1@example 2@c man begin SYNOPSIS 3@command{qemu-img} [@var{standard} @var{options}] @var{command} [@var{command} @var{options}] 4@c man end 5@end example 6 7@c man begin DESCRIPTION 8qemu-img allows you to create, convert and modify images offline. It can handle 9all image formats supported by QEMU. 10 11@b{Warning:} Never use qemu-img to modify images in use by a running virtual 12machine or any other process; this may destroy the image. Also, be aware that 13querying an image that is being modified by another process may encounter 14inconsistent state. 15@c man end 16 17@c man begin OPTIONS 18 19Standard options: 20@table @option 21@item -h, --help 22Display this help and exit 23@item -V, --version 24Display version information and exit 25@item -T, --trace [[enable=]@var{pattern}][,events=@var{file}][,file=@var{file}] 26@findex --trace 27@include qemu-option-trace.texi 28@end table 29 30The following commands are supported: 31 32@include qemu-img-cmds.texi 33 34Command parameters: 35@table @var 36 37@item filename 38is a disk image filename 39 40@item fmt 41is the disk image format. It is guessed automatically in most cases. See below 42for a description of the supported disk formats. 43 44@item size 45is the disk image size in bytes. Optional suffixes @code{k} or @code{K} 46(kilobyte, 1024) @code{M} (megabyte, 1024k) and @code{G} (gigabyte, 1024M) 47and T (terabyte, 1024G) are supported. @code{b} is ignored. 48 49@item output_filename 50is the destination disk image filename 51 52@item output_fmt 53is the destination format 54 55@item options 56is a comma separated list of format specific options in a 57name=value format. Use @code{-o ?} for an overview of the options supported 58by the used format or see the format descriptions below for details. 59 60@item snapshot_param 61is param used for internal snapshot, format is 62'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]' 63 64@item snapshot_id_or_name 65is deprecated, use snapshot_param instead 66 67@end table 68 69@table @option 70 71@item --object @var{objectdef} 72is a QEMU user creatable object definition. See the @code{qemu(1)} manual 73page for a description of the object properties. The most common object 74type is a @code{secret}, which is used to supply passwords and/or encryption 75keys. 76 77@item --image-opts 78Indicates that the source @var{filename} parameter is to be interpreted as a 79full option string, not a plain filename. This parameter is mutually 80exclusive with the @var{-f} parameter. 81 82@item --target-image-opts 83Indicates that the @var{output_filename} parameter(s) are to be interpreted as 84a full option string, not a plain filename. This parameter is mutually 85exclusive with the @var{-O} parameters. It is currently required to also use 86the @var{-n} parameter to skip image creation. This restriction may be relaxed 87in a future release. 88 89@item --force-share (-U) 90If specified, @code{qemu-img} will open the image in shared mode, allowing 91other QEMU processes to open it in write mode. For example, this can be used to 92get the image information (with 'info' subcommand) when the image is used by a 93running guest. Note that this could produce inconsistent results because of 94concurrent metadata changes, etc. This option is only allowed when opening 95images in read-only mode. 96 97@item --backing-chain 98will enumerate information about backing files in a disk image chain. Refer 99below for further description. 100 101@item -c 102indicates that target image must be compressed (qcow format only) 103 104@item -h 105with or without a command shows help and lists the supported formats 106 107@item -p 108display progress bar (compare, convert and rebase commands only). 109If the @var{-p} option is not used for a command that supports it, the 110progress is reported when the process receives a @code{SIGUSR1} or 111@code{SIGINFO} signal. 112 113@item -q 114Quiet mode - do not print any output (except errors). There's no progress bar 115in case both @var{-q} and @var{-p} options are used. 116 117@item -S @var{size} 118indicates the consecutive number of bytes that must contain only zeros 119for qemu-img to create a sparse image during conversion. This value is rounded 120down to the nearest 512 bytes. You may use the common size suffixes like 121@code{k} for kilobytes. 122 123@item -t @var{cache} 124specifies the cache mode that should be used with the (destination) file. See 125the documentation of the emulator's @code{-drive cache=...} option for allowed 126values. 127 128@item -T @var{src_cache} 129specifies the cache mode that should be used with the source file(s). See 130the documentation of the emulator's @code{-drive cache=...} option for allowed 131values. 132 133@end table 134 135Parameters to snapshot subcommand: 136 137@table @option 138 139@item snapshot 140is the name of the snapshot to create, apply or delete 141@item -a 142applies a snapshot (revert disk to saved state) 143@item -c 144creates a snapshot 145@item -d 146deletes a snapshot 147@item -l 148lists all snapshots in the given image 149@end table 150 151Parameters to compare subcommand: 152 153@table @option 154 155@item -f 156First image format 157@item -F 158Second image format 159@item -s 160Strict mode - fail on different image size or sector allocation 161@end table 162 163Parameters to convert subcommand: 164 165@table @option 166 167@item -n 168Skip the creation of the target volume 169@item -m 170Number of parallel coroutines for the convert process 171@item -W 172Allow out-of-order writes to the destination. This option improves performance, 173but is only recommended for preallocated devices like host devices or other 174raw block devices. 175@end table 176 177Parameters to dd subcommand: 178 179@table @option 180 181@item bs=@var{block_size} 182defines the block size 183@item count=@var{blocks} 184sets the number of input blocks to copy 185@item if=@var{input} 186sets the input file 187@item of=@var{output} 188sets the output file 189@item skip=@var{blocks} 190sets the number of input blocks to skip 191@end table 192 193Command description: 194 195@table @option 196@item bench [-c @var{count}] [-d @var{depth}] [-f @var{fmt}] [--flush-interval=@var{flush_interval}] [-n] [--no-drain] [-o @var{offset}] [--pattern=@var{pattern}] [-q] [-s @var{buffer_size}] [-S @var{step_size}] [-t @var{cache}] [-w] @var{filename} 197 198Run a simple sequential I/O benchmark on the specified image. If @code{-w} is 199specified, a write test is performed, otherwise a read test is performed. 200 201A total number of @var{count} I/O requests is performed, each @var{buffer_size} 202bytes in size, and with @var{depth} requests in parallel. The first request 203starts at the position given by @var{offset}, each following request increases 204the current position by @var{step_size}. If @var{step_size} is not given, 205@var{buffer_size} is used for its value. 206 207If @var{flush_interval} is specified for a write test, the request queue is 208drained and a flush is issued before new writes are made whenever the number of 209remaining requests is a multiple of @var{flush_interval}. If additionally 210@code{--no-drain} is specified, a flush is issued without draining the request 211queue first. 212 213If @code{-n} is specified, the native AIO backend is used if possible. On 214Linux, this option only works if @code{-t none} or @code{-t directsync} is 215specified as well. 216 217For write tests, by default a buffer filled with zeros is written. This can be 218overridden with a pattern byte specified by @var{pattern}. 219 220@item check [-f @var{fmt}] [--output=@var{ofmt}] [-r [leaks | all]] [-T @var{src_cache}] @var{filename} 221 222Perform a consistency check on the disk image @var{filename}. The command can 223output in the format @var{ofmt} which is either @code{human} or @code{json}. 224 225If @code{-r} is specified, qemu-img tries to repair any inconsistencies found 226during the check. @code{-r leaks} repairs only cluster leaks, whereas 227@code{-r all} fixes all kinds of errors, with a higher risk of choosing the 228wrong fix or hiding corruption that has already occurred. 229 230Only the formats @code{qcow2}, @code{qed} and @code{vdi} support 231consistency checks. 232 233In case the image does not have any inconsistencies, check exits with @code{0}. 234Other exit codes indicate the kind of inconsistency found or if another error 235occurred. The following table summarizes all exit codes of the check subcommand: 236 237@table @option 238 239@item 0 240Check completed, the image is (now) consistent 241@item 1 242Check not completed because of internal errors 243@item 2 244Check completed, image is corrupted 245@item 3 246Check completed, image has leaked clusters, but is not corrupted 247@item 63 248Checks are not supported by the image format 249 250@end table 251 252If @code{-r} is specified, exit codes representing the image state refer to the 253state after (the attempt at) repairing it. That is, a successful @code{-r all} 254will yield the exit code 0, independently of the image state before. 255 256@item create [-f @var{fmt}] [-b @var{backing_file}] [-F @var{backing_fmt}] [-u] [-o @var{options}] @var{filename} [@var{size}] 257 258Create the new disk image @var{filename} of size @var{size} and format 259@var{fmt}. Depending on the file format, you can add one or more @var{options} 260that enable additional features of this format. 261 262If the option @var{backing_file} is specified, then the image will record 263only the differences from @var{backing_file}. No size needs to be specified in 264this case. @var{backing_file} will never be modified unless you use the 265@code{commit} monitor command (or qemu-img commit). 266 267If a relative path name is given, the backing file is looked up relative to 268the directory containing @var{filename}. 269 270Note that a given backing file will be opened to check that it is valid. Use 271the @code{-u} option to enable unsafe backing file mode, which means that the 272image will be created even if the associated backing file cannot be opened. A 273matching backing file must be created or additional options be used to make the 274backing file specification valid when you want to use an image created this 275way. 276 277The size can also be specified using the @var{size} option with @code{-o}, 278it doesn't need to be specified separately in this case. 279 280@item commit [-q] [-f @var{fmt}] [-t @var{cache}] [-b @var{base}] [-d] [-p] @var{filename} 281 282Commit the changes recorded in @var{filename} in its base image or backing file. 283If the backing file is smaller than the snapshot, then the backing file will be 284resized to be the same size as the snapshot. If the snapshot is smaller than 285the backing file, the backing file will not be truncated. If you want the 286backing file to match the size of the smaller snapshot, you can safely truncate 287it yourself once the commit operation successfully completes. 288 289The image @var{filename} is emptied after the operation has succeeded. If you do 290not need @var{filename} afterwards and intend to drop it, you may skip emptying 291@var{filename} by specifying the @code{-d} flag. 292 293If the backing chain of the given image file @var{filename} has more than one 294layer, the backing file into which the changes will be committed may be 295specified as @var{base} (which has to be part of @var{filename}'s backing 296chain). If @var{base} is not specified, the immediate backing file of the top 297image (which is @var{filename}) will be used. Note that after a commit operation 298all images between @var{base} and the top image will be invalid and may return 299garbage data when read. For this reason, @code{-b} implies @code{-d} (so that 300the top image stays valid). 301 302@item compare [-f @var{fmt}] [-F @var{fmt}] [-T @var{src_cache}] [-p] [-s] [-q] @var{filename1} @var{filename2} 303 304Check if two images have the same content. You can compare images with 305different format or settings. 306 307The format is probed unless you specify it by @var{-f} (used for 308@var{filename1}) and/or @var{-F} (used for @var{filename2}) option. 309 310By default, images with different size are considered identical if the larger 311image contains only unallocated and/or zeroed sectors in the area after the end 312of the other image. In addition, if any sector is not allocated in one image 313and contains only zero bytes in the second one, it is evaluated as equal. You 314can use Strict mode by specifying the @var{-s} option. When compare runs in 315Strict mode, it fails in case image size differs or a sector is allocated in 316one image and is not allocated in the second one. 317 318By default, compare prints out a result message. This message displays 319information that both images are same or the position of the first different 320byte. In addition, result message can report different image size in case 321Strict mode is used. 322 323Compare exits with @code{0} in case the images are equal and with @code{1} 324in case the images differ. Other exit codes mean an error occurred during 325execution and standard error output should contain an error message. 326The following table sumarizes all exit codes of the compare subcommand: 327 328@table @option 329 330@item 0 331Images are identical 332@item 1 333Images differ 334@item 2 335Error on opening an image 336@item 3 337Error on checking a sector allocation 338@item 4 339Error on reading data 340 341@end table 342 343@item convert [-c] [-p] [-n] [-f @var{fmt}] [-t @var{cache}] [-T @var{src_cache}] [-O @var{output_fmt}] [-B @var{backing_file}] [-o @var{options}] [-s @var{snapshot_id_or_name}] [-l @var{snapshot_param}] [-m @var{num_coroutines}] [-W] [-S @var{sparse_size}] @var{filename} [@var{filename2} [...]] @var{output_filename} 344 345Convert the disk image @var{filename} or a snapshot @var{snapshot_param}(@var{snapshot_id_or_name} is deprecated) 346to disk image @var{output_filename} using format @var{output_fmt}. It can be optionally compressed (@code{-c} 347option) or use any format specific options like encryption (@code{-o} option). 348 349Only the formats @code{qcow} and @code{qcow2} support compression. The 350compression is read-only. It means that if a compressed sector is 351rewritten, then it is rewritten as uncompressed data. 352 353Image conversion is also useful to get smaller image when using a 354growable format such as @code{qcow}: the empty sectors are detected and 355suppressed from the destination image. 356 357@var{sparse_size} indicates the consecutive number of bytes (defaults to 4k) 358that must contain only zeros for qemu-img to create a sparse image during 359conversion. If @var{sparse_size} is 0, the source will not be scanned for 360unallocated or zero sectors, and the destination image will always be 361fully allocated. 362 363You can use the @var{backing_file} option to force the output image to be 364created as a copy on write image of the specified base image; the 365@var{backing_file} should have the same content as the input's base image, 366however the path, image format, etc may differ. 367 368If a relative path name is given, the backing file is looked up relative to 369the directory containing @var{output_filename}. 370 371If the @code{-n} option is specified, the target volume creation will be 372skipped. This is useful for formats such as @code{rbd} if the target 373volume has already been created with site specific options that cannot 374be supplied through qemu-img. 375 376Out of order writes can be enabled with @code{-W} to improve performance. 377This is only recommended for preallocated devices like host devices or other 378raw block devices. Out of order write does not work in combination with 379creating compressed images. 380 381@var{num_coroutines} specifies how many coroutines work in parallel during 382the convert process (defaults to 8). 383 384@item dd [-f @var{fmt}] [-O @var{output_fmt}] [bs=@var{block_size}] [count=@var{blocks}] [skip=@var{blocks}] if=@var{input} of=@var{output} 385 386Dd copies from @var{input} file to @var{output} file converting it from 387@var{fmt} format to @var{output_fmt} format. 388 389The data is by default read and written using blocks of 512 bytes but can be 390modified by specifying @var{block_size}. If count=@var{blocks} is specified 391dd will stop reading input after reading @var{blocks} input blocks. 392 393The size syntax is similar to dd(1)'s size syntax. 394 395@item info [-f @var{fmt}] [--output=@var{ofmt}] [--backing-chain] @var{filename} 396 397Give information about the disk image @var{filename}. Use it in 398particular to know the size reserved on disk which can be different 399from the displayed size. If VM snapshots are stored in the disk image, 400they are displayed too. The command can output in the format @var{ofmt} 401which is either @code{human} or @code{json}. 402 403If a disk image has a backing file chain, information about each disk image in 404the chain can be recursively enumerated by using the option @code{--backing-chain}. 405 406For instance, if you have an image chain like: 407 408@example 409base.qcow2 <- snap1.qcow2 <- snap2.qcow2 410@end example 411 412To enumerate information about each disk image in the above chain, starting from top to base, do: 413 414@example 415qemu-img info --backing-chain snap2.qcow2 416@end example 417 418@item map [-f @var{fmt}] [--output=@var{ofmt}] @var{filename} 419 420Dump the metadata of image @var{filename} and its backing file chain. 421In particular, this commands dumps the allocation state of every sector 422of @var{filename}, together with the topmost file that allocates it in 423the backing file chain. 424 425Two option formats are possible. The default format (@code{human}) 426only dumps known-nonzero areas of the file. Known-zero parts of the 427file are omitted altogether, and likewise for parts that are not allocated 428throughout the chain. @command{qemu-img} output will identify a file 429from where the data can be read, and the offset in the file. Each line 430will include four fields, the first three of which are hexadecimal 431numbers. For example the first line of: 432@example 433Offset Length Mapped to File 4340 0x20000 0x50000 /tmp/overlay.qcow2 4350x100000 0x10000 0x95380000 /tmp/backing.qcow2 436@end example 437@noindent 438means that 0x20000 (131072) bytes starting at offset 0 in the image are 439available in /tmp/overlay.qcow2 (opened in @code{raw} format) starting 440at offset 0x50000 (327680). Data that is compressed, encrypted, or 441otherwise not available in raw format will cause an error if @code{human} 442format is in use. Note that file names can include newlines, thus it is 443not safe to parse this output format in scripts. 444 445The alternative format @code{json} will return an array of dictionaries 446in JSON format. It will include similar information in 447the @code{start}, @code{length}, @code{offset} fields; 448it will also include other more specific information: 449@itemize @minus 450@item 451whether the sectors contain actual data or not (boolean field @code{data}; 452if false, the sectors are either unallocated or stored as optimized 453all-zero clusters); 454 455@item 456whether the data is known to read as zero (boolean field @code{zero}); 457 458@item 459in order to make the output shorter, the target file is expressed as 460a @code{depth}; for example, a depth of 2 refers to the backing file 461of the backing file of @var{filename}. 462@end itemize 463 464In JSON format, the @code{offset} field is optional; it is absent in 465cases where @code{human} format would omit the entry or exit with an error. 466If @code{data} is false and the @code{offset} field is present, the 467corresponding sectors in the file are not yet in use, but they are 468preallocated. 469 470For more information, consult @file{include/block/block.h} in QEMU's 471source code. 472 473@item measure [--output=@var{ofmt}] [-O @var{output_fmt}] [-o @var{options}] [--size @var{N} | [--object @var{objectdef}] [--image-opts] [-f @var{fmt}] [-l @var{snapshot_param}] @var{filename}] 474 475Calculate the file size required for a new image. This information can be used 476to size logical volumes or SAN LUNs appropriately for the image that will be 477placed in them. The values reported are guaranteed to be large enough to fit 478the image. The command can output in the format @var{ofmt} which is either 479@code{human} or @code{json}. 480 481If the size @var{N} is given then act as if creating a new empty image file 482using @command{qemu-img create}. If @var{filename} is given then act as if 483converting an existing image file using @command{qemu-img convert}. The format 484of the new file is given by @var{output_fmt} while the format of an existing 485file is given by @var{fmt}. 486 487A snapshot in an existing image can be specified using @var{snapshot_param}. 488 489The following fields are reported: 490@example 491required size: 524288 492fully allocated size: 1074069504 493@end example 494 495The @code{required size} is the file size of the new image. It may be smaller 496than the virtual disk size if the image format supports compact representation. 497 498The @code{fully allocated size} is the file size of the new image once data has 499been written to all sectors. This is the maximum size that the image file can 500occupy with the exception of internal snapshots, dirty bitmaps, vmstate data, 501and other advanced image format features. 502 503@item snapshot [-l | -a @var{snapshot} | -c @var{snapshot} | -d @var{snapshot} ] @var{filename} 504 505List, apply, create or delete snapshots in image @var{filename}. 506 507@item rebase [-f @var{fmt}] [-t @var{cache}] [-T @var{src_cache}] [-p] [-u] -b @var{backing_file} [-F @var{backing_fmt}] @var{filename} 508 509Changes the backing file of an image. Only the formats @code{qcow2} and 510@code{qed} support changing the backing file. 511 512The backing file is changed to @var{backing_file} and (if the image format of 513@var{filename} supports this) the backing file format is changed to 514@var{backing_fmt}. If @var{backing_file} is specified as ``'' (the empty 515string), then the image is rebased onto no backing file (i.e. it will exist 516independently of any backing file). 517 518If a relative path name is given, the backing file is looked up relative to 519the directory containing @var{filename}. 520 521@var{cache} specifies the cache mode to be used for @var{filename}, whereas 522@var{src_cache} specifies the cache mode for reading backing files. 523 524There are two different modes in which @code{rebase} can operate: 525@table @option 526@item Safe mode 527This is the default mode and performs a real rebase operation. The new backing 528file may differ from the old one and qemu-img rebase will take care of keeping 529the guest-visible content of @var{filename} unchanged. 530 531In order to achieve this, any clusters that differ between @var{backing_file} 532and the old backing file of @var{filename} are merged into @var{filename} 533before actually changing the backing file. 534 535Note that the safe mode is an expensive operation, comparable to converting 536an image. It only works if the old backing file still exists. 537 538@item Unsafe mode 539qemu-img uses the unsafe mode if @code{-u} is specified. In this mode, only the 540backing file name and format of @var{filename} is changed without any checks 541on the file contents. The user must take care of specifying the correct new 542backing file, or the guest-visible content of the image will be corrupted. 543 544This mode is useful for renaming or moving the backing file to somewhere else. 545It can be used without an accessible old backing file, i.e. you can use it to 546fix an image whose backing file has already been moved/renamed. 547@end table 548 549You can use @code{rebase} to perform a ``diff'' operation on two 550disk images. This can be useful when you have copied or cloned 551a guest, and you want to get back to a thin image on top of a 552template or base image. 553 554Say that @code{base.img} has been cloned as @code{modified.img} by 555copying it, and that the @code{modified.img} guest has run so there 556are now some changes compared to @code{base.img}. To construct a thin 557image called @code{diff.qcow2} that contains just the differences, do: 558 559@example 560qemu-img create -f qcow2 -b modified.img diff.qcow2 561qemu-img rebase -b base.img diff.qcow2 562@end example 563 564At this point, @code{modified.img} can be discarded, since 565@code{base.img + diff.qcow2} contains the same information. 566 567@item resize [--shrink] [--preallocation=@var{prealloc}] @var{filename} [+ | -]@var{size} 568 569Change the disk image as if it had been created with @var{size}. 570 571Before using this command to shrink a disk image, you MUST use file system and 572partitioning tools inside the VM to reduce allocated file systems and partition 573sizes accordingly. Failure to do so will result in data loss! 574 575When shrinking images, the @code{--shrink} option must be given. This informs 576qemu-img that the user acknowledges all loss of data beyond the truncated 577image's end. 578 579After using this command to grow a disk image, you must use file system and 580partitioning tools inside the VM to actually begin using the new space on the 581device. 582 583When growing an image, the @code{--preallocation} option may be used to specify 584how the additional image area should be allocated on the host. See the format 585description in the @code{NOTES} section which values are allowed. Using this 586option may result in slightly more data being allocated than necessary. 587 588@item amend [-p] [-f @var{fmt}] [-t @var{cache}] -o @var{options} @var{filename} 589 590Amends the image format specific @var{options} for the image file 591@var{filename}. Not all file formats support this operation. 592@end table 593@c man end 594 595@ignore 596@c man begin NOTES 597Supported image file formats: 598 599@table @option 600@item raw 601 602Raw disk image format (default). This format has the advantage of 603being simple and easily exportable to all other emulators. If your 604file system supports @emph{holes} (for example in ext2 or ext3 on 605Linux or NTFS on Windows), then only the written sectors will reserve 606space. Use @code{qemu-img info} to know the real size used by the 607image or @code{ls -ls} on Unix/Linux. 608 609Supported options: 610@table @code 611@item preallocation 612Preallocation mode (allowed values: @code{off}, @code{falloc}, @code{full}). 613@code{falloc} mode preallocates space for image by calling posix_fallocate(). 614@code{full} mode preallocates space for image by writing zeros to underlying 615storage. 616@end table 617 618@item qcow2 619QEMU image format, the most versatile format. Use it to have smaller 620images (useful if your filesystem does not supports holes, for example 621on Windows), optional AES encryption, zlib based compression and 622support of multiple VM snapshots. 623 624Supported options: 625@table @code 626@item compat 627Determines the qcow2 version to use. @code{compat=0.10} uses the 628traditional image format that can be read by any QEMU since 0.10. 629@code{compat=1.1} enables image format extensions that only QEMU 1.1 and 630newer understand (this is the default). Amongst others, this includes zero 631clusters, which allow efficient copy-on-read for sparse images. 632 633@item backing_file 634File name of a base image (see @option{create} subcommand) 635@item backing_fmt 636Image format of the base image 637@item encryption 638If this option is set to @code{on}, the image is encrypted with 128-bit AES-CBC. 639 640The use of encryption in qcow and qcow2 images is considered to be flawed by 641modern cryptography standards, suffering from a number of design problems: 642 643@itemize @minus 644@item 645The AES-CBC cipher is used with predictable initialization vectors based 646on the sector number. This makes it vulnerable to chosen plaintext attacks 647which can reveal the existence of encrypted data. 648@item 649The user passphrase is directly used as the encryption key. A poorly 650chosen or short passphrase will compromise the security of the encryption. 651@item 652In the event of the passphrase being compromised there is no way to 653change the passphrase to protect data in any qcow images. The files must 654be cloned, using a different encryption passphrase in the new file. The 655original file must then be securely erased using a program like shred, 656though even this is ineffective with many modern storage technologies. 657@item 658Initialization vectors used to encrypt sectors are based on the 659guest virtual sector number, instead of the host physical sector. When 660a disk image has multiple internal snapshots this means that data in 661multiple physical sectors is encrypted with the same initialization 662vector. With the CBC mode, this opens the possibility of watermarking 663attacks if the attack can collect multiple sectors encrypted with the 664same IV and some predictable data. Having multiple qcow2 images with 665the same passphrase also exposes this weakness since the passphrase 666is directly used as the key. 667@end itemize 668 669Use of qcow / qcow2 encryption is thus strongly discouraged. Users are 670recommended to use an alternative encryption technology such as the 671Linux dm-crypt / LUKS system. 672 673@item cluster_size 674Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster 675sizes can improve the image file size whereas larger cluster sizes generally 676provide better performance. 677 678@item preallocation 679Preallocation mode (allowed values: @code{off}, @code{metadata}, @code{falloc}, 680@code{full}). An image with preallocated metadata is initially larger but can 681improve performance when the image needs to grow. @code{falloc} and @code{full} 682preallocations are like the same options of @code{raw} format, but sets up 683metadata also. 684 685@item lazy_refcounts 686If this option is set to @code{on}, reference count updates are postponed with 687the goal of avoiding metadata I/O and improving performance. This is 688particularly interesting with @option{cache=writethrough} which doesn't batch 689metadata updates. The tradeoff is that after a host crash, the reference count 690tables must be rebuilt, i.e. on the next open an (automatic) @code{qemu-img 691check -r all} is required, which may take some time. 692 693This option can only be enabled if @code{compat=1.1} is specified. 694 695@item nocow 696If this option is set to @code{on}, it will turn off COW of the file. It's only 697valid on btrfs, no effect on other file systems. 698 699Btrfs has low performance when hosting a VM image file, even more when the guest 700on the VM also using btrfs as file system. Turning off COW is a way to mitigate 701this bad performance. Generally there are two ways to turn off COW on btrfs: 702a) Disable it by mounting with nodatacow, then all newly created files will be 703NOCOW. b) For an empty file, add the NOCOW file attribute. That's what this option 704does. 705 706Note: this option is only valid to new or empty files. If there is an existing 707file which is COW and has data blocks already, it couldn't be changed to NOCOW 708by setting @code{nocow=on}. One can issue @code{lsattr filename} to check if 709the NOCOW flag is set or not (Capital 'C' is NOCOW flag). 710 711@end table 712 713@item Other 714QEMU also supports various other image file formats for compatibility with 715older QEMU versions or other hypervisors, including VMDK, VDI, VHD (vpc), VHDX, 716qcow1 and QED. For a full list of supported formats see @code{qemu-img --help}. 717For a more detailed description of these formats, see the QEMU Emulation User 718Documentation. 719 720The main purpose of the block drivers for these formats is image conversion. 721For running VMs, it is recommended to convert the disk images to either raw or 722qcow2 in order to achieve good performance. 723@end table 724 725 726@c man end 727 728@setfilename qemu-img 729@settitle QEMU disk image utility 730 731@c man begin SEEALSO 732The HTML documentation of QEMU for more precise information and Linux 733user mode emulator invocation. 734@c man end 735 736@c man begin AUTHOR 737Fabrice Bellard 738@c man end 739 740@end ignore 741