your filesystem does not supports holes, for example 779 on Windows), optional AES encryption, zlib based compression and 780 support of multiple VM snapshots. 781 782 Supported options: 783 784 ``compat`` 785 Determines the qcow2 version to use. ``compat=0.10`` uses the 786 traditional image format that can be read by any QEMU since 0.10. 787 ``compat=1.1`` enables image format extensions that only QEMU 1.1 and 788 newer understand (this is the default). Amongst others, this includes zero 789 clusters, which allow efficient copy-on-read for sparse images. 790 791 ``backing_file`` 792 File name of a base image (see ``create`` subcommand) 793 794 ``backing_fmt`` 795 Image format of the base image 796 797 ``encryption`` 798 If this option is set to ``on``, the image is encrypted with 799 128-bit AES-CBC. 800 801 The use of encryption in qcow and qcow2 images is considered to be 802 flawed by modern cryptography standards, suffering from a number 803 of design problems: 804 805 - The AES-CBC cipher is used with predictable initialization 806 vectors based on the sector number. This makes it vulnerable to 807 chosen plaintext attacks which can reveal the existence of 808 encrypted data. 809 810 - The user passphrase is directly used as the encryption key. A 811 poorly chosen or short passphrase will compromise the security 812 of the encryption. 813 814 - In the event of the passphrase being compromised there is no way 815 to change the passphrase to protect data in any qcow images. The 816 files must be cloned, using a different encryption passphrase in 817 the new file. The original file must then be securely erased 818 using a program like shred, though even this is ineffective with 819 many modern storage technologies. 820 821 - Initialization vectors used to encrypt sectors are based on the 822 guest virtual sector number, instead of the host physical 823 sector. When a disk image has multiple internal snapshots this 824 means that data in multiple physical sectors is encrypted with 825 the same initialization vector. With the CBC mode, this opens 826 the possibility of watermarking attacks if the attack can 827 collect multiple sectors encrypted with the same IV and some 828 predictable data. Having multiple qcow2 images with the same 829 passphrase also exposes this weakness since the passphrase is 830 directly used as the key. 831 832 Use of qcow / qcow2 encryption is thus strongly discouraged. Users are 833 recommended to use an alternative encryption technology such as the 834 Linux dm-crypt / LUKS system. 835 836 ``cluster_size`` 837 Changes the qcow2 cluster size (must be between 512 and 838 2M). Smaller cluster sizes can improve the image file size whereas 839 larger cluster sizes generally provide better performance. 840 841 ``preallocation`` 842 Preallocation mode (allowed values: ``off``, ``metadata``, 843 ``falloc``, ``full``). An image with preallocated metadata is 844 initially larger but can improve performance when the image needs 845 to grow. ``falloc`` and ``full`` preallocations are like the same 846 options of ``raw`` format, but sets up metadata also. 847 848 ``lazy_refcounts`` 849 If this option is set to ``on``, reference count updates are 850 postponed with the goal of avoiding metadata I/O and improving 851 performance. This is particularly interesting with 852 ``cache=writethrough`` which doesn't batch metadata 853 updates. The tradeoff is that after a host crash, the reference 854 count tables must be rebuilt, i.e. on the next open an (automatic) 855 ``qemu-img check -r all`` is required, which may take some time. 856 857 This option can only be enabled if ``compat=1.1`` is specified. 858 859 ``nocow`` 860 If this option is set to ``on``, it will turn off COW of the file. It's 861 only valid on btrfs, no effect on other file systems. 862 863 Btrfs has low performance when hosting a VM image file, even more 864 when the guest on the VM also using btrfs as file system. Turning 865 off COW is a way to mitigate this bad performance. Generally there 866 are two ways to turn off COW on btrfs: 867 868 - Disable it by mounting with nodatacow, then all newly created files 869 will be NOCOW 870 - For an empty file, add the NOCOW file attribute. That's what this 871 option does. 872 873 Note: this option is only valid to new or empty files. If there is 874 an existing file which is COW and has data blocks already, it 875 couldn't be changed to NOCOW by setting ``nocow=on``. One can 876 issue ``lsattr filename`` to check if the NOCOW flag is set or not 877 (Capital 'C' is NOCOW flag). 878 879 ``data_file`` 880 Filename where all guest data will be stored. If this option is used, 881 the qcow2 file will only contain the image's metadata. 882 883 Note: Data loss will occur if the given filename already exists when 884 using this option with ``qemu-img create`` since ``qemu-img`` will create 885 the data file anew, overwriting the file's original contents. To simply 886 update the reference to point to the given pre-existing file, use 887 ``qemu-img amend``. 888 889 ``data_file_raw`` 890 If this option is set to ``on``, QEMU will always keep the external data 891 file consistent as a standalone read-only raw image. 892 893 It does this by forwarding all write accesses to the qcow2 file through to 894 the raw data file, including their offsets. Therefore, data that is visible 895 on the qcow2 node (i.e., to the guest) at some offset is visible at the same 896 offset in the raw data file. This results in a read-only raw image. Writes 897 that bypass the qcow2 metadata may corrupt the qcow2 metadata because the 898 out-of-band writes may result in the metadata falling out of sync with the 899 raw image. 900 901 If this option is ``off``, QEMU will use the data file to store data in an 902 arbitrary manner. The file’s content will not make sense without the 903 accompanying qcow2 metadata. Where data is written will have no relation to 904 its offset as seen by the guest, and some writes (specifically zero writes) 905 may not be forwarded to the data file at all, but will only be handled by 906 modifying qcow2 metadata. 907 908 This option can only be enabled if ``data_file`` is set. 909 910``Other`` 911 912 QEMU also supports various other image file formats for 913 compatibility with older QEMU versions or other hypervisors, 914 including VMDK, VDI, VHD (vpc), VHDX, qcow1 and QED. For a full list 915 of supported formats see ``qemu-img --help``. For a more detailed 916 description of these formats, see the QEMU block drivers reference 917 documentation. 918 919 The main purpose of the block drivers for these formats is image 920 conversion. For running VMs, it is recommended to convert the disk 921 images to either raw or qcow2 in order to achieve good performance. 922