1QEMU disk image utility 2======================= 3 4Synopsis 5-------- 6 7**qemu-img** [*standard options*] *command* [*command options*] 8 9Description 10----------- 11 12qemu-img allows you to create, convert and modify images offline. It can handle 13all image formats supported by QEMU. 14 15**Warning:** Never use qemu-img to modify images in use by a running virtual 16machine or any other process; this may destroy the image. Also, be aware that 17querying an image that is being modified by another process may encounter 18inconsistent state. 19 20Options 21------- 22 23.. program:: qemu-img 24 25Standard options: 26 27.. option:: -h, --help 28 29 Display this help and exit 30 31.. option:: -V, --version 32 33 Display version information and exit 34 35.. option:: -T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE] 36 37 .. include:: ../qemu-option-trace.rst.inc 38 39The following commands are supported: 40 41.. hxtool-doc:: qemu-img-cmds.hx 42 43Command parameters: 44 45*FILENAME* is a disk image filename. 46 47*FMT* is the disk image format. It is guessed automatically in most 48cases. See below for a description of the supported disk formats. 49 50*SIZE* is the disk image size in bytes. Optional suffixes ``k`` or 51``K`` (kilobyte, 1024) ``M`` (megabyte, 1024k) and ``G`` (gigabyte, 521024M) and T (terabyte, 1024G) are supported. ``b`` is ignored. 53 54*OUTPUT_FILENAME* is the destination disk image filename. 55 56*OUTPUT_FMT* is the destination format. 57 58*OPTIONS* is a comma separated list of format specific options in a 59name=value format. Use ``-o ?`` for an overview of the options supported 60by the used format or see the format descriptions below for details. 61 62*SNAPSHOT_PARAM* is param used for internal snapshot, format is 63'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'. 64 65.. 66 Note the use of a new 'program'; otherwise Sphinx complains about 67 the -h option appearing both in the above option list and this one. 68 69.. program:: qemu-img-common-opts 70 71.. option:: --object OBJECTDEF 72 73 is a QEMU user creatable object definition. See the :manpage:`qemu(1)` 74 manual page for a description of the object properties. The most common 75 object type is a ``secret``, which is used to supply passwords and/or 76 encryption keys. 77 78.. option:: --image-opts 79 80 Indicates that the source *FILENAME* parameter is to be interpreted as a 81 full option string, not a plain filename. This parameter is mutually 82 exclusive with the *-f* parameter. 83 84.. option:: --target-image-opts 85 86 Indicates that the OUTPUT_FILENAME parameter(s) are to be interpreted as 87 a full option string, not a plain filename. This parameter is mutually 88 exclusive with the *-O* parameters. It is currently required to also use 89 the *-n* parameter to skip image creation. This restriction may be relaxed 90 in a future release. 91 92.. option:: --force-share (-U) 93 94 If specified, ``qemu-img`` will open the image in shared mode, allowing 95 other QEMU processes to open it in write mode. For example, this can be used to 96 get the image information (with 'info' subcommand) when the image is used by a 97 running guest. Note that this could produce inconsistent results because of 98 concurrent metadata changes, etc. This option is only allowed when opening 99 images in read-only mode. 100 101.. option:: --backing-chain 102 103 Will enumerate information about backing files in a disk image chain. Refer 104 below for further description. 105 106.. option:: -c 107 108 Indicates that target image must be compressed (qcow format only). 109 110.. option:: -h 111 112 With or without a command, shows help and lists the supported formats. 113 114.. option:: -p 115 116 Display progress bar (compare, convert and rebase commands only). 117 If the *-p* option is not used for a command that supports it, the 118 progress is reported when the process receives a ``SIGUSR1`` or 119 ``SIGINFO`` signal. 120 121.. option:: -q 122 123 Quiet mode - do not print any output (except errors). There's no progress bar 124 in case both *-q* and *-p* options are used. 125 126.. option:: -S SIZE 127 128 Indicates the consecutive number of bytes that must contain only zeros 129 for qemu-img to create a sparse image during conversion. This value is rounded 130 down to the nearest 512 bytes. You may use the common size suffixes like 131 ``k`` for kilobytes. 132 133.. option:: -t CACHE 134 135 Specifies the cache mode that should be used with the (destination) file. See 136 the documentation of the emulator's ``-drive cache=...`` option for allowed 137 values. 138 139.. option:: -T SRC_CACHE 140 141 Specifies the cache mode that should be used with the source file(s). See 142 the documentation of the emulator's ``-drive cache=...`` option for allowed 143 values. 144 145Parameters to compare subcommand: 146 147.. program:: qemu-img-compare 148 149.. option:: -f 150 151 First image format 152 153.. option:: -F 154 155 Second image format 156 157.. option:: -s 158 159 Strict mode - fail on different image size or sector allocation 160 161Parameters to convert subcommand: 162 163.. program:: qemu-img-convert 164 165.. option:: --bitmaps 166 167 Additionally copy all persistent bitmaps from the top layer of the source 168 169.. option:: -n 170 171 Skip the creation of the target volume 172 173.. option:: -m 174 175 Number of parallel coroutines for the convert process 176 177.. option:: -W 178 179 Allow out-of-order writes to the destination. This option improves performance, 180 but is only recommended for preallocated devices like host devices or other 181 raw block devices. 182 183.. option:: -C 184 185 Try to use copy offloading to move data from source image to target. This may 186 improve performance if the data is remote, such as with NFS or iSCSI backends, 187 but will not automatically sparsify zero sectors, and may result in a fully 188 allocated target image depending on the host support for getting allocation 189 information. 190 191.. option:: -r 192 193 Rate limit for the convert process 194 195.. option:: --salvage 196 197 Try to ignore I/O errors when reading. Unless in quiet mode (``-q``), errors 198 will still be printed. Areas that cannot be read from the source will be 199 treated as containing only zeroes. 200 201.. option:: --target-is-zero 202 203 Assume that reading the destination image will always return 204 zeros. This parameter is mutually exclusive with a destination image 205 that has a backing file. It is required to also use the ``-n`` 206 parameter to skip image creation. 207 208Parameters to dd subcommand: 209 210.. program:: qemu-img-dd 211 212.. option:: bs=BLOCK_SIZE 213 214 Defines the block size 215 216.. option:: count=BLOCKS 217 218 Sets the number of input blocks to copy 219 220.. option:: if=INPUT 221 222 Sets the input file 223 224.. option:: of=OUTPUT 225 226 Sets the output file 227 228.. option:: skip=BLOCKS 229 230 Sets the number of input blocks to skip 231 232Parameters to snapshot subcommand: 233 234.. program:: qemu-img-snapshot 235 236.. option:: snapshot 237 238 Is the name of the snapshot to create, apply or delete 239 240.. option:: -a 241 242 Applies a snapshot (revert disk to saved state) 243 244.. option:: -c 245 246 Creates a snapshot 247 248.. option:: -d 249 250 Deletes a snapshot 251 252.. option:: -l 253 254 Lists all snapshots in the given image 255 256Command description: 257 258.. program:: qemu-img-commands 259 260.. option:: amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE] [--force] -o OPTIONS FILENAME 261 262 Amends the image format specific *OPTIONS* for the image file 263 *FILENAME*. Not all file formats support this operation. 264 265 The set of options that can be amended are dependent on the image 266 format, but note that amending the backing chain relationship should 267 instead be performed with ``qemu-img rebase``. 268 269 --force allows some unsafe operations. Currently for -f luks, it allows to 270 erase the last encryption key, and to overwrite an active encryption key. 271 272.. 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 273 274 Run a simple sequential I/O benchmark on the specified image. If ``-w`` is 275 specified, a write test is performed, otherwise a read test is performed. 276 277 A total number of *COUNT* I/O requests is performed, each *BUFFER_SIZE* 278 bytes in size, and with *DEPTH* requests in parallel. The first request 279 starts at the position given by *OFFSET*, each following request increases 280 the current position by *STEP_SIZE*. If *STEP_SIZE* is not given, 281 *BUFFER_SIZE* is used for its value. 282 283 If *FLUSH_INTERVAL* is specified for a write test, the request queue is 284 drained and a flush is issued before new writes are made whenever the number of 285 remaining requests is a multiple of *FLUSH_INTERVAL*. If additionally 286 ``--no-drain`` is specified, a flush is issued without draining the request 287 queue first. 288 289 if ``-i`` is specified, *AIO* option can be used to specify different 290 AIO backends: ``threads``, ``native`` or ``io_uring``. 291 292 If ``-n`` is specified, the native AIO backend is used if possible. On 293 Linux, this option only works if ``-t none`` or ``-t directsync`` is 294 specified as well. 295 296 For write tests, by default a buffer filled with zeros is written. This can be 297 overridden with a pattern byte specified by *PATTERN*. 298 299.. 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 300 301 Perform one or more modifications of the persistent bitmap *BITMAP* 302 in the disk image *FILENAME*. The various modifications are: 303 304 ``--add`` to create *BITMAP*, enabled to record future edits. 305 306 ``--remove`` to remove *BITMAP*. 307 308 ``--clear`` to clear *BITMAP*. 309 310 ``--enable`` to change *BITMAP* to start recording future edits. 311 312 ``--disable`` to change *BITMAP* to stop recording future edits. 313 314 ``--merge`` to merge the contents of the *SOURCE* bitmap into *BITMAP*. 315 316 Additional options include ``-g`` which sets a non-default 317 *GRANULARITY* for ``--add``, and ``-b`` and ``-F`` which select an 318 alternative source file for all *SOURCE* bitmaps used by 319 ``--merge``. 320 321 To see what bitmaps are present in an image, use ``qemu-img info``. 322 323.. option:: check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT] [-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME 324 325 Perform a consistency check on the disk image *FILENAME*. The command can 326 output in the format *OFMT* which is either ``human`` or ``json``. 327 The JSON output is an object of QAPI type ``ImageCheck``. 328 329 If ``-r`` is specified, qemu-img tries to repair any inconsistencies found 330 during the check. ``-r leaks`` repairs only cluster leaks, whereas 331 ``-r all`` fixes all kinds of errors, with a higher risk of choosing the 332 wrong fix or hiding corruption that has already occurred. 333 334 Only the formats ``qcow2``, ``qed`` and ``vdi`` support 335 consistency checks. 336 337 In case the image does not have any inconsistencies, check exits with ``0``. 338 Other exit codes indicate the kind of inconsistency found or if another error 339 occurred. The following table summarizes all exit codes of the check subcommand: 340 341 0 342 Check completed, the image is (now) consistent 343 1 344 Check not completed because of internal errors 345 2 346 Check completed, image is corrupted 347 3 348 Check completed, image has leaked clusters, but is not corrupted 349 63 350 Checks are not supported by the image format 351 352 If ``-r`` is specified, exit codes representing the image state refer to the 353 state after (the attempt at) repairing it. That is, a successful ``-r all`` 354 will yield the exit code 0, independently of the image state before. 355 356.. option:: commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b BASE] [-r RATE_LIMIT] [-d] [-p] FILENAME 357 358 Commit the changes recorded in *FILENAME* in its base image or backing file. 359 If the backing file is smaller than the snapshot, then the backing file will be 360 resized to be the same size as the snapshot. If the snapshot is smaller than 361 the backing file, the backing file will not be truncated. If you want the 362 backing file to match the size of the smaller snapshot, you can safely truncate 363 it yourself once the commit operation successfully completes. 364 365 The image *FILENAME* is emptied after the operation has succeeded. If you do 366 not need *FILENAME* afterwards and intend to drop it, you may skip emptying 367 *FILENAME* by specifying the ``-d`` flag. 368 369 If the backing chain of the given image file *FILENAME* has more than one 370 layer, the backing file into which the changes will be committed may be 371 specified as *BASE* (which has to be part of *FILENAME*'s backing 372 chain). If *BASE* is not specified, the immediate backing file of the top 373 image (which is *FILENAME*) will be used. Note that after a commit operation 374 all images between *BASE* and the top image will be invalid and may return 375 garbage data when read. For this reason, ``-b`` implies ``-d`` (so that 376 the top image stays valid). 377 378 The rate limit for the commit process is specified by ``-r``. 379 380.. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 FILENAME2 381 382 Check if two images have the same content. You can compare images with 383 different format or settings. 384 385 The format is probed unless you specify it by ``-f`` (used for 386 *FILENAME1*) and/or ``-F`` (used for *FILENAME2*) option. 387 388 By default, images with different size are considered identical if the larger 389 image contains only unallocated and/or zeroed sectors in the area after the end 390 of the other image. In addition, if any sector is not allocated in one image 391 and contains only zero bytes in the second one, it is evaluated as equal. You 392 can use Strict mode by specifying the ``-s`` option. When compare runs in 393 Strict mode, it fails in case image size differs or a sector is allocated in 394 one image and is not allocated in the second one. 395 396 By default, compare prints out a result message. This message displays 397 information that both images are same or the position of the first different 398 byte. In addition, result message can report different image size in case 399 Strict mode is used. 400 401 Compare exits with ``0`` in case the images are equal and with ``1`` 402 in case the images differ. Other exit codes mean an error occurred during 403 execution and standard error output should contain an error message. 404 The following table sumarizes all exit codes of the compare subcommand: 405 406 0 407 Images are identical (or requested help was printed) 408 1 409 Images differ 410 2 411 Error on opening an image 412 3 413 Error on checking a sector allocation 414 4 415 Error on reading data 416 417.. option:: convert [--object OBJECTDEF] [--image-opts] [--target-image-opts] [--target-is-zero] [--bitmaps] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-O OUTPUT_FMT] [-B BACKING_FILE] [-o OPTIONS] [-l SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-r RATE_LIMIT] [-m NUM_COROUTINES] [-W] FILENAME [FILENAME2 [...]] OUTPUT_FILENAME 418 419 Convert the disk image *FILENAME* or a snapshot *SNAPSHOT_PARAM* 420 to disk image *OUTPUT_FILENAME* using format *OUTPUT_FMT*. It can 421 be optionally compressed (``-c`` option) or use any format specific 422 options like encryption (``-o`` option). 423 424 Only the formats ``qcow`` and ``qcow2`` support compression. The 425 compression is read-only. It means that if a compressed sector is 426 rewritten, then it is rewritten as uncompressed data. 427 428 Image conversion is also useful to get smaller image when using a 429 growable format such as ``qcow``: the empty sectors are detected and 430 suppressed from the destination image. 431 432 *SPARSE_SIZE* indicates the consecutive number of bytes (defaults to 4k) 433 that must contain only zeros for qemu-img to create a sparse image during 434 conversion. If *SPARSE_SIZE* is 0, the source will not be scanned for 435 unallocated or zero sectors, and the destination image will always be 436 fully allocated. 437 438 You can use the *BACKING_FILE* option to force the output image to be 439 created as a copy on write image of the specified base image; the 440 *BACKING_FILE* should have the same content as the input's base image, 441 however the path, image format, etc may differ. 442 443 If a relative path name is given, the backing file is looked up relative to 444 the directory containing *OUTPUT_FILENAME*. 445 446 If the ``-n`` option is specified, the target volume creation will be 447 skipped. This is useful for formats such as ``rbd`` if the target 448 volume has already been created with site specific options that cannot 449 be supplied through qemu-img. 450 451 Out of order writes can be enabled with ``-W`` to improve performance. 452 This is only recommended for preallocated devices like host devices or other 453 raw block devices. Out of order write does not work in combination with 454 creating compressed images. 455 456 *NUM_COROUTINES* specifies how many coroutines work in parallel during 457 the convert process (defaults to 8). 458 459.. option:: create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE] [-F BACKING_FMT] [-u] [-o OPTIONS] FILENAME [SIZE] 460 461 Create the new disk image *FILENAME* of size *SIZE* and format 462 *FMT*. Depending on the file format, you can add one or more *OPTIONS* 463 that enable additional features of this format. 464 465 If the option *BACKING_FILE* is specified, then the image will record 466 only the differences from *BACKING_FILE*. No size needs to be specified in 467 this case. *BACKING_FILE* will never be modified unless you use the 468 ``commit`` monitor command (or qemu-img commit). 469 470 If a relative path name is given, the backing file is looked up relative to 471 the directory containing *FILENAME*. 472 473 Note that a given backing file will be opened to check that it is valid. Use 474 the ``-u`` option to enable unsafe backing file mode, which means that the 475 image will be created even if the associated backing file cannot be opened. A 476 matching backing file must be created or additional options be used to make the 477 backing file specification valid when you want to use an image created this 478 way. 479 480 The size can also be specified using the *SIZE* option with ``-o``, 481 it doesn't need to be specified separately in this case. 482 483 484.. option:: dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE] [count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT 485 486 dd copies from *INPUT* file to *OUTPUT* file converting it from 487 *FMT* format to *OUTPUT_FMT* format. 488 489 The data is by default read and written using blocks of 512 bytes but can be 490 modified by specifying *BLOCK_SIZE*. If count=\ *BLOCKS* is specified 491 dd will stop reading input after reading *BLOCKS* input blocks. 492 493 The size syntax is similar to :manpage:`dd(1)`'s size syntax. 494 495.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [-U] FILENAME 496 497 Give information about the disk image *FILENAME*. Use it in 498 particular to know the size reserved on disk which can be different 499 from the displayed size. If VM snapshots are stored in the disk image, 500 they are displayed too. 501 502 If a disk image has a backing file chain, information about each disk image in 503 the chain can be recursively enumerated by using the option ``--backing-chain``. 504 505 For instance, if you have an image chain like: 506 507 :: 508 509 base.qcow2 <- snap1.qcow2 <- snap2.qcow2 510 511 To enumerate information about each disk image in the above chain, starting from top to base, do: 512 513 :: 514 515 qemu-img info --backing-chain snap2.qcow2 516 517 The command can output in the format *OFMT* which is either ``human`` or 518 ``json``. The JSON output is an object of QAPI type ``ImageInfo``; with 519 ``--backing-chain``, it is an array of ``ImageInfo`` objects. 520 521 ``--output=human`` reports the following information (for every image in the 522 chain): 523 524 *image* 525 The image file name 526 527 *file format* 528 The image format 529 530 *virtual size* 531 The size of the guest disk 532 533 *disk size* 534 How much space the image file occupies on the host file system (may be 535 shown as 0 if this information is unavailable, e.g. because there is no 536 file system) 537 538 *cluster_size* 539 Cluster size of the image format, if applicable 540 541 *encrypted* 542 Whether the image is encrypted (only present if so) 543 544 *cleanly shut down* 545 This is shown as ``no`` if the image is dirty and will have to be 546 auto-repaired the next time it is opened in qemu. 547 548 *backing file* 549 The backing file name, if present 550 551 *backing file format* 552 The format of the backing file, if the image enforces it 553 554 *Snapshot list* 555 A list of all internal snapshots 556 557 *Format specific information* 558 Further information whose structure depends on the image format. This 559 section is a textual representation of the respective 560 ``ImageInfoSpecific*`` QAPI object (e.g. ``ImageInfoSpecificQCow2`` 561 for qcow2 images). 562 563.. option:: map [--object OBJECTDEF] [--image-opts] [-f FMT] [--start-offset=OFFSET] [--max-length=LEN] [--output=OFMT] [-U] FILENAME 564 565 Dump the metadata of image *FILENAME* and its backing file chain. 566 In particular, this commands dumps the allocation state of every sector 567 of *FILENAME*, together with the topmost file that allocates it in 568 the backing file chain. 569 570 Two option formats are possible. The default format (``human``) 571 only dumps known-nonzero areas of the file. Known-zero parts of the 572 file are omitted altogether, and likewise for parts that are not allocated 573 throughout the chain. ``qemu-img`` output will identify a file 574 from where the data can be read, and the offset in the file. Each line 575 will include four fields, the first three of which are hexadecimal 576 numbers. For example the first line of: 577 578 :: 579 580 Offset Length Mapped to File 581 0 0x20000 0x50000 /tmp/overlay.qcow2 582 0x100000 0x10000 0x95380000 /tmp/backing.qcow2 583 584 means that 0x20000 (131072) bytes starting at offset 0 in the image are 585 available in /tmp/overlay.qcow2 (opened in ``raw`` format) starting 586 at offset 0x50000 (327680). Data that is compressed, encrypted, or 587 otherwise not available in raw format will cause an error if ``human`` 588 format is in use. Note that file names can include newlines, thus it is 589 not safe to parse this output format in scripts. 590 591 The alternative format ``json`` will return an array of dictionaries 592 in JSON format. It will include similar information in 593 the ``start``, ``length``, ``offset`` fields; 594 it will also include other more specific information: 595 596 - whether the sectors contain actual data or not (boolean field ``data``; 597 if false, the sectors are either unallocated or stored as optimized 598 all-zero clusters); 599 - whether the data is known to read as zero (boolean field ``zero``); 600 - in order to make the output shorter, the target file is expressed as 601 a ``depth``; for example, a depth of 2 refers to the backing file 602 of the backing file of *FILENAME*. 603 604 In JSON format, the ``offset`` field is optional; it is absent in 605 cases where ``human`` format would omit the entry or exit with an error. 606 If ``data`` is false and the ``offset`` field is present, the 607 corresponding sectors in the file are not yet in use, but they are 608 preallocated. 609 610 For more information, consult ``include/block/block.h`` in QEMU's 611 source code. 612 613.. option:: measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--object OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME] 614 615 Calculate the file size required for a new image. This information 616 can be used to size logical volumes or SAN LUNs appropriately for 617 the image that will be placed in them. The values reported are 618 guaranteed to be large enough to fit the image. The command can 619 output in the format *OFMT* which is either ``human`` or ``json``. 620 The JSON output is an object of QAPI type ``BlockMeasureInfo``. 621 622 If the size *N* is given then act as if creating a new empty image file 623 using ``qemu-img create``. If *FILENAME* is given then act as if 624 converting an existing image file using ``qemu-img convert``. The format 625 of the new file is given by *OUTPUT_FMT* while the format of an existing 626 file is given by *FMT*. 627 628 A snapshot in an existing image can be specified using *SNAPSHOT_PARAM*. 629 630 The following fields are reported: 631 632 :: 633 634 required size: 524288 635 fully allocated size: 1074069504 636 bitmaps size: 0 637 638 The ``required size`` is the file size of the new image. It may be smaller 639 than the virtual disk size if the image format supports compact representation. 640 641 The ``fully allocated size`` is the file size of the new image once data has 642 been written to all sectors. This is the maximum size that the image file can 643 occupy with the exception of internal snapshots, dirty bitmaps, vmstate data, 644 and other advanced image format features. 645 646 The ``bitmaps size`` is the additional size required in order to 647 copy bitmaps from a source image in addition to the guest-visible 648 data; the line is omitted if either source or destination lacks 649 bitmap support, or 0 if bitmaps are supported but there is nothing 650 to copy. 651 652.. option:: snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAPSHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME 653 654 List, apply, create or delete snapshots in image *FILENAME*. 655 656.. option:: rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-p] [-u] -b BACKING_FILE [-F BACKING_FMT] FILENAME 657 658 Changes the backing file of an image. Only the formats ``qcow2`` and 659 ``qed`` support changing the backing file. 660 661 The backing file is changed to *BACKING_FILE* and (if the image format of 662 *FILENAME* supports this) the backing file format is changed to 663 *BACKING_FMT*. If *BACKING_FILE* is specified as "" (the empty 664 string), then the image is rebased onto no backing file (i.e. it will exist 665 independently of any backing file). 666 667 If a relative path name is given, the backing file is looked up relative to 668 the directory containing *FILENAME*. 669 670 *CACHE* specifies the cache mode to be used for *FILENAME*, whereas 671 *SRC_CACHE* specifies the cache mode for reading backing files. 672 673 There are two different modes in which ``rebase`` can operate: 674 675 Safe mode 676 This is the default mode and performs a real rebase operation. The 677 new backing file may differ from the old one and qemu-img rebase 678 will take care of keeping the guest-visible content of *FILENAME* 679 unchanged. 680 681 In order to achieve this, any clusters that differ between 682 *BACKING_FILE* and the old backing file of *FILENAME* are merged 683 into *FILENAME* before actually changing the backing file. 684 685 Note that the safe mode is an expensive operation, comparable to 686 converting an image. It only works if the old backing file still 687 exists. 688 689 Unsafe mode 690 qemu-img uses the unsafe mode if ``-u`` is specified. In this 691 mode, only the backing file name and format of *FILENAME* is changed 692 without any checks on the file contents. The user must take care of 693 specifying the correct new backing file, or the guest-visible 694 content of the image will be corrupted. 695 696 This mode is useful for renaming or moving the backing file to 697 somewhere else. It can be used without an accessible old backing 698 file, i.e. you can use it to fix an image whose backing file has 699 already been moved/renamed. 700 701 You can use ``rebase`` to perform a "diff" operation on two 702 disk images. This can be useful when you have copied or cloned 703 a guest, and you want to get back to a thin image on top of a 704 template or base image. 705 706 Say that ``base.img`` has been cloned as ``modified.img`` by 707 copying it, and that the ``modified.img`` guest has run so there 708 are now some changes compared to ``base.img``. To construct a thin 709 image called ``diff.qcow2`` that contains just the differences, do: 710 711 :: 712 713 qemu-img create -f qcow2 -b modified.img diff.qcow2 714 qemu-img rebase -b base.img diff.qcow2 715 716 At this point, ``modified.img`` can be discarded, since 717 ``base.img + diff.qcow2`` contains the same information. 718 719.. option:: resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--preallocation=PREALLOC] [-q] [--shrink] FILENAME [+ | -]SIZE 720 721 Change the disk image as if it had been created with *SIZE*. 722 723 Before using this command to shrink a disk image, you MUST use file system and 724 partitioning tools inside the VM to reduce allocated file systems and partition 725 sizes accordingly. Failure to do so will result in data loss! 726 727 When shrinking images, the ``--shrink`` option must be given. This informs 728 qemu-img that the user acknowledges all loss of data beyond the truncated 729 image's end. 730 731 After using this command to grow a disk image, you must use file system and 732 partitioning tools inside the VM to actually begin using the new space on the 733 device. 734 735 When growing an image, the ``--preallocation`` option may be used to specify 736 how the additional image area should be allocated on the host. See the format 737 description in the :ref:`notes` section which values are allowed. Using this 738 option may result in slightly more data being allocated than necessary. 739 740.. _notes: 741 742Notes 743----- 744 745Supported image file formats: 746 747``raw`` 748 749 Raw disk image format (default). This format has the advantage of 750 being simple and easily exportable to all other emulators. If your 751 file system supports *holes* (for example in ext2 or ext3 on 752 Linux or NTFS on Windows), then only the written sectors will reserve 753 space. Use ``qemu-img info`` to know the real size used by the 754 image or ``ls -ls`` on Unix/Linux. 755 756 Supported options: 757 758 ``preallocation`` 759 Preallocation mode (allowed values: ``off``, ``falloc``, 760 ``full``). ``falloc`` mode preallocates space for image by 761 calling ``posix_fallocate()``. ``full`` mode preallocates space 762 for image by writing data to underlying storage. This data may or 763 may not be zero, depending on the storage location. 764 765``qcow2`` 766 767 QEMU image format, the most versatile format. Use it to have smaller 768 images (useful if your filesystem does not supports holes, for example 769 on Windows), optional AES encryption, zlib based compression and 770 support of multiple VM snapshots. 771 772 Supported options: 773 774 ``compat`` 775 Determines the qcow2 version to use. ``compat=0.10`` uses the 776 traditional image format that can be read by any QEMU since 0.10. 777 ``compat=1.1`` enables image format extensions that only QEMU 1.1 and 778 newer understand (this is the default). Amongst others, this includes zero 779 clusters, which allow efficient copy-on-read for sparse images. 780 781 ``backing_file`` 782 File name of a base image (see ``create`` subcommand) 783 784 ``backing_fmt`` 785 Image format of the base image 786 787 ``encryption`` 788 If this option is set to ``on``, the image is encrypted with 789 128-bit AES-CBC. 790 791 The use of encryption in qcow and qcow2 images is considered to be 792 flawed by modern cryptography standards, suffering from a number 793 of design problems: 794 795 - The AES-CBC cipher is used with predictable initialization 796 vectors based on the sector number. This makes it vulnerable to 797 chosen plaintext attacks which can reveal the existence of 798 encrypted data. 799 800 - The user passphrase is directly used as the encryption key. A 801 poorly chosen or short passphrase will compromise the security 802 of the encryption. 803 804 - In the event of the passphrase being compromised there is no way 805 to change the passphrase to protect data in any qcow images. The 806 files must be cloned, using a different encryption passphrase in 807 the new file. The original file must then be securely erased 808 using a program like shred, though even this is ineffective with 809 many modern storage technologies. 810 811 - Initialization vectors used to encrypt sectors are based on the 812 guest virtual sector number, instead of the host physical 813 sector. When a disk image has multiple internal snapshots this 814 means that data in multiple physical sectors is encrypted with 815 the same initialization vector. With the CBC mode, this opens 816 the possibility of watermarking attacks if the attack can 817 collect multiple sectors encrypted with the same IV and some 818 predictable data. Having multiple qcow2 images with the same 819 passphrase also exposes this weakness since the passphrase is 820 directly used as the key. 821 822 Use of qcow / qcow2 encryption is thus strongly discouraged. Users are 823 recommended to use an alternative encryption technology such as the 824 Linux dm-crypt / LUKS system. 825 826 ``cluster_size`` 827 Changes the qcow2 cluster size (must be between 512 and 828 2M). Smaller cluster sizes can improve the image file size whereas 829 larger cluster sizes generally provide better performance. 830 831 ``preallocation`` 832 Preallocation mode (allowed values: ``off``, ``metadata``, 833 ``falloc``, ``full``). An image with preallocated metadata is 834 initially larger but can improve performance when the image needs 835 to grow. ``falloc`` and ``full`` preallocations are like the same 836 options of ``raw`` format, but sets up metadata also. 837 838 ``lazy_refcounts`` 839 If this option is set to ``on``, reference count updates are 840 postponed with the goal of avoiding metadata I/O and improving 841 performance. This is particularly interesting with 842 ``cache=writethrough`` which doesn't batch metadata 843 updates. The tradeoff is that after a host crash, the reference 844 count tables must be rebuilt, i.e. on the next open an (automatic) 845 ``qemu-img check -r all`` is required, which may take some time. 846 847 This option can only be enabled if ``compat=1.1`` is specified. 848 849 ``nocow`` 850 If this option is set to ``on``, it will turn off COW of the file. It's 851 only valid on btrfs, no effect on other file systems. 852 853 Btrfs has low performance when hosting a VM image file, even more 854 when the guest on the VM also using btrfs as file system. Turning 855 off COW is a way to mitigate this bad performance. Generally there 856 are two ways to turn off COW on btrfs: 857 858 - Disable it by mounting with nodatacow, then all newly created files 859 will be NOCOW 860 - For an empty file, add the NOCOW file attribute. That's what this 861 option does. 862 863 Note: this option is only valid to new or empty files. If there is 864 an existing file which is COW and has data blocks already, it 865 couldn't be changed to NOCOW by setting ``nocow=on``. One can 866 issue ``lsattr filename`` to check if the NOCOW flag is set or not 867 (Capital 'C' is NOCOW flag). 868 869 ``data_file`` 870 Filename where all guest data will be stored. If this option is used, 871 the qcow2 file will only contain the image's metadata. 872 873 Note: Data loss will occur if the given filename already exists when 874 using this option with ``qemu-img create`` since ``qemu-img`` will create 875 the data file anew, overwriting the file's original contents. To simply 876 update the reference to point to the given pre-existing file, use 877 ``qemu-img amend``. 878 879 ``data_file_raw`` 880 If this option is set to ``on``, QEMU will always keep the external data 881 file consistent as a standalone read-only raw image. 882 883 It does this by forwarding all write accesses to the qcow2 file through to 884 the raw data file, including their offsets. Therefore, data that is visible 885 on the qcow2 node (i.e., to the guest) at some offset is visible at the same 886 offset in the raw data file. This results in a read-only raw image. Writes 887 that bypass the qcow2 metadata may corrupt the qcow2 metadata because the 888 out-of-band writes may result in the metadata falling out of sync with the 889 raw image. 890 891 If this option is ``off``, QEMU will use the data file to store data in an 892 arbitrary manner. The file’s content will not make sense without the 893 accompanying qcow2 metadata. Where data is written will have no relation to 894 its offset as seen by the guest, and some writes (specifically zero writes) 895 may not be forwarded to the data file at all, but will only be handled by 896 modifying qcow2 metadata. 897 898 This option can only be enabled if ``data_file`` is set. 899 900``Other`` 901 902 QEMU also supports various other image file formats for 903 compatibility with older QEMU versions or other hypervisors, 904 including VMDK, VDI, VHD (vpc), VHDX, qcow1 and QED. For a full list 905 of supported formats see ``qemu-img --help``. For a more detailed 906 description of these formats, see the QEMU block drivers reference 907 documentation. 908 909 The main purpose of the block drivers for these formats is image 910 conversion. For running VMs, it is recommended to convert the disk 911 images to either raw or qcow2 in order to achieve good performance. 912