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] [--force] -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 The set of options that can be amended are dependent on the image 262 format, but note that amending the backing chain relationship should 263 instead be performed with ``qemu-img rebase``. 264 265 --force allows some unsafe operations. Currently for -f luks, it allows to 266 erase the last encryption key, and to overwrite an active encryption key. 267 268.. 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 269 270 Run a simple sequential I/O benchmark on the specified image. If ``-w`` is 271 specified, a write test is performed, otherwise a read test is performed. 272 273 A total number of *COUNT* I/O requests is performed, each *BUFFER_SIZE* 274 bytes in size, and with *DEPTH* requests in parallel. The first request 275 starts at the position given by *OFFSET*, each following request increases 276 the current position by *STEP_SIZE*. If *STEP_SIZE* is not given, 277 *BUFFER_SIZE* is used for its value. 278 279 If *FLUSH_INTERVAL* is specified for a write test, the request queue is 280 drained and a flush is issued before new writes are made whenever the number of 281 remaining requests is a multiple of *FLUSH_INTERVAL*. If additionally 282 ``--no-drain`` is specified, a flush is issued without draining the request 283 queue first. 284 285 if ``-i`` is specified, *AIO* option can be used to specify different 286 AIO backends: ``threads``, ``native`` or ``io_uring``. 287 288 If ``-n`` is specified, the native AIO backend is used if possible. On 289 Linux, this option only works if ``-t none`` or ``-t directsync`` is 290 specified as well. 291 292 For write tests, by default a buffer filled with zeros is written. This can be 293 overridden with a pattern byte specified by *PATTERN*. 294 295.. 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 296 297 Perform one or more modifications of the persistent bitmap *BITMAP* 298 in the disk image *FILENAME*. The various modifications are: 299 300 ``--add`` to create *BITMAP*, enabled to record future edits. 301 302 ``--remove`` to remove *BITMAP*. 303 304 ``--clear`` to clear *BITMAP*. 305 306 ``--enable`` to change *BITMAP* to start recording future edits. 307 308 ``--disable`` to change *BITMAP* to stop recording future edits. 309 310 ``--merge`` to merge the contents of the *SOURCE* bitmap into *BITMAP*. 311 312 Additional options include ``-g`` which sets a non-default 313 *GRANULARITY* for ``--add``, and ``-b`` and ``-F`` which select an 314 alternative source file for all *SOURCE* bitmaps used by 315 ``--merge``. 316 317 To see what bitmaps are present in an image, use ``qemu-img info``. 318 319.. option:: check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT] [-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME 320 321 Perform a consistency check on the disk image *FILENAME*. The command can 322 output in the format *OFMT* which is either ``human`` or ``json``. 323 The JSON output is an object of QAPI type ``ImageCheck``. 324 325 If ``-r`` is specified, qemu-img tries to repair any inconsistencies found 326 during the check. ``-r leaks`` repairs only cluster leaks, whereas 327 ``-r all`` fixes all kinds of errors, with a higher risk of choosing the 328 wrong fix or hiding corruption that has already occurred. 329 330 Only the formats ``qcow2``, ``qed`` and ``vdi`` support 331 consistency checks. 332 333 In case the image does not have any inconsistencies, check exits with ``0``. 334 Other exit codes indicate the kind of inconsistency found or if another error 335 occurred. The following table summarizes all exit codes of the check subcommand: 336 337 0 338 Check completed, the image is (now) consistent 339 1 340 Check not completed because of internal errors 341 2 342 Check completed, image is corrupted 343 3 344 Check completed, image has leaked clusters, but is not corrupted 345 63 346 Checks are not supported by the image format 347 348 If ``-r`` is specified, exit codes representing the image state refer to the 349 state after (the attempt at) repairing it. That is, a successful ``-r all`` 350 will yield the exit code 0, independently of the image state before. 351 352.. option:: commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b BASE] [-d] [-p] FILENAME 353 354 Commit the changes recorded in *FILENAME* in its base image or backing file. 355 If the backing file is smaller than the snapshot, then the backing file will be 356 resized to be the same size as the snapshot. If the snapshot is smaller than 357 the backing file, the backing file will not be truncated. If you want the 358 backing file to match the size of the smaller snapshot, you can safely truncate 359 it yourself once the commit operation successfully completes. 360 361 The image *FILENAME* is emptied after the operation has succeeded. If you do 362 not need *FILENAME* afterwards and intend to drop it, you may skip emptying 363 *FILENAME* by specifying the ``-d`` flag. 364 365 If the backing chain of the given image file *FILENAME* has more than one 366 layer, the backing file into which the changes will be committed may be 367 specified as *BASE* (which has to be part of *FILENAME*'s backing 368 chain). If *BASE* is not specified, the immediate backing file of the top 369 image (which is *FILENAME*) will be used. Note that after a commit operation 370 all images between *BASE* and the top image will be invalid and may return 371 garbage data when read. For this reason, ``-b`` implies ``-d`` (so that 372 the top image stays valid). 373 374.. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 FILENAME2 375 376 Check if two images have the same content. You can compare images with 377 different format or settings. 378 379 The format is probed unless you specify it by ``-f`` (used for 380 *FILENAME1*) and/or ``-F`` (used for *FILENAME2*) option. 381 382 By default, images with different size are considered identical if the larger 383 image contains only unallocated and/or zeroed sectors in the area after the end 384 of the other image. In addition, if any sector is not allocated in one image 385 and contains only zero bytes in the second one, it is evaluated as equal. You 386 can use Strict mode by specifying the ``-s`` option. When compare runs in 387 Strict mode, it fails in case image size differs or a sector is allocated in 388 one image and is not allocated in the second one. 389 390 By default, compare prints out a result message. This message displays 391 information that both images are same or the position of the first different 392 byte. In addition, result message can report different image size in case 393 Strict mode is used. 394 395 Compare exits with ``0`` in case the images are equal and with ``1`` 396 in case the images differ. Other exit codes mean an error occurred during 397 execution and standard error output should contain an error message. 398 The following table sumarizes all exit codes of the compare subcommand: 399 400 0 401 Images are identical 402 1 403 Images differ 404 2 405 Error on opening an image 406 3 407 Error on checking a sector allocation 408 4 409 Error on reading data 410 411.. 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 412 413 Convert the disk image *FILENAME* or a snapshot *SNAPSHOT_PARAM* 414 to disk image *OUTPUT_FILENAME* using format *OUTPUT_FMT*. It can 415 be optionally compressed (``-c`` option) or use any format specific 416 options like encryption (``-o`` option). 417 418 Only the formats ``qcow`` and ``qcow2`` support compression. The 419 compression is read-only. It means that if a compressed sector is 420 rewritten, then it is rewritten as uncompressed data. 421 422 Image conversion is also useful to get smaller image when using a 423 growable format such as ``qcow``: the empty sectors are detected and 424 suppressed from the destination image. 425 426 *SPARSE_SIZE* indicates the consecutive number of bytes (defaults to 4k) 427 that must contain only zeros for qemu-img to create a sparse image during 428 conversion. If *SPARSE_SIZE* is 0, the source will not be scanned for 429 unallocated or zero sectors, and the destination image will always be 430 fully allocated. 431 432 You can use the *BACKING_FILE* option to force the output image to be 433 created as a copy on write image of the specified base image; the 434 *BACKING_FILE* should have the same content as the input's base image, 435 however the path, image format, etc may differ. 436 437 If a relative path name is given, the backing file is looked up relative to 438 the directory containing *OUTPUT_FILENAME*. 439 440 If the ``-n`` option is specified, the target volume creation will be 441 skipped. This is useful for formats such as ``rbd`` if the target 442 volume has already been created with site specific options that cannot 443 be supplied through qemu-img. 444 445 Out of order writes can be enabled with ``-W`` to improve performance. 446 This is only recommended for preallocated devices like host devices or other 447 raw block devices. Out of order write does not work in combination with 448 creating compressed images. 449 450 *NUM_COROUTINES* specifies how many coroutines work in parallel during 451 the convert process (defaults to 8). 452 453.. option:: create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE] [-F BACKING_FMT] [-u] [-o OPTIONS] FILENAME [SIZE] 454 455 Create the new disk image *FILENAME* of size *SIZE* and format 456 *FMT*. Depending on the file format, you can add one or more *OPTIONS* 457 that enable additional features of this format. 458 459 If the option *BACKING_FILE* is specified, then the image will record 460 only the differences from *BACKING_FILE*. No size needs to be specified in 461 this case. *BACKING_FILE* will never be modified unless you use the 462 ``commit`` monitor command (or qemu-img commit). 463 464 If a relative path name is given, the backing file is looked up relative to 465 the directory containing *FILENAME*. 466 467 Note that a given backing file will be opened to check that it is valid. Use 468 the ``-u`` option to enable unsafe backing file mode, which means that the 469 image will be created even if the associated backing file cannot be opened. A 470 matching backing file must be created or additional options be used to make the 471 backing file specification valid when you want to use an image created this 472 way. 473 474 The size can also be specified using the *SIZE* option with ``-o``, 475 it doesn't need to be specified separately in this case. 476 477 478.. option:: dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE] [count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT 479 480 dd copies from *INPUT* file to *OUTPUT* file converting it from 481 *FMT* format to *OUTPUT_FMT* format. 482 483 The data is by default read and written using blocks of 512 bytes but can be 484 modified by specifying *BLOCK_SIZE*. If count=\ *BLOCKS* is specified 485 dd will stop reading input after reading *BLOCKS* input blocks. 486 487 The size syntax is similar to :manpage:`dd(1)`'s size syntax. 488 489.. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [-U] FILENAME 490 491 Give information about the disk image *FILENAME*. Use it in 492 particular to know the size reserved on disk which can be different 493 from the displayed size. If VM snapshots are stored in the disk image, 494 they are displayed too. 495 496 If a disk image has a backing file chain, information about each disk image in 497 the chain can be recursively enumerated by using the option ``--backing-chain``. 498 499 For instance, if you have an image chain like: 500 501 :: 502 503 base.qcow2 <- snap1.qcow2 <- snap2.qcow2 504 505 To enumerate information about each disk image in the above chain, starting from top to base, do: 506 507 :: 508 509 qemu-img info --backing-chain snap2.qcow2 510 511 The command can output in the format *OFMT* which is either ``human`` or 512 ``json``. The JSON output is an object of QAPI type ``ImageInfo``; with 513 ``--backing-chain``, it is an array of ``ImageInfo`` objects. 514 515 ``--output=human`` reports the following information (for every image in the 516 chain): 517 518 *image* 519 The image file name 520 521 *file format* 522 The image format 523 524 *virtual size* 525 The size of the guest disk 526 527 *disk size* 528 How much space the image file occupies on the host file system (may be 529 shown as 0 if this information is unavailable, e.g. because there is no 530 file system) 531 532 *cluster_size* 533 Cluster size of the image format, if applicable 534 535 *encrypted* 536 Whether the image is encrypted (only present if so) 537 538 *cleanly shut down* 539 This is shown as ``no`` if the image is dirty and will have to be 540 auto-repaired the next time it is opened in qemu. 541 542 *backing file* 543 The backing file name, if present 544 545 *backing file format* 546 The format of the backing file, if the image enforces it 547 548 *Snapshot list* 549 A list of all internal snapshots 550 551 *Format specific information* 552 Further information whose structure depends on the image format. This 553 section is a textual representation of the respective 554 ``ImageInfoSpecific*`` QAPI object (e.g. ``ImageInfoSpecificQCow2`` 555 for qcow2 images). 556 557.. option:: map [--object OBJECTDEF] [--image-opts] [-f FMT] [--start-offset=OFFSET] [--max-length=LEN] [--output=OFMT] [-U] FILENAME 558 559 Dump the metadata of image *FILENAME* and its backing file chain. 560 In particular, this commands dumps the allocation state of every sector 561 of *FILENAME*, together with the topmost file that allocates it in 562 the backing file chain. 563 564 Two option formats are possible. The default format (``human``) 565 only dumps known-nonzero areas of the file. Known-zero parts of the 566 file are omitted altogether, and likewise for parts that are not allocated 567 throughout the chain. ``qemu-img`` output will identify a file 568 from where the data can be read, and the offset in the file. Each line 569 will include four fields, the first three of which are hexadecimal 570 numbers. For example the first line of: 571 572 :: 573 574 Offset Length Mapped to File 575 0 0x20000 0x50000 /tmp/overlay.qcow2 576 0x100000 0x10000 0x95380000 /tmp/backing.qcow2 577 578 means that 0x20000 (131072) bytes starting at offset 0 in the image are 579 available in /tmp/overlay.qcow2 (opened in ``raw`` format) starting 580 at offset 0x50000 (327680). Data that is compressed, encrypted, or 581 otherwise not available in raw format will cause an error if ``human`` 582 format is in use. Note that file names can include newlines, thus it is 583 not safe to parse this output format in scripts. 584 585 The alternative format ``json`` will return an array of dictionaries 586 in JSON format. It will include similar information in 587 the ``start``, ``length``, ``offset`` fields; 588 it will also include other more specific information: 589 590 - whether the sectors contain actual data or not (boolean field ``data``; 591 if false, the sectors are either unallocated or stored as optimized 592 all-zero clusters); 593 - whether the data is known to read as zero (boolean field ``zero``); 594 - in order to make the output shorter, the target file is expressed as 595 a ``depth``; for example, a depth of 2 refers to the backing file 596 of the backing file of *FILENAME*. 597 598 In JSON format, the ``offset`` field is optional; it is absent in 599 cases where ``human`` format would omit the entry or exit with an error. 600 If ``data`` is false and the ``offset`` field is present, the 601 corresponding sectors in the file are not yet in use, but they are 602 preallocated. 603 604 For more information, consult ``include/block/block.h`` in QEMU's 605 source code. 606 607.. option:: measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--object OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME] 608 609 Calculate the file size required for a new image. This information 610 can be used to size logical volumes or SAN LUNs appropriately for 611 the image that will be placed in them. The values reported are 612 guaranteed to be large enough to fit the image. The command can 613 output in the format *OFMT* which is either ``human`` or ``json``. 614 The JSON output is an object of QAPI type ``BlockMeasureInfo``. 615 616 If the size *N* is given then act as if creating a new empty image file 617 using ``qemu-img create``. If *FILENAME* is given then act as if 618 converting an existing image file using ``qemu-img convert``. The format 619 of the new file is given by *OUTPUT_FMT* while the format of an existing 620 file is given by *FMT*. 621 622 A snapshot in an existing image can be specified using *SNAPSHOT_PARAM*. 623 624 The following fields are reported: 625 626 :: 627 628 required size: 524288 629 fully allocated size: 1074069504 630 bitmaps size: 0 631 632 The ``required size`` is the file size of the new image. It may be smaller 633 than the virtual disk size if the image format supports compact representation. 634 635 The ``fully allocated size`` is the file size of the new image once data has 636 been written to all sectors. This is the maximum size that the image file can 637 occupy with the exception of internal snapshots, dirty bitmaps, vmstate data, 638 and other advanced image format features. 639 640 The ``bitmaps size`` is the additional size required in order to 641 copy bitmaps from a source image in addition to the guest-visible 642 data; the line is omitted if either source or destination lacks 643 bitmap support, or 0 if bitmaps are supported but there is nothing 644 to copy. 645 646.. option:: snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAPSHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME 647 648 List, apply, create or delete snapshots in image *FILENAME*. 649 650.. option:: rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-p] [-u] -b BACKING_FILE [-F BACKING_FMT] FILENAME 651 652 Changes the backing file of an image. Only the formats ``qcow2`` and 653 ``qed`` support changing the backing file. 654 655 The backing file is changed to *BACKING_FILE* and (if the image format of 656 *FILENAME* supports this) the backing file format is changed to 657 *BACKING_FMT*. If *BACKING_FILE* is specified as "" (the empty 658 string), then the image is rebased onto no backing file (i.e. it will exist 659 independently of any backing file). 660 661 If a relative path name is given, the backing file is looked up relative to 662 the directory containing *FILENAME*. 663 664 *CACHE* specifies the cache mode to be used for *FILENAME*, whereas 665 *SRC_CACHE* specifies the cache mode for reading backing files. 666 667 There are two different modes in which ``rebase`` can operate: 668 669 Safe mode 670 This is the default mode and performs a real rebase operation. The 671 new backing file may differ from the old one and qemu-img rebase 672 will take care of keeping the guest-visible content of *FILENAME* 673 unchanged. 674 675 In order to achieve this, any clusters that differ between 676 *BACKING_FILE* and the old backing file of *FILENAME* are merged 677 into *FILENAME* before actually changing the backing file. 678 679 Note that the safe mode is an expensive operation, comparable to 680 converting an image. It only works if the old backing file still 681 exists. 682 683 Unsafe mode 684 qemu-img uses the unsafe mode if ``-u`` is specified. In this 685 mode, only the backing file name and format of *FILENAME* is changed 686 without any checks on the file contents. The user must take care of 687 specifying the correct new backing file, or the guest-visible 688 content of the image will be corrupted. 689 690 This mode is useful for renaming or moving the backing file to 691 somewhere else. It can be used without an accessible old backing 692 file, i.e. you can use it to fix an image whose backing file has 693 already been moved/renamed. 694 695 You can use ``rebase`` to perform a "diff" operation on two 696 disk images. This can be useful when you have copied or cloned 697 a guest, and you want to get back to a thin image on top of a 698 template or base image. 699 700 Say that ``base.img`` has been cloned as ``modified.img`` by 701 copying it, and that the ``modified.img`` guest has run so there 702 are now some changes compared to ``base.img``. To construct a thin 703 image called ``diff.qcow2`` that contains just the differences, do: 704 705 :: 706 707 qemu-img create -f qcow2 -b modified.img diff.qcow2 708 qemu-img rebase -b base.img diff.qcow2 709 710 At this point, ``modified.img`` can be discarded, since 711 ``base.img + diff.qcow2`` contains the same information. 712 713.. option:: resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--preallocation=PREALLOC] [-q] [--shrink] FILENAME [+ | -]SIZE 714 715 Change the disk image as if it had been created with *SIZE*. 716 717 Before using this command to shrink a disk image, you MUST use file system and 718 partitioning tools inside the VM to reduce allocated file systems and partition 719 sizes accordingly. Failure to do so will result in data loss! 720 721 When shrinking images, the ``--shrink`` option must be given. This informs 722 qemu-img that the user acknowledges all loss of data beyond the truncated 723 image's end. 724 725 After using this command to grow a disk image, you must use file system and 726 partitioning tools inside the VM to actually begin using the new space on the 727 device. 728 729 When growing an image, the ``--preallocation`` option may be used to specify 730 how the additional image area should be allocated on the host. See the format 731 description in the :ref:`notes` section which values are allowed. Using this 732 option may result in slightly more data being allocated than necessary. 733 734.. _notes: 735 736Notes 737----- 738 739Supported image file formats: 740 741``raw`` 742 743 Raw disk image format (default). This format has the advantage of 744 being simple and easily exportable to all other emulators. If your 745 file system supports *holes* (for example in ext2 or ext3 on 746 Linux or NTFS on Windows), then only the written sectors will reserve 747 space. Use ``qemu-img info`` to know the real size used by the 748 image or ``ls -ls`` on Unix/Linux. 749 750 Supported options: 751 752 ``preallocation`` 753 Preallocation mode (allowed values: ``off``, ``falloc``, 754 ``full``). ``falloc`` mode preallocates space for image by 755 calling ``posix_fallocate()``. ``full`` mode preallocates space 756 for image by writing data to underlying storage. This data may or 757 may not be zero, depending on the storage location. 758 759``qcow2`` 760 761 QEMU image format, the most versatile format. Use it to have smaller 762 images (useful if your filesystem does not supports holes, for example 763 on Windows), optional AES encryption, zlib based compression and 764 support of multiple VM snapshots. 765 766 Supported options: 767 768 ``compat`` 769 Determines the qcow2 version to use. ``compat=0.10`` uses the 770 traditional image format that can be read by any QEMU since 0.10. 771 ``compat=1.1`` enables image format extensions that only QEMU 1.1 and 772 newer understand (this is the default). Amongst others, this includes zero 773 clusters, which allow efficient copy-on-read for sparse images. 774 775 ``backing_file`` 776 File name of a base image (see ``create`` subcommand) 777 778 ``backing_fmt`` 779 Image format of the base image 780 781 ``encryption`` 782 If this option is set to ``on``, the image is encrypted with 783 128-bit AES-CBC. 784 785 The use of encryption in qcow and qcow2 images is considered to be 786 flawed by modern cryptography standards, suffering from a number 787 of design problems: 788 789 - The AES-CBC cipher is used with predictable initialization 790 vectors based on the sector number. This makes it vulnerable to 791 chosen plaintext attacks which can reveal the existence of 792 encrypted data. 793 794 - The user passphrase is directly used as the encryption key. A 795 poorly chosen or short passphrase will compromise the security 796 of the encryption. 797 798 - In the event of the passphrase being compromised there is no way 799 to change the passphrase to protect data in any qcow images. The 800 files must be cloned, using a different encryption passphrase in 801 the new file. The original file must then be securely erased 802 using a program like shred, though even this is ineffective with 803 many modern storage technologies. 804 805 - Initialization vectors used to encrypt sectors are based on the 806 guest virtual sector number, instead of the host physical 807 sector. When a disk image has multiple internal snapshots this 808 means that data in multiple physical sectors is encrypted with 809 the same initialization vector. With the CBC mode, this opens 810 the possibility of watermarking attacks if the attack can 811 collect multiple sectors encrypted with the same IV and some 812 predictable data. Having multiple qcow2 images with the same 813 passphrase also exposes this weakness since the passphrase is 814 directly used as the key. 815 816 Use of qcow / qcow2 encryption is thus strongly discouraged. Users are 817 recommended to use an alternative encryption technology such as the 818 Linux dm-crypt / LUKS system. 819 820 ``cluster_size`` 821 Changes the qcow2 cluster size (must be between 512 and 822 2M). Smaller cluster sizes can improve the image file size whereas 823 larger cluster sizes generally provide better performance. 824 825 ``preallocation`` 826 Preallocation mode (allowed values: ``off``, ``metadata``, 827 ``falloc``, ``full``). An image with preallocated metadata is 828 initially larger but can improve performance when the image needs 829 to grow. ``falloc`` and ``full`` preallocations are like the same 830 options of ``raw`` format, but sets up metadata also. 831 832 ``lazy_refcounts`` 833 If this option is set to ``on``, reference count updates are 834 postponed with the goal of avoiding metadata I/O and improving 835 performance. This is particularly interesting with 836 ``cache=writethrough`` which doesn't batch metadata 837 updates. The tradeoff is that after a host crash, the reference 838 count tables must be rebuilt, i.e. on the next open an (automatic) 839 ``qemu-img check -r all`` is required, which may take some time. 840 841 This option can only be enabled if ``compat=1.1`` is specified. 842 843 ``nocow`` 844 If this option is set to ``on``, it will turn off COW of the file. It's 845 only valid on btrfs, no effect on other file systems. 846 847 Btrfs has low performance when hosting a VM image file, even more 848 when the guest on the VM also using btrfs as file system. Turning 849 off COW is a way to mitigate this bad performance. Generally there 850 are two ways to turn off COW on btrfs: 851 852 - Disable it by mounting with nodatacow, then all newly created files 853 will be NOCOW 854 - For an empty file, add the NOCOW file attribute. That's what this 855 option does. 856 857 Note: this option is only valid to new or empty files. If there is 858 an existing file which is COW and has data blocks already, it 859 couldn't be changed to NOCOW by setting ``nocow=on``. One can 860 issue ``lsattr filename`` to check if the NOCOW flag is set or not 861 (Capital 'C' is NOCOW flag). 862 863``Other`` 864 865 QEMU also supports various other image file formats for 866 compatibility with older QEMU versions or other hypervisors, 867 including VMDK, VDI, VHD (vpc), VHDX, qcow1 and QED. For a full list 868 of supported formats see ``qemu-img --help``. For a more detailed 869 description of these formats, see the QEMU block drivers reference 870 documentation. 871 872 The main purpose of the block drivers for these formats is image 873 conversion. For running VMs, it is recommended to convert the disk 874 images to either raw or qcow2 in order to achieve good performance. 875