1QEMU disk image utility 2======================= 3 4Synopsis 5-------- 6 7**qemu-img** [*standard options*] *command* [*command options*] 8 9Description 10----------- 11 12qemu-img allows you to create, convert and modify images offline. It can handle 13all image formats supported by QEMU. 14 15**Warning:** Never use qemu-img to modify images in use by a running virtual 16machine or any other process; this may destroy the image. Also, be aware that 17querying an image that is being modified by another process may encounter 18inconsistent state. 19 20Options 21------- 22 23.. program:: qemu-img 24 25Standard options: 26 27.. option:: -h, --help 28 29 Display this help and exit 30 31.. option:: -V, --version 32 33 Display version information and exit 34 35.. option:: -T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE] 36 37 .. include:: ../qemu-option-trace.rst.inc 38 39The following commands are supported: 40 41.. hxtool-doc:: qemu-img-cmds.hx 42 43Command parameters: 44 45*FILENAME* is a disk image filename. 46 47*FMT* is the disk image format. It is guessed automatically in most 48cases. See below for a description of the supported disk formats. 49 50*SIZE* is the disk image size in bytes. Optional suffixes ``k`` or 51``K`` (kilobyte, 1024) ``M`` (megabyte, 1024k) and ``G`` (gigabyte, 521024M) and T (terabyte, 1024G) are supported. ``b`` is ignored. 53 54*OUTPUT_FILENAME* is the destination disk image filename. 55 56*OUTPUT_FMT* is the destination format. 57 58*OPTIONS* is a comma separated list of format specific options in a 59name=value format. Use ``-o ?`` for an overview of the options supported 60by the used format or see the format descriptions below for details. 61 62*SNAPSHOT_PARAM* is param used for internal snapshot, format is 63'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'. 64 65.. 66 Note the use of a new 'program'; otherwise Sphinx complains about 67 the -h option appearing both in the above option list and this one. 68 69.. program:: qemu-img-common-opts 70 71.. option:: --object OBJECTDEF 72 73 is a QEMU user creatable object definition. See the :manpage:`qemu(1)` 74 manual page for a description of the object properties. The most common 75 object type is a ``secret``, which is used to supply passwords and/or 76 encryption keys. 77 78.. option:: --image-opts 79 80 Indicates that the source *FILENAME* parameter is to be interpreted as a 81 full option string, not a plain filename. This parameter is mutually 82 exclusive with the *-f* parameter. 83 84.. option:: --target-image-opts 85 86 Indicates that the OUTPUT_FILENAME parameter(s) are to be interpreted as 87 a full option string, not a plain filename. This parameter is mutually 88 exclusive with the *-O* parameters. It is currently required to also use 89 the *-n* parameter to skip image creation. This restriction may be relaxed 90 in a future release. 91 92.. option:: --force-share (-U) 93 94 If specified, ``qemu-img`` will open the image in shared mode, allowing 95 other QEMU processes to open it in write mode. For example, this can be used to 96 get the image information (with 'info' subcommand) when the image is used by a 97 running guest. Note that this could produce inconsistent results because of 98 concurrent metadata changes, etc. This option is only allowed when opening 99 images in read-only mode. 100 101.. option:: --backing-chain 102 103 Will enumerate information about backing files in a disk image chain. Refer 104 below for further description. 105 106.. option:: -c 107 108 Indicates that target image must be compressed (qcow format only). 109 110.. option:: -h 111 112 With or without a command, shows help and lists the supported formats. 113 114.. option:: -p 115 116 Display progress bar (compare, convert and rebase commands only). 117 If the *-p* option is not used for a command that supports it, the 118 progress is reported when the process receives a ``SIGUSR1`` or 119 ``SIGINFO`` signal. 120 121.. option:: -q 122 123 Quiet mode - do not print any output (except errors). There's no progress bar 124 in case both *-q* and *-p* options are used. 125 126.. option:: -S SIZE 127 128 Indicates the consecutive number of bytes that must contain only zeros 129 for qemu-img to create a sparse image during conversion. This value is rounded 130 down to the nearest 512 bytes. You may use the common size suffixes like 131 ``k`` for kilobytes. 132 133.. option:: -t CACHE 134 135 Specifies the cache mode that should be used with the (destination) file. See 136 the documentation of the emulator's ``-drive cache=...`` option for allowed 137 values. 138 139.. option:: -T SRC_CACHE 140 141 Specifies the cache mode that should be used with the source file(s). See 142 the documentation of the emulator's ``-drive cache=...`` option for allowed 143 values. 144 145Parameters to snapshot subcommand: 146 147.. program:: qemu-img-snapshot 148 149.. option:: snapshot 150 151 Is the name of the snapshot to create, apply or delete 152 153.. option:: -a 154 155 Applies a snapshot (revert disk to saved state) 156 157.. option:: -c 158 159 Creates a snapshot 160 161.. option:: -d 162 163 Deletes a snapshot 164 165.. option:: -l 166 167 Lists all snapshots in the given image 168 169Parameters to compare subcommand: 170 171.. program:: qemu-img-compare 172 173.. option:: -f 174 175 First image format 176 177.. option:: -F 178 179 Second image format 180 181.. option:: -s 182 183 Strict mode - fail on different image size or sector allocation 184 185Parameters to convert subcommand: 186 187.. program:: qemu-img-convert 188 189.. option:: -n 190 191 Skip the creation of the target volume 192 193.. option:: -m 194 195 Number of parallel coroutines for the convert process 196 197.. option:: -W 198 199 Allow out-of-order writes to the destination. This option improves performance, 200 but is only recommended for preallocated devices like host devices or other 201 raw block devices. 202 203.. option:: -C 204 205 Try to use copy offloading to move data from source image to target. This may 206 improve performance if the data is remote, such as with NFS or iSCSI backends, 207 but will not automatically sparsify zero sectors, and may result in a fully 208 allocated target image depending on the host support for getting allocation 209 information. 210 211.. option:: --salvage 212 213 Try to ignore I/O errors when reading. Unless in quiet mode (``-q``), errors 214 will still be printed. Areas that cannot be read from the source will be 215 treated as containing only zeroes. 216 217.. option:: --target-is-zero 218 219 Assume that reading the destination image will always return 220 zeros. This parameter is mutually exclusive with a destination image 221 that has a backing file. It is required to also use the ``-n`` 222 parameter to skip image creation. 223 224Parameters to dd subcommand: 225 226.. program:: qemu-img-dd 227 228.. option:: bs=BLOCK_SIZE 229 230 Defines the block size 231 232.. option:: count=BLOCKS 233 234 Sets the number of input blocks to copy 235 236.. option:: if=INPUT 237 238 Sets the input file 239 240.. option:: of=OUTPUT 241 242 Sets the output file 243 244.. option:: skip=BLOCKS 245 246 Sets the number of input blocks to skip 247 248Command description: 249 250.. program:: qemu-img-commands 251 252.. option:: amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE] -o OPTIONS FILENAME 253 254 Amends the image format specific *OPTIONS* for the image file 255 *FILENAME*. Not all file formats support this operation. 256 257.. 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 258 259 Run a simple sequential I/O benchmark on the specified image. If ``-w`` is 260 specified, a write test is performed, otherwise a read test is performed. 261 262 A total number of *COUNT* I/O requests is performed, each *BUFFER_SIZE* 263 bytes in size, and with *DEPTH* requests in parallel. The first request 264 starts at the position given by *OFFSET*, each following request increases 265 the current position by *STEP_SIZE*. If *STEP_SIZE* is not given, 266 *BUFFER_SIZE* is used for its value. 267 268 If *FLUSH_INTERVAL* is specified for a write test, the request queue is 269 drained and a flush is issued before new writes are made whenever the number of 270 remaining requests is a multiple of *FLUSH_INTERVAL*. If additionally 271 ``--no-drain`` is specified, a flush is issued without draining the request 272 queue first. 273 274 if ``-i`` is specified, *AIO* option can be used to specify different 275 AIO backends: ``threads``, ``native`` or ``io_uring``. 276 277 If ``-n`` is specified, the native AIO backend is used if possible. On 278 Linux, this option only works if ``-t none`` or ``-t directsync`` is 279 specified as well. 280 281 For write tests, by default a buffer filled with zeros is written. This can be 282 overridden with a pattern byte specified by *PATTERN*. 283 284.. option:: check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT] [-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME 285 286 Perform a consistency check on the disk image *FILENAME*. The command can 287 output in the format *OFMT* which is either ``human`` or ``json``. 288 The JSON output is an object of QAPI type ``ImageCheck``. 289 290 If ``-r`` is specified, qemu-img tries to repair any inconsistencies found 291 during the check. ``-r leaks`` repairs only cluster leaks, whereas 292 ``-r all`` fixes all kinds of errors, with a higher risk of choosing the 293 wrong fix or hiding corruption that has already occurred. 294 295 Only the formats ``qcow2``, ``qed`` and ``vdi`` support 296 consistency checks. 297 298 In case the image does not have any inconsistencies, check exits with ``0``. 299 Other exit codes indicate the kind of inconsistency found or if another error 300 occurred. The following table summarizes all exit codes of the check subcommand: 301 302 0 303 Check completed, the image is (now) consistent 304 1 305 Check not completed because of internal errors 306 2 307 Check completed, image is corrupted 308 3 309 Check completed, image has leaked clusters, but is not corrupted 310 63 311 Checks are not supported by the image format 312 313 If ``-r`` is specified, exit codes representing the image state refer to the 314 state after (the attempt at) repairing it. That is, a successful ``-r all`` 315 will yield the exit code 0, independently of the image state before. 316 317.. option:: commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b BASE] [-d] [-p] FILENAME 318 319 Commit the changes recorded in *FILENAME* in its base image or backing file. 320 If the backing file is smaller than the snapshot, then the backing file will be 321 resized to be the same size as the snapshot. If the snapshot is smaller than 322 the backing file, the backing file will not be truncated. If you want the 323 backing file to match the size of the smaller snapshot, you can safely truncate 324 it yourself once the commit operation successfully completes. 325 326 The image *FILENAME* is emptied after the operation has succeeded. If you do 327 not need *FILENAME* afterwards and intend to drop it, you may skip emptying 328 *FILENAME* by specifying the ``-d`` flag. 329 330 If the backing chain of the given image file *FILENAME* has more than one 331 layer, the backing file into which the changes will be committed may be 332 specified as *BASE* (which has to be part of *FILENAME*'s backing 333 chain). If *BASE* is not specified, the immediate backing file of the top 334 image (which is *FILENAME*) will be used. Note that after a commit operation 335 all images between *BASE* and the top image will be invalid and may return 336 garbage data when read. For this reason, ``-b`` implies ``-d`` (so that 337 the top image stays valid). 338 339.. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 FILENAME2 340 341 Check if two images have the same content. You can compare images with 342 different format or settings. 343 344 The format is probed unless you specify it by ``-f`` (used for 345 *FILENAME1*) and/or ``-F`` (used for *FILENAME2*) option. 346 347 By default, images with different size are considered identical if the larger 348 image contains only unallocated and/or zeroed sectors in the area after the end 349 of the other image. In addition, if any sector is not allocated in one image 350 and contains only zero bytes in the second one, it is evaluated as equal. You 351 can use Strict mode by specifying the ``-s`` option. When compare runs in 352 Strict mode, it fails in case image size differs or a sector is allocated in 353 one image and is not allocated in the second one. 354 355 By default, compare prints out a result message. This message displays 356 information that both images are same or the position of the first different 357 byte. In addition, result message can report different image size in case 358 Strict mode is used. 359 360 Compare exits with ``0`` in case the images are equal and with ``1`` 361 in case the images differ. Other exit codes mean an error occurred during 362 execution and standard error output should contain an error message. 363 The following table sumarizes all exit codes of the compare subcommand: 364 365 0 366 Images are identical 367 1 368 Images differ 369 2 370 Error on opening an image 371 3 372 Error on checking a sector allocation 373 4 374 Error on reading data 375 376.. option:: convert [--object OBJECTDEF] [--image-opts] [--target-image-opts] [--target-is-zero] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-O OUTPUT_FMT] [-B BACKING_FILE] [-o OPTIONS] [-l SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-m NUM_COROUTINES] [-W] FILENAME [FILENAME2 [...]] OUTPUT_FILENAME 377 378 Convert the disk image *FILENAME* or a snapshot *SNAPSHOT_PARAM* 379 to disk image *OUTPUT_FILENAME* using format *OUTPUT_FMT*. It can 380 be optionally compressed (``-c`` option) or use any format specific 381 options like encryption (``-o`` option). 382 383 Only the formats ``qcow`` and ``qcow2`` support compression. The 384 compression is read-only. It means that if a compressed sector is 385 rewritten, then it is rewritten as uncompressed data. 386 387 Image conversion is also useful to get smaller image when using a 388 growable format such as ``qcow``: the empty sectors are detected and 389 suppressed from the destination image. 390 391 *SPARSE_SIZE* indicates the consecutive number of bytes (defaults to 4k) 392 that must contain only zeros for qemu-img to create a sparse image during 393 conversion. If *SPARSE_SIZE* is 0, the source will not be scanned for 394 unallocated or zero sectors, and the destination image will always be 395 fully allocated. 396 397 You can use the *BACKING_FILE* option to force the output image to be 398 created as a copy on write image of the specified base image; the 399 *BACKING_FILE* should have the same content as the input's base image, 400 however the path, image format, etc may differ. 401 402 If a relative path name is given, the backing file is looked up relative to 403 the directory containing *OUTPUT_FILENAME*. 404 405 If the ``-n`` option is specified, the target volume creation will be 406 skipped. This is useful for formats such as ``rbd`` if the target 407 volume has already been created with site specific options that cannot 408 be supplied through qemu-img. 409 410 Out of order writes can be enabled with ``-W`` to improve performance. 411 This is only recommended for preallocated devices like host devices or other 412 raw block devices. Out of order write does not work in combination with 413 creating compressed images. 414 415 *NUM_COROUTINES* specifies how many coroutines work in parallel during 416 the convert process (defaults to 8). 417 418.. option:: create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE] [-F BACKING_FMT] [-u] [-o OPTIONS] FILENAME [SIZE] 419 420 Create the new disk image *FILENAME* of size *SIZE* and format 421 *FMT*. Depending on the file format, you can add one or more *OPTIONS* 422 that enable additional features of this format. 423 424 If the option *BACKING_FILE* is specified, then the image will record 425 only the differences from *BACKING_FILE*. No size needs to be specified in 426 this case. *BACKING_FILE* will never be modified unless you use the 427 ``commit`` monitor command (or qemu-img commit). 428 429 If a relative path name is given, the backing file is looked up relative to 430 the directory containing *FILENAME*. 431 432 Note that a given backing file will be opened to check that it is valid. Use 433 the ``-u`` option to enable unsafe backing file mode, which means that the 434 image will be created even if the associated backing file cannot be opened. A 435 matching backing file must be created or additional options be used to make the 436 backing file specification valid when you want to use an image created this 437 way. 438 439 The size can also be specified using the *SIZE* option with ``-o``, 440 it doesn't need to be specified separately in this case. 441 442 443.. option:: dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE] [count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT 444 445 dd copies from *INPUT* file to *OUTPUT* file converting it from 446 *FMT* format to *OUTPUT_FMT* format. 447 448 The data is by default read and written using blocks of 512 bytes but can be 449 modified by specifying *BLOCK_SIZE*. If count=\ *BLOCKS* is specified 450 dd will stop reading input after reading *BLOCKS* input blocks. 451 452 The size syntax is similar to :manpage:`dd(1)`'s size syntax. 453 454.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [-U] FILENAME 455 456 Give information about the disk image *FILENAME*. Use it in 457 particular to know the size reserved on disk which can be different 458 from the displayed size. If VM snapshots are stored in the disk image, 459 they are displayed too. 460 461 If a disk image has a backing file chain, information about each disk image in 462 the chain can be recursively enumerated by using the option ``--backing-chain``. 463 464 For instance, if you have an image chain like: 465 466 :: 467 468 base.qcow2 <- snap1.qcow2 <- snap2.qcow2 469 470 To enumerate information about each disk image in the above chain, starting from top to base, do: 471 472 :: 473 474 qemu-img info --backing-chain snap2.qcow2 475 476 The command can output in the format *OFMT* which is either ``human`` or 477 ``json``. The JSON output is an object of QAPI type ``ImageInfo``; with 478 ``--backing-chain``, it is an array of ``ImageInfo`` objects. 479 480 ``--output=human`` reports the following information (for every image in the 481 chain): 482 483 *image* 484 The image file name 485 486 *file format* 487 The image format 488 489 *virtual size* 490 The size of the guest disk 491 492 *disk size* 493 How much space the image file occupies on the host file system (may be 494 shown as 0 if this information is unavailable, e.g. because there is no 495 file system) 496 497 *cluster_size* 498 Cluster size of the image format, if applicable 499 500 *encrypted* 501 Whether the image is encrypted (only present if so) 502 503 *cleanly shut down* 504 This is shown as ``no`` if the image is dirty and will have to be 505 auto-repaired the next time it is opened in qemu. 506 507 *backing file* 508 The backing file name, if present 509 510 *backing file format* 511 The format of the backing file, if the image enforces it 512 513 *Snapshot list* 514 A list of all internal snapshots 515 516 *Format specific information* 517 Further information whose structure depends on the image format. This 518 section is a textual representation of the respective 519 ``ImageInfoSpecific*`` QAPI object (e.g. ``ImageInfoSpecificQCow2`` 520 for qcow2 images). 521 522.. option:: map [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [-U] FILENAME 523 524 Dump the metadata of image *FILENAME* and its backing file chain. 525 In particular, this commands dumps the allocation state of every sector 526 of *FILENAME*, together with the topmost file that allocates it in 527 the backing file chain. 528 529 Two option formats are possible. The default format (``human``) 530 only dumps known-nonzero areas of the file. Known-zero parts of the 531 file are omitted altogether, and likewise for parts that are not allocated 532 throughout the chain. ``qemu-img`` output will identify a file 533 from where the data can be read, and the offset in the file. Each line 534 will include four fields, the first three of which are hexadecimal 535 numbers. For example the first line of: 536 537 :: 538 539 Offset Length Mapped to File 540 0 0x20000 0x50000 /tmp/overlay.qcow2 541 0x100000 0x10000 0x95380000 /tmp/backing.qcow2 542 543 means that 0x20000 (131072) bytes starting at offset 0 in the image are 544 available in /tmp/overlay.qcow2 (opened in ``raw`` format) starting 545 at offset 0x50000 (327680). Data that is compressed, encrypted, or 546 otherwise not available in raw format will cause an error if ``human`` 547 format is in use. Note that file names can include newlines, thus it is 548 not safe to parse this output format in scripts. 549 550 The alternative format ``json`` will return an array of dictionaries 551 in JSON format. It will include similar information in 552 the ``start``, ``length``, ``offset`` fields; 553 it will also include other more specific information: 554 555 - whether the sectors contain actual data or not (boolean field ``data``; 556 if false, the sectors are either unallocated or stored as optimized 557 all-zero clusters); 558 - whether the data is known to read as zero (boolean field ``zero``); 559 - in order to make the output shorter, the target file is expressed as 560 a ``depth``; for example, a depth of 2 refers to the backing file 561 of the backing file of *FILENAME*. 562 563 In JSON format, the ``offset`` field is optional; it is absent in 564 cases where ``human`` format would omit the entry or exit with an error. 565 If ``data`` is false and the ``offset`` field is present, the 566 corresponding sectors in the file are not yet in use, but they are 567 preallocated. 568 569 For more information, consult ``include/block/block.h`` in QEMU's 570 source code. 571 572.. option:: measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--object OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME] 573 574 Calculate the file size required for a new image. This information 575 can be used to size logical volumes or SAN LUNs appropriately for 576 the image that will be placed in them. The values reported are 577 guaranteed to be large enough to fit the image. The command can 578 output in the format *OFMT* which is either ``human`` or ``json``. 579 The JSON output is an object of QAPI type ``BlockMeasureInfo``. 580 581 If the size *N* is given then act as if creating a new empty image file 582 using ``qemu-img create``. If *FILENAME* is given then act as if 583 converting an existing image file using ``qemu-img convert``. The format 584 of the new file is given by *OUTPUT_FMT* while the format of an existing 585 file is given by *FMT*. 586 587 A snapshot in an existing image can be specified using *SNAPSHOT_PARAM*. 588 589 The following fields are reported: 590 591 :: 592 593 required size: 524288 594 fully allocated size: 1074069504 595 596 The ``required size`` is the file size of the new image. It may be smaller 597 than the virtual disk size if the image format supports compact representation. 598 599 The ``fully allocated size`` is the file size of the new image once data has 600 been written to all sectors. This is the maximum size that the image file can 601 occupy with the exception of internal snapshots, dirty bitmaps, vmstate data, 602 and other advanced image format features. 603 604.. option:: snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAPSHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME 605 606 List, apply, create or delete snapshots in image *FILENAME*. 607 608.. option:: rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-p] [-u] -b BACKING_FILE [-F BACKING_FMT] FILENAME 609 610 Changes the backing file of an image. Only the formats ``qcow2`` and 611 ``qed`` support changing the backing file. 612 613 The backing file is changed to *BACKING_FILE* and (if the image format of 614 *FILENAME* supports this) the backing file format is changed to 615 *BACKING_FMT*. If *BACKING_FILE* is specified as "" (the empty 616 string), then the image is rebased onto no backing file (i.e. it will exist 617 independently of any backing file). 618 619 If a relative path name is given, the backing file is looked up relative to 620 the directory containing *FILENAME*. 621 622 *CACHE* specifies the cache mode to be used for *FILENAME*, whereas 623 *SRC_CACHE* specifies the cache mode for reading backing files. 624 625 There are two different modes in which ``rebase`` can operate: 626 627 Safe mode 628 This is the default mode and performs a real rebase operation. The 629 new backing file may differ from the old one and qemu-img rebase 630 will take care of keeping the guest-visible content of *FILENAME* 631 unchanged. 632 633 In order to achieve this, any clusters that differ between 634 *BACKING_FILE* and the old backing file of *FILENAME* are merged 635 into *FILENAME* before actually changing the backing file. 636 637 Note that the safe mode is an expensive operation, comparable to 638 converting an image. It only works if the old backing file still 639 exists. 640 641 Unsafe mode 642 qemu-img uses the unsafe mode if ``-u`` is specified. In this 643 mode, only the backing file name and format of *FILENAME* is changed 644 without any checks on the file contents. The user must take care of 645 specifying the correct new backing file, or the guest-visible 646 content of the image will be corrupted. 647 648 This mode is useful for renaming or moving the backing file to 649 somewhere else. It can be used without an accessible old backing 650 file, i.e. you can use it to fix an image whose backing file has 651 already been moved/renamed. 652 653 You can use ``rebase`` to perform a "diff" operation on two 654 disk images. This can be useful when you have copied or cloned 655 a guest, and you want to get back to a thin image on top of a 656 template or base image. 657 658 Say that ``base.img`` has been cloned as ``modified.img`` by 659 copying it, and that the ``modified.img`` guest has run so there 660 are now some changes compared to ``base.img``. To construct a thin 661 image called ``diff.qcow2`` that contains just the differences, do: 662 663 :: 664 665 qemu-img create -f qcow2 -b modified.img diff.qcow2 666 qemu-img rebase -b base.img diff.qcow2 667 668 At this point, ``modified.img`` can be discarded, since 669 ``base.img + diff.qcow2`` contains the same information. 670 671.. option:: resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--preallocation=PREALLOC] [-q] [--shrink] FILENAME [+ | -]SIZE 672 673 Change the disk image as if it had been created with *SIZE*. 674 675 Before using this command to shrink a disk image, you MUST use file system and 676 partitioning tools inside the VM to reduce allocated file systems and partition 677 sizes accordingly. Failure to do so will result in data loss! 678 679 When shrinking images, the ``--shrink`` option must be given. This informs 680 qemu-img that the user acknowledges all loss of data beyond the truncated 681 image's end. 682 683 After using this command to grow a disk image, you must use file system and 684 partitioning tools inside the VM to actually begin using the new space on the 685 device. 686 687 When growing an image, the ``--preallocation`` option may be used to specify 688 how the additional image area should be allocated on the host. See the format 689 description in the :ref:`notes` section which values are allowed. Using this 690 option may result in slightly more data being allocated than necessary. 691 692.. _notes: 693 694Notes 695----- 696 697Supported image file formats: 698 699``raw`` 700 701 Raw disk image format (default). This format has the advantage of 702 being simple and easily exportable to all other emulators. If your 703 file system supports *holes* (for example in ext2 or ext3 on 704 Linux or NTFS on Windows), then only the written sectors will reserve 705 space. Use ``qemu-img info`` to know the real size used by the 706 image or ``ls -ls`` on Unix/Linux. 707 708 Supported options: 709 710 ``preallocation`` 711 Preallocation mode (allowed values: ``off``, ``falloc``, 712 ``full``). ``falloc`` mode preallocates space for image by 713 calling ``posix_fallocate()``. ``full`` mode preallocates space 714 for image by writing data to underlying storage. This data may or 715 may not be zero, depending on the storage location. 716 717``qcow2`` 718 719 QEMU image format, the most versatile format. Use it to have smaller 720 images (useful if your filesystem does not supports holes, for example 721 on Windows), optional AES encryption, zlib based compression and 722 support of multiple VM snapshots. 723 724 Supported options: 725 726 ``compat`` 727 Determines the qcow2 version to use. ``compat=0.10`` uses the 728 traditional image format that can be read by any QEMU since 0.10. 729 ``compat=1.1`` enables image format extensions that only QEMU 1.1 and 730 newer understand (this is the default). Amongst others, this includes zero 731 clusters, which allow efficient copy-on-read for sparse images. 732 733 ``backing_file`` 734 File name of a base image (see ``create`` subcommand) 735 736 ``backing_fmt`` 737 Image format of the base image 738 739 ``encryption`` 740 If this option is set to ``on``, the image is encrypted with 741 128-bit AES-CBC. 742 743 The use of encryption in qcow and qcow2 images is considered to be 744 flawed by modern cryptography standards, suffering from a number 745 of design problems: 746 747 - The AES-CBC cipher is used with predictable initialization 748 vectors based on the sector number. This makes it vulnerable to 749 chosen plaintext attacks which can reveal the existence of 750 encrypted data. 751 752 - The user passphrase is directly used as the encryption key. A 753 poorly chosen or short passphrase will compromise the security 754 of the encryption. 755 756 - In the event of the passphrase being compromised there is no way 757 to change the passphrase to protect data in any qcow images. The 758 files must be cloned, using a different encryption passphrase in 759 the new file. The original file must then be securely erased 760 using a program like shred, though even this is ineffective with 761 many modern storage technologies. 762 763 - Initialization vectors used to encrypt sectors are based on the 764 guest virtual sector number, instead of the host physical 765 sector. When a disk image has multiple internal snapshots this 766 means that data in multiple physical sectors is encrypted with 767 the same initialization vector. With the CBC mode, this opens 768 the possibility of watermarking attacks if the attack can 769 collect multiple sectors encrypted with the same IV and some 770 predictable data. Having multiple qcow2 images with the same 771 passphrase also exposes this weakness since the passphrase is 772 directly used as the key. 773 774 Use of qcow / qcow2 encryption is thus strongly discouraged. Users are 775 recommended to use an alternative encryption technology such as the 776 Linux dm-crypt / LUKS system. 777 778 ``cluster_size`` 779 Changes the qcow2 cluster size (must be between 512 and 780 2M). Smaller cluster sizes can improve the image file size whereas 781 larger cluster sizes generally provide better performance. 782 783 ``preallocation`` 784 Preallocation mode (allowed values: ``off``, ``metadata``, 785 ``falloc``, ``full``). An image with preallocated metadata is 786 initially larger but can improve performance when the image needs 787 to grow. ``falloc`` and ``full`` preallocations are like the same 788 options of ``raw`` format, but sets up metadata also. 789 790 ``lazy_refcounts`` 791 If this option is set to ``on``, reference count updates are 792 postponed with the goal of avoiding metadata I/O and improving 793 performance. This is particularly interesting with 794 ``cache=writethrough`` which doesn't batch metadata 795 updates. The tradeoff is that after a host crash, the reference 796 count tables must be rebuilt, i.e. on the next open an (automatic) 797 ``qemu-img check -r all`` is required, which may take some time. 798 799 This option can only be enabled if ``compat=1.1`` is specified. 800 801 ``nocow`` 802 If this option is set to ``on``, it will turn off COW of the file. It's 803 only valid on btrfs, no effect on other file systems. 804 805 Btrfs has low performance when hosting a VM image file, even more 806 when the guest on the VM also using btrfs as file system. Turning 807 off COW is a way to mitigate this bad performance. Generally there 808 are two ways to turn off COW on btrfs: 809 810 - Disable it by mounting with nodatacow, then all newly created files 811 will be NOCOW 812 - For an empty file, add the NOCOW file attribute. That's what this 813 option does. 814 815 Note: this option is only valid to new or empty files. If there is 816 an existing file which is COW and has data blocks already, it 817 couldn't be changed to NOCOW by setting ``nocow=on``. One can 818 issue ``lsattr filename`` to check if the NOCOW flag is set or not 819 (Capital 'C' is NOCOW flag). 820 821``Other`` 822 823 QEMU also supports various other image file formats for 824 compatibility with older QEMU versions or other hypervisors, 825 including VMDK, VDI, VHD (vpc), VHDX, qcow1 and QED. For a full list 826 of supported formats see ``qemu-img --help``. For a more detailed 827 description of these formats, see the QEMU block drivers reference 828 documentation. 829 830 The main purpose of the block drivers for these formats is image 831 conversion. For running VMs, it is recommended to convert the disk 832 images to either raw or qcow2 in order to achieve good performance. 833