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:: -n 166 167 Skip the creation of the target volume 168 169.. option:: -m 170 171 Number of parallel coroutines for the convert process 172 173.. option:: -W 174 175 Allow out-of-order writes to the destination. This option improves performance, 176 but is only recommended for preallocated devices like host devices or other 177 raw block devices. 178 179.. option:: -C 180 181 Try to use copy offloading to move data from source image to target. This may 182 improve performance if the data is remote, such as with NFS or iSCSI backends, 183 but will not automatically sparsify zero sectors, and may result in a fully 184 allocated target image depending on the host support for getting allocation 185 information. 186 187.. option:: --salvage 188 189 Try to ignore I/O errors when reading. Unless in quiet mode (``-q``), errors 190 will still be printed. Areas that cannot be read from the source will be 191 treated as containing only zeroes. 192 193.. option:: --target-is-zero 194 195 Assume that reading the destination image will always return 196 zeros. This parameter is mutually exclusive with a destination image 197 that has a backing file. It is required to also use the ``-n`` 198 parameter to skip image creation. 199 200Parameters to dd subcommand: 201 202.. program:: qemu-img-dd 203 204.. option:: bs=BLOCK_SIZE 205 206 Defines the block size 207 208.. option:: count=BLOCKS 209 210 Sets the number of input blocks to copy 211 212.. option:: if=INPUT 213 214 Sets the input file 215 216.. option:: of=OUTPUT 217 218 Sets the output file 219 220.. option:: skip=BLOCKS 221 222 Sets the number of input blocks to skip 223 224Parameters to snapshot subcommand: 225 226.. program:: qemu-img-snapshot 227 228.. option:: snapshot 229 230 Is the name of the snapshot to create, apply or delete 231 232.. option:: -a 233 234 Applies a snapshot (revert disk to saved state) 235 236.. option:: -c 237 238 Creates a snapshot 239 240.. option:: -d 241 242 Deletes a snapshot 243 244.. option:: -l 245 246 Lists all snapshots in the given image 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:: bitmap (--merge SOURCE | --add | --remove | --clear | --enable | --disable)... [-b SOURCE_FILE [-F SOURCE_FMT]] [-g GRANULARITY] [--object OBJECTDEF] [--image-opts | -f FMT] FILENAME BITMAP 285 286 Perform one or more modifications of the persistent bitmap *BITMAP* 287 in the disk image *FILENAME*. The various modifications are: 288 289 ``--add`` to create *BITMAP*, enabled to record future edits. 290 291 ``--remove`` to remove *BITMAP*. 292 293 ``--clear`` to clear *BITMAP*. 294 295 ``--enable`` to change *BITMAP* to start recording future edits. 296 297 ``--disable`` to change *BITMAP* to stop recording future edits. 298 299 ``--merge`` to merge the contents of *SOURCE_BITMAP* into *BITMAP*. 300 301 Additional options include ``-g`` which sets a non-default 302 *GRANULARITY* for ``--add``, and ``-b`` and ``-F`` which select an 303 alternative source file for all *SOURCE* bitmaps used by 304 ``--merge``. 305 306 To see what bitmaps are present in an image, use ``qemu-img info``. 307 308.. option:: check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT] [-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME 309 310 Perform a consistency check on the disk image *FILENAME*. The command can 311 output in the format *OFMT* which is either ``human`` or ``json``. 312 The JSON output is an object of QAPI type ``ImageCheck``. 313 314 If ``-r`` is specified, qemu-img tries to repair any inconsistencies found 315 during the check. ``-r leaks`` repairs only cluster leaks, whereas 316 ``-r all`` fixes all kinds of errors, with a higher risk of choosing the 317 wrong fix or hiding corruption that has already occurred. 318 319 Only the formats ``qcow2``, ``qed`` and ``vdi`` support 320 consistency checks. 321 322 In case the image does not have any inconsistencies, check exits with ``0``. 323 Other exit codes indicate the kind of inconsistency found or if another error 324 occurred. The following table summarizes all exit codes of the check subcommand: 325 326 0 327 Check completed, the image is (now) consistent 328 1 329 Check not completed because of internal errors 330 2 331 Check completed, image is corrupted 332 3 333 Check completed, image has leaked clusters, but is not corrupted 334 63 335 Checks are not supported by the image format 336 337 If ``-r`` is specified, exit codes representing the image state refer to the 338 state after (the attempt at) repairing it. That is, a successful ``-r all`` 339 will yield the exit code 0, independently of the image state before. 340 341.. option:: commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b BASE] [-d] [-p] FILENAME 342 343 Commit the changes recorded in *FILENAME* in its base image or backing file. 344 If the backing file is smaller than the snapshot, then the backing file will be 345 resized to be the same size as the snapshot. If the snapshot is smaller than 346 the backing file, the backing file will not be truncated. If you want the 347 backing file to match the size of the smaller snapshot, you can safely truncate 348 it yourself once the commit operation successfully completes. 349 350 The image *FILENAME* is emptied after the operation has succeeded. If you do 351 not need *FILENAME* afterwards and intend to drop it, you may skip emptying 352 *FILENAME* by specifying the ``-d`` flag. 353 354 If the backing chain of the given image file *FILENAME* has more than one 355 layer, the backing file into which the changes will be committed may be 356 specified as *BASE* (which has to be part of *FILENAME*'s backing 357 chain). If *BASE* is not specified, the immediate backing file of the top 358 image (which is *FILENAME*) will be used. Note that after a commit operation 359 all images between *BASE* and the top image will be invalid and may return 360 garbage data when read. For this reason, ``-b`` implies ``-d`` (so that 361 the top image stays valid). 362 363.. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 FILENAME2 364 365 Check if two images have the same content. You can compare images with 366 different format or settings. 367 368 The format is probed unless you specify it by ``-f`` (used for 369 *FILENAME1*) and/or ``-F`` (used for *FILENAME2*) option. 370 371 By default, images with different size are considered identical if the larger 372 image contains only unallocated and/or zeroed sectors in the area after the end 373 of the other image. In addition, if any sector is not allocated in one image 374 and contains only zero bytes in the second one, it is evaluated as equal. You 375 can use Strict mode by specifying the ``-s`` option. When compare runs in 376 Strict mode, it fails in case image size differs or a sector is allocated in 377 one image and is not allocated in the second one. 378 379 By default, compare prints out a result message. This message displays 380 information that both images are same or the position of the first different 381 byte. In addition, result message can report different image size in case 382 Strict mode is used. 383 384 Compare exits with ``0`` in case the images are equal and with ``1`` 385 in case the images differ. Other exit codes mean an error occurred during 386 execution and standard error output should contain an error message. 387 The following table sumarizes all exit codes of the compare subcommand: 388 389 0 390 Images are identical 391 1 392 Images differ 393 2 394 Error on opening an image 395 3 396 Error on checking a sector allocation 397 4 398 Error on reading data 399 400.. 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 401 402 Convert the disk image *FILENAME* or a snapshot *SNAPSHOT_PARAM* 403 to disk image *OUTPUT_FILENAME* using format *OUTPUT_FMT*. It can 404 be optionally compressed (``-c`` option) or use any format specific 405 options like encryption (``-o`` option). 406 407 Only the formats ``qcow`` and ``qcow2`` support compression. The 408 compression is read-only. It means that if a compressed sector is 409 rewritten, then it is rewritten as uncompressed data. 410 411 Image conversion is also useful to get smaller image when using a 412 growable format such as ``qcow``: the empty sectors are detected and 413 suppressed from the destination image. 414 415 *SPARSE_SIZE* indicates the consecutive number of bytes (defaults to 4k) 416 that must contain only zeros for qemu-img to create a sparse image during 417 conversion. If *SPARSE_SIZE* is 0, the source will not be scanned for 418 unallocated or zero sectors, and the destination image will always be 419 fully allocated. 420 421 You can use the *BACKING_FILE* option to force the output image to be 422 created as a copy on write image of the specified base image; the 423 *BACKING_FILE* should have the same content as the input's base image, 424 however the path, image format, etc may differ. 425 426 If a relative path name is given, the backing file is looked up relative to 427 the directory containing *OUTPUT_FILENAME*. 428 429 If the ``-n`` option is specified, the target volume creation will be 430 skipped. This is useful for formats such as ``rbd`` if the target 431 volume has already been created with site specific options that cannot 432 be supplied through qemu-img. 433 434 Out of order writes can be enabled with ``-W`` to improve performance. 435 This is only recommended for preallocated devices like host devices or other 436 raw block devices. Out of order write does not work in combination with 437 creating compressed images. 438 439 *NUM_COROUTINES* specifies how many coroutines work in parallel during 440 the convert process (defaults to 8). 441 442.. option:: create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE] [-F BACKING_FMT] [-u] [-o OPTIONS] FILENAME [SIZE] 443 444 Create the new disk image *FILENAME* of size *SIZE* and format 445 *FMT*. Depending on the file format, you can add one or more *OPTIONS* 446 that enable additional features of this format. 447 448 If the option *BACKING_FILE* is specified, then the image will record 449 only the differences from *BACKING_FILE*. No size needs to be specified in 450 this case. *BACKING_FILE* will never be modified unless you use the 451 ``commit`` monitor command (or qemu-img commit). 452 453 If a relative path name is given, the backing file is looked up relative to 454 the directory containing *FILENAME*. 455 456 Note that a given backing file will be opened to check that it is valid. Use 457 the ``-u`` option to enable unsafe backing file mode, which means that the 458 image will be created even if the associated backing file cannot be opened. A 459 matching backing file must be created or additional options be used to make the 460 backing file specification valid when you want to use an image created this 461 way. 462 463 The size can also be specified using the *SIZE* option with ``-o``, 464 it doesn't need to be specified separately in this case. 465 466 467.. option:: dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE] [count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT 468 469 dd copies from *INPUT* file to *OUTPUT* file converting it from 470 *FMT* format to *OUTPUT_FMT* format. 471 472 The data is by default read and written using blocks of 512 bytes but can be 473 modified by specifying *BLOCK_SIZE*. If count=\ *BLOCKS* is specified 474 dd will stop reading input after reading *BLOCKS* input blocks. 475 476 The size syntax is similar to :manpage:`dd(1)`'s size syntax. 477 478.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [-U] FILENAME 479 480 Give information about the disk image *FILENAME*. Use it in 481 particular to know the size reserved on disk which can be different 482 from the displayed size. If VM snapshots are stored in the disk image, 483 they are displayed too. 484 485 If a disk image has a backing file chain, information about each disk image in 486 the chain can be recursively enumerated by using the option ``--backing-chain``. 487 488 For instance, if you have an image chain like: 489 490 :: 491 492 base.qcow2 <- snap1.qcow2 <- snap2.qcow2 493 494 To enumerate information about each disk image in the above chain, starting from top to base, do: 495 496 :: 497 498 qemu-img info --backing-chain snap2.qcow2 499 500 The command can output in the format *OFMT* which is either ``human`` or 501 ``json``. The JSON output is an object of QAPI type ``ImageInfo``; with 502 ``--backing-chain``, it is an array of ``ImageInfo`` objects. 503 504 ``--output=human`` reports the following information (for every image in the 505 chain): 506 507 *image* 508 The image file name 509 510 *file format* 511 The image format 512 513 *virtual size* 514 The size of the guest disk 515 516 *disk size* 517 How much space the image file occupies on the host file system (may be 518 shown as 0 if this information is unavailable, e.g. because there is no 519 file system) 520 521 *cluster_size* 522 Cluster size of the image format, if applicable 523 524 *encrypted* 525 Whether the image is encrypted (only present if so) 526 527 *cleanly shut down* 528 This is shown as ``no`` if the image is dirty and will have to be 529 auto-repaired the next time it is opened in qemu. 530 531 *backing file* 532 The backing file name, if present 533 534 *backing file format* 535 The format of the backing file, if the image enforces it 536 537 *Snapshot list* 538 A list of all internal snapshots 539 540 *Format specific information* 541 Further information whose structure depends on the image format. This 542 section is a textual representation of the respective 543 ``ImageInfoSpecific*`` QAPI object (e.g. ``ImageInfoSpecificQCow2`` 544 for qcow2 images). 545 546.. option:: map [--object OBJECTDEF] [--image-opts] [-f FMT] [--start-offset=OFFSET] [--max-length=LEN] [--output=OFMT] [-U] FILENAME 547 548 Dump the metadata of image *FILENAME* and its backing file chain. 549 In particular, this commands dumps the allocation state of every sector 550 of *FILENAME*, together with the topmost file that allocates it in 551 the backing file chain. 552 553 Two option formats are possible. The default format (``human``) 554 only dumps known-nonzero areas of the file. Known-zero parts of the 555 file are omitted altogether, and likewise for parts that are not allocated 556 throughout the chain. ``qemu-img`` output will identify a file 557 from where the data can be read, and the offset in the file. Each line 558 will include four fields, the first three of which are hexadecimal 559 numbers. For example the first line of: 560 561 :: 562 563 Offset Length Mapped to File 564 0 0x20000 0x50000 /tmp/overlay.qcow2 565 0x100000 0x10000 0x95380000 /tmp/backing.qcow2 566 567 means that 0x20000 (131072) bytes starting at offset 0 in the image are 568 available in /tmp/overlay.qcow2 (opened in ``raw`` format) starting 569 at offset 0x50000 (327680). Data that is compressed, encrypted, or 570 otherwise not available in raw format will cause an error if ``human`` 571 format is in use. Note that file names can include newlines, thus it is 572 not safe to parse this output format in scripts. 573 574 The alternative format ``json`` will return an array of dictionaries 575 in JSON format. It will include similar information in 576 the ``start``, ``length``, ``offset`` fields; 577 it will also include other more specific information: 578 579 - whether the sectors contain actual data or not (boolean field ``data``; 580 if false, the sectors are either unallocated or stored as optimized 581 all-zero clusters); 582 - whether the data is known to read as zero (boolean field ``zero``); 583 - in order to make the output shorter, the target file is expressed as 584 a ``depth``; for example, a depth of 2 refers to the backing file 585 of the backing file of *FILENAME*. 586 587 In JSON format, the ``offset`` field is optional; it is absent in 588 cases where ``human`` format would omit the entry or exit with an error. 589 If ``data`` is false and the ``offset`` field is present, the 590 corresponding sectors in the file are not yet in use, but they are 591 preallocated. 592 593 For more information, consult ``include/block/block.h`` in QEMU's 594 source code. 595 596.. option:: measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--object OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME] 597 598 Calculate the file size required for a new image. This information 599 can be used to size logical volumes or SAN LUNs appropriately for 600 the image that will be placed in them. The values reported are 601 guaranteed to be large enough to fit the image. The command can 602 output in the format *OFMT* which is either ``human`` or ``json``. 603 The JSON output is an object of QAPI type ``BlockMeasureInfo``. 604 605 If the size *N* is given then act as if creating a new empty image file 606 using ``qemu-img create``. If *FILENAME* is given then act as if 607 converting an existing image file using ``qemu-img convert``. The format 608 of the new file is given by *OUTPUT_FMT* while the format of an existing 609 file is given by *FMT*. 610 611 A snapshot in an existing image can be specified using *SNAPSHOT_PARAM*. 612 613 The following fields are reported: 614 615 :: 616 617 required size: 524288 618 fully allocated size: 1074069504 619 620 The ``required size`` is the file size of the new image. It may be smaller 621 than the virtual disk size if the image format supports compact representation. 622 623 The ``fully allocated size`` is the file size of the new image once data has 624 been written to all sectors. This is the maximum size that the image file can 625 occupy with the exception of internal snapshots, dirty bitmaps, vmstate data, 626 and other advanced image format features. 627 628.. option:: snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAPSHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME 629 630 List, apply, create or delete snapshots in image *FILENAME*. 631 632.. option:: rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-p] [-u] -b BACKING_FILE [-F BACKING_FMT] FILENAME 633 634 Changes the backing file of an image. Only the formats ``qcow2`` and 635 ``qed`` support changing the backing file. 636 637 The backing file is changed to *BACKING_FILE* and (if the image format of 638 *FILENAME* supports this) the backing file format is changed to 639 *BACKING_FMT*. If *BACKING_FILE* is specified as "" (the empty 640 string), then the image is rebased onto no backing file (i.e. it will exist 641 independently of any backing file). 642 643 If a relative path name is given, the backing file is looked up relative to 644 the directory containing *FILENAME*. 645 646 *CACHE* specifies the cache mode to be used for *FILENAME*, whereas 647 *SRC_CACHE* specifies the cache mode for reading backing files. 648 649 There are two different modes in which ``rebase`` can operate: 650 651 Safe mode 652 This is the default mode and performs a real rebase operation. The 653 new backing file may differ from the old one and qemu-img rebase 654 will take care of keeping the guest-visible content of *FILENAME* 655 unchanged. 656 657 In order to achieve this, any clusters that differ between 658 *BACKING_FILE* and the old backing file of *FILENAME* are merged 659 into *FILENAME* before actually changing the backing file. 660 661 Note that the safe mode is an expensive operation, comparable to 662 converting an image. It only works if the old backing file still 663 exists. 664 665 Unsafe mode 666 qemu-img uses the unsafe mode if ``-u`` is specified. In this 667 mode, only the backing file name and format of *FILENAME* is changed 668 without any checks on the file contents. The user must take care of 669 specifying the correct new backing file, or the guest-visible 670 content of the image will be corrupted. 671 672 This mode is useful for renaming or moving the backing file to 673 somewhere else. It can be used without an accessible old backing 674 file, i.e. you can use it to fix an image whose backing file has 675 already been moved/renamed. 676 677 You can use ``rebase`` to perform a "diff" operation on two 678 disk images. This can be useful when you have copied or cloned 679 a guest, and you want to get back to a thin image on top of a 680 template or base image. 681 682 Say that ``base.img`` has been cloned as ``modified.img`` by 683 copying it, and that the ``modified.img`` guest has run so there 684 are now some changes compared to ``base.img``. To construct a thin 685 image called ``diff.qcow2`` that contains just the differences, do: 686 687 :: 688 689 qemu-img create -f qcow2 -b modified.img diff.qcow2 690 qemu-img rebase -b base.img diff.qcow2 691 692 At this point, ``modified.img`` can be discarded, since 693 ``base.img + diff.qcow2`` contains the same information. 694 695.. option:: resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--preallocation=PREALLOC] [-q] [--shrink] FILENAME [+ | -]SIZE 696 697 Change the disk image as if it had been created with *SIZE*. 698 699 Before using this command to shrink a disk image, you MUST use file system and 700 partitioning tools inside the VM to reduce allocated file systems and partition 701 sizes accordingly. Failure to do so will result in data loss! 702 703 When shrinking images, the ``--shrink`` option must be given. This informs 704 qemu-img that the user acknowledges all loss of data beyond the truncated 705 image's end. 706 707 After using this command to grow a disk image, you must use file system and 708 partitioning tools inside the VM to actually begin using the new space on the 709 device. 710 711 When growing an image, the ``--preallocation`` option may be used to specify 712 how the additional image area should be allocated on the host. See the format 713 description in the :ref:`notes` section which values are allowed. Using this 714 option may result in slightly more data being allocated than necessary. 715 716.. _notes: 717 718Notes 719----- 720 721Supported image file formats: 722 723``raw`` 724 725 Raw disk image format (default). This format has the advantage of 726 being simple and easily exportable to all other emulators. If your 727 file system supports *holes* (for example in ext2 or ext3 on 728 Linux or NTFS on Windows), then only the written sectors will reserve 729 space. Use ``qemu-img info`` to know the real size used by the 730 image or ``ls -ls`` on Unix/Linux. 731 732 Supported options: 733 734 ``preallocation`` 735 Preallocation mode (allowed values: ``off``, ``falloc``, 736 ``full``). ``falloc`` mode preallocates space for image by 737 calling ``posix_fallocate()``. ``full`` mode preallocates space 738 for image by writing data to underlying storage. This data may or 739 may not be zero, depending on the storage location. 740 741``qcow2`` 742 743 QEMU image format, the most versatile format. Use it to have smaller 744 images (useful if your filesystem does not supports holes, for example 745 on Windows), optional AES encryption, zlib based compression and 746 support of multiple VM snapshots. 747 748 Supported options: 749 750 ``compat`` 751 Determines the qcow2 version to use. ``compat=0.10`` uses the 752 traditional image format that can be read by any QEMU since 0.10. 753 ``compat=1.1`` enables image format extensions that only QEMU 1.1 and 754 newer understand (this is the default). Amongst others, this includes zero 755 clusters, which allow efficient copy-on-read for sparse images. 756 757 ``backing_file`` 758 File name of a base image (see ``create`` subcommand) 759 760 ``backing_fmt`` 761 Image format of the base image 762 763 ``encryption`` 764 If this option is set to ``on``, the image is encrypted with 765 128-bit AES-CBC. 766 767 The use of encryption in qcow and qcow2 images is considered to be 768 flawed by modern cryptography standards, suffering from a number 769 of design problems: 770 771 - The AES-CBC cipher is used with predictable initialization 772 vectors based on the sector number. This makes it vulnerable to 773 chosen plaintext attacks which can reveal the existence of 774 encrypted data. 775 776 - The user passphrase is directly used as the encryption key. A 777 poorly chosen or short passphrase will compromise the security 778 of the encryption. 779 780 - In the event of the passphrase being compromised there is no way 781 to change the passphrase to protect data in any qcow images. The 782 files must be cloned, using a different encryption passphrase in 783 the new file. The original file must then be securely erased 784 using a program like shred, though even this is ineffective with 785 many modern storage technologies. 786 787 - Initialization vectors used to encrypt sectors are based on the 788 guest virtual sector number, instead of the host physical 789 sector. When a disk image has multiple internal snapshots this 790 means that data in multiple physical sectors is encrypted with 791 the same initialization vector. With the CBC mode, this opens 792 the possibility of watermarking attacks if the attack can 793 collect multiple sectors encrypted with the same IV and some 794 predictable data. Having multiple qcow2 images with the same 795 passphrase also exposes this weakness since the passphrase is 796 directly used as the key. 797 798 Use of qcow / qcow2 encryption is thus strongly discouraged. Users are 799 recommended to use an alternative encryption technology such as the 800 Linux dm-crypt / LUKS system. 801 802 ``cluster_size`` 803 Changes the qcow2 cluster size (must be between 512 and 804 2M). Smaller cluster sizes can improve the image file size whereas 805 larger cluster sizes generally provide better performance. 806 807 ``preallocation`` 808 Preallocation mode (allowed values: ``off``, ``metadata``, 809 ``falloc``, ``full``). An image with preallocated metadata is 810 initially larger but can improve performance when the image needs 811 to grow. ``falloc`` and ``full`` preallocations are like the same 812 options of ``raw`` format, but sets up metadata also. 813 814 ``lazy_refcounts`` 815 If this option is set to ``on``, reference count updates are 816 postponed with the goal of avoiding metadata I/O and improving 817 performance. This is particularly interesting with 818 ``cache=writethrough`` which doesn't batch metadata 819 updates. The tradeoff is that after a host crash, the reference 820 count tables must be rebuilt, i.e. on the next open an (automatic) 821 ``qemu-img check -r all`` is required, which may take some time. 822 823 This option can only be enabled if ``compat=1.1`` is specified. 824 825 ``nocow`` 826 If this option is set to ``on``, it will turn off COW of the file. It's 827 only valid on btrfs, no effect on other file systems. 828 829 Btrfs has low performance when hosting a VM image file, even more 830 when the guest on the VM also using btrfs as file system. Turning 831 off COW is a way to mitigate this bad performance. Generally there 832 are two ways to turn off COW on btrfs: 833 834 - Disable it by mounting with nodatacow, then all newly created files 835 will be NOCOW 836 - For an empty file, add the NOCOW file attribute. That's what this 837 option does. 838 839 Note: this option is only valid to new or empty files. If there is 840 an existing file which is COW and has data blocks already, it 841 couldn't be changed to NOCOW by setting ``nocow=on``. One can 842 issue ``lsattr filename`` to check if the NOCOW flag is set or not 843 (Capital 'C' is NOCOW flag). 844 845``Other`` 846 847 QEMU also supports various other image file formats for 848 compatibility with older QEMU versions or other hypervisors, 849 including VMDK, VDI, VHD (vpc), VHDX, qcow1 and QED. For a full list 850 of supported formats see ``qemu-img --help``. For a more detailed 851 description of these formats, see the QEMU block drivers reference 852 documentation. 853 854 The main purpose of the block drivers for these formats is image 855 conversion. For running VMs, it is recommended to convert the disk 856 images to either raw or qcow2 in order to achieve good performance. 857