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