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@end table 65 66@table @option 67 68@item --object @var{objectdef} 69is a QEMU user creatable object definition. See the @code{qemu(1)} manual 70page for a description of the object properties. The most common object 71type is a @code{secret}, which is used to supply passwords and/or encryption 72keys. 73 74@item --image-opts 75Indicates that the source @var{filename} parameter is to be interpreted as a 76full option string, not a plain filename. This parameter is mutually 77exclusive with the @var{-f} parameter. 78 79@item --target-image-opts 80Indicates that the @var{output_filename} parameter(s) are to be interpreted as 81a full option string, not a plain filename. This parameter is mutually 82exclusive with the @var{-O} parameters. It is currently required to also use 83the @var{-n} parameter to skip image creation. This restriction may be relaxed 84in a future release. 85 86@item --force-share (-U) 87If specified, @code{qemu-img} will open the image in shared mode, allowing 88other QEMU processes to open it in write mode. For example, this can be used to 89get the image information (with 'info' subcommand) when the image is used by a 90running guest. Note that this could produce inconsistent results because of 91concurrent metadata changes, etc. This option is only allowed when opening 92images in read-only mode. 93 94@item --backing-chain 95will enumerate information about backing files in a disk image chain. Refer 96below for further description. 97 98@item -c 99indicates that target image must be compressed (qcow format only) 100 101@item -h 102with or without a command shows help and lists the supported formats 103 104@item -p 105display progress bar (compare, convert and rebase commands only). 106If the @var{-p} option is not used for a command that supports it, the 107progress is reported when the process receives a @code{SIGUSR1} or 108@code{SIGINFO} signal. 109 110@item -q 111Quiet mode - do not print any output (except errors). There's no progress bar 112in case both @var{-q} and @var{-p} options are used. 113 114@item -S @var{size} 115indicates the consecutive number of bytes that must contain only zeros 116for qemu-img to create a sparse image during conversion. This value is rounded 117down to the nearest 512 bytes. You may use the common size suffixes like 118@code{k} for kilobytes. 119 120@item -t @var{cache} 121specifies the cache mode that should be used with the (destination) file. See 122the documentation of the emulator's @code{-drive cache=...} option for allowed 123values. 124 125@item -T @var{src_cache} 126specifies the cache mode that should be used with the source file(s). See 127the documentation of the emulator's @code{-drive cache=...} option for allowed 128values. 129 130@end table 131 132Parameters to snapshot subcommand: 133 134@table @option 135 136@item snapshot 137is the name of the snapshot to create, apply or delete 138@item -a 139applies a snapshot (revert disk to saved state) 140@item -c 141creates a snapshot 142@item -d 143deletes a snapshot 144@item -l 145lists all snapshots in the given image 146@end table 147 148Parameters to compare subcommand: 149 150@table @option 151 152@item -f 153First image format 154@item -F 155Second image format 156@item -s 157Strict mode - fail on different image size or sector allocation 158@end table 159 160Parameters to convert subcommand: 161 162@table @option 163 164@item -n 165Skip the creation of the target volume 166@item -m 167Number of parallel coroutines for the convert process 168@item -W 169Allow out-of-order writes to the destination. This option improves performance, 170but is only recommended for preallocated devices like host devices or other 171raw block devices. 172@item -C 173Try to use copy offloading to move data from source image to target. This may 174improve performance if the data is remote, such as with NFS or iSCSI backends, 175but will not automatically sparsify zero sectors, and may result in a fully 176allocated target image depending on the host support for getting allocation 177information. 178@end table 179 180Parameters to dd subcommand: 181 182@table @option 183 184@item bs=@var{block_size} 185defines the block size 186@item count=@var{blocks} 187sets the number of input blocks to copy 188@item if=@var{input} 189sets the input file 190@item of=@var{output} 191sets the output file 192@item skip=@var{blocks} 193sets the number of input blocks to skip 194@end table 195 196Command description: 197 198@table @option 199 200@item amend [--object @var{objectdef}] [--image-opts] [-p] [-p] [-f @var{fmt}] [-t @var{cache}] -o @var{options} @var{filename} 201 202Amends the image format specific @var{options} for the image file 203@var{filename}. Not all file formats support this operation. 204 205@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] [-U] @var{filename} 206 207Run a simple sequential I/O benchmark on the specified image. If @code{-w} is 208specified, a write test is performed, otherwise a read test is performed. 209 210A total number of @var{count} I/O requests is performed, each @var{buffer_size} 211bytes in size, and with @var{depth} requests in parallel. The first request 212starts at the position given by @var{offset}, each following request increases 213the current position by @var{step_size}. If @var{step_size} is not given, 214@var{buffer_size} is used for its value. 215 216If @var{flush_interval} is specified for a write test, the request queue is 217drained and a flush is issued before new writes are made whenever the number of 218remaining requests is a multiple of @var{flush_interval}. If additionally 219@code{--no-drain} is specified, a flush is issued without draining the request 220queue first. 221 222If @code{-n} is specified, the native AIO backend is used if possible. On 223Linux, this option only works if @code{-t none} or @code{-t directsync} is 224specified as well. 225 226For write tests, by default a buffer filled with zeros is written. This can be 227overridden with a pattern byte specified by @var{pattern}. 228 229@item check [--object @var{objectdef}] [--image-opts] [-q] [-f @var{fmt}] [--output=@var{ofmt}] [-r [leaks | all]] [-T @var{src_cache}] [-U] @var{filename} 230 231Perform a consistency check on the disk image @var{filename}. The command can 232output in the format @var{ofmt} which is either @code{human} or @code{json}. 233 234If @code{-r} is specified, qemu-img tries to repair any inconsistencies found 235during the check. @code{-r leaks} repairs only cluster leaks, whereas 236@code{-r all} fixes all kinds of errors, with a higher risk of choosing the 237wrong fix or hiding corruption that has already occurred. 238 239Only the formats @code{qcow2}, @code{qed} and @code{vdi} support 240consistency checks. 241 242In case the image does not have any inconsistencies, check exits with @code{0}. 243Other exit codes indicate the kind of inconsistency found or if another error 244occurred. The following table summarizes all exit codes of the check subcommand: 245 246@table @option 247 248@item 0 249Check completed, the image is (now) consistent 250@item 1 251Check not completed because of internal errors 252@item 2 253Check completed, image is corrupted 254@item 3 255Check completed, image has leaked clusters, but is not corrupted 256@item 63 257Checks are not supported by the image format 258 259@end table 260 261If @code{-r} is specified, exit codes representing the image state refer to the 262state after (the attempt at) repairing it. That is, a successful @code{-r all} 263will yield the exit code 0, independently of the image state before. 264 265@item commit [--object @var{objectdef}] [--image-opts] [-q] [-f @var{fmt}] [-t @var{cache}] [-b @var{base}] [-d] [-p] @var{filename} 266 267Commit the changes recorded in @var{filename} in its base image or backing file. 268If the backing file is smaller than the snapshot, then the backing file will be 269resized to be the same size as the snapshot. If the snapshot is smaller than 270the backing file, the backing file will not be truncated. If you want the 271backing file to match the size of the smaller snapshot, you can safely truncate 272it yourself once the commit operation successfully completes. 273 274The image @var{filename} is emptied after the operation has succeeded. If you do 275not need @var{filename} afterwards and intend to drop it, you may skip emptying 276@var{filename} by specifying the @code{-d} flag. 277 278If the backing chain of the given image file @var{filename} has more than one 279layer, the backing file into which the changes will be committed may be 280specified as @var{base} (which has to be part of @var{filename}'s backing 281chain). If @var{base} is not specified, the immediate backing file of the top 282image (which is @var{filename}) will be used. Note that after a commit operation 283all images between @var{base} and the top image will be invalid and may return 284garbage data when read. For this reason, @code{-b} implies @code{-d} (so that 285the top image stays valid). 286 287@item compare [--object @var{objectdef}] [--image-opts] [-f @var{fmt}] [-F @var{fmt}] [-T @var{src_cache}] [-p] [-q] [-s] [-U] @var{filename1} @var{filename2} 288 289Check if two images have the same content. You can compare images with 290different format or settings. 291 292The format is probed unless you specify it by @var{-f} (used for 293@var{filename1}) and/or @var{-F} (used for @var{filename2}) option. 294 295By default, images with different size are considered identical if the larger 296image contains only unallocated and/or zeroed sectors in the area after the end 297of the other image. In addition, if any sector is not allocated in one image 298and contains only zero bytes in the second one, it is evaluated as equal. You 299can use Strict mode by specifying the @var{-s} option. When compare runs in 300Strict mode, it fails in case image size differs or a sector is allocated in 301one image and is not allocated in the second one. 302 303By default, compare prints out a result message. This message displays 304information that both images are same or the position of the first different 305byte. In addition, result message can report different image size in case 306Strict mode is used. 307 308Compare exits with @code{0} in case the images are equal and with @code{1} 309in case the images differ. Other exit codes mean an error occurred during 310execution and standard error output should contain an error message. 311The following table sumarizes all exit codes of the compare subcommand: 312 313@table @option 314 315@item 0 316Images are identical 317@item 1 318Images differ 319@item 2 320Error on opening an image 321@item 3 322Error on checking a sector allocation 323@item 4 324Error on reading data 325 326@end table 327 328@item convert [--object @var{objectdef}] [--image-opts] [--target-image-opts] [-U] [-C] [-c] [-p] [-q] [-n] [-f @var{fmt}] [-t @var{cache}] [-T @var{src_cache}] [-O @var{output_fmt}] [-B @var{backing_file}] [-o @var{options}] [-l @var{snapshot_param}] [-S @var{sparse_size}] [-m @var{num_coroutines}] [-W] @var{filename} [@var{filename2} [...]] @var{output_filename} 329 330Convert the disk image @var{filename} or a snapshot @var{snapshot_param} 331to disk image @var{output_filename} using format @var{output_fmt}. It can be optionally compressed (@code{-c} 332option) or use any format specific options like encryption (@code{-o} option). 333 334Only the formats @code{qcow} and @code{qcow2} support compression. The 335compression is read-only. It means that if a compressed sector is 336rewritten, then it is rewritten as uncompressed data. 337 338Image conversion is also useful to get smaller image when using a 339growable format such as @code{qcow}: the empty sectors are detected and 340suppressed from the destination image. 341 342@var{sparse_size} indicates the consecutive number of bytes (defaults to 4k) 343that must contain only zeros for qemu-img to create a sparse image during 344conversion. If @var{sparse_size} is 0, the source will not be scanned for 345unallocated or zero sectors, and the destination image will always be 346fully allocated. 347 348You can use the @var{backing_file} option to force the output image to be 349created as a copy on write image of the specified base image; the 350@var{backing_file} should have the same content as the input's base image, 351however the path, image format, etc may differ. 352 353If a relative path name is given, the backing file is looked up relative to 354the directory containing @var{output_filename}. 355 356If the @code{-n} option is specified, the target volume creation will be 357skipped. This is useful for formats such as @code{rbd} if the target 358volume has already been created with site specific options that cannot 359be supplied through qemu-img. 360 361Out of order writes can be enabled with @code{-W} to improve performance. 362This is only recommended for preallocated devices like host devices or other 363raw block devices. Out of order write does not work in combination with 364creating compressed images. 365 366@var{num_coroutines} specifies how many coroutines work in parallel during 367the convert process (defaults to 8). 368 369@item create [--object @var{objectdef}] [-q] [-f @var{fmt}] [-b @var{backing_file}] [-F @var{backing_fmt}] [-u] [-o @var{options}] @var{filename} [@var{size}] 370 371Create the new disk image @var{filename} of size @var{size} and format 372@var{fmt}. Depending on the file format, you can add one or more @var{options} 373that enable additional features of this format. 374 375If the option @var{backing_file} is specified, then the image will record 376only the differences from @var{backing_file}. No size needs to be specified in 377this case. @var{backing_file} will never be modified unless you use the 378@code{commit} monitor command (or qemu-img commit). 379 380If a relative path name is given, the backing file is looked up relative to 381the directory containing @var{filename}. 382 383Note that a given backing file will be opened to check that it is valid. Use 384the @code{-u} option to enable unsafe backing file mode, which means that the 385image will be created even if the associated backing file cannot be opened. A 386matching backing file must be created or additional options be used to make the 387backing file specification valid when you want to use an image created this 388way. 389 390The size can also be specified using the @var{size} option with @code{-o}, 391it doesn't need to be specified separately in this case. 392 393@item dd [--image-opts] [-U] [-f @var{fmt}] [-O @var{output_fmt}] [bs=@var{block_size}] [count=@var{blocks}] [skip=@var{blocks}] if=@var{input} of=@var{output} 394 395Dd copies from @var{input} file to @var{output} file converting it from 396@var{fmt} format to @var{output_fmt} format. 397 398The data is by default read and written using blocks of 512 bytes but can be 399modified by specifying @var{block_size}. If count=@var{blocks} is specified 400dd will stop reading input after reading @var{blocks} input blocks. 401 402The size syntax is similar to dd(1)'s size syntax. 403 404@item info [--object @var{objectdef}] [--image-opts] [-f @var{fmt}] [--output=@var{ofmt}] [--backing-chain] [-U] @var{filename} 405 406Give information about the disk image @var{filename}. Use it in 407particular to know the size reserved on disk which can be different 408from the displayed size. If VM snapshots are stored in the disk image, 409they are displayed too. The command can output in the format @var{ofmt} 410which is either @code{human} or @code{json}. 411 412If a disk image has a backing file chain, information about each disk image in 413the chain can be recursively enumerated by using the option @code{--backing-chain}. 414 415For instance, if you have an image chain like: 416 417@example 418base.qcow2 <- snap1.qcow2 <- snap2.qcow2 419@end example 420 421To enumerate information about each disk image in the above chain, starting from top to base, do: 422 423@example 424qemu-img info --backing-chain snap2.qcow2 425@end example 426 427@item map [-f @var{fmt}] [--output=@var{ofmt}] @var{filename} 428 429Dump the metadata of image @var{filename} and its backing file chain. 430In particular, this commands dumps the allocation state of every sector 431of @var{filename}, together with the topmost file that allocates it in 432the backing file chain. 433 434Two option formats are possible. The default format (@code{human}) 435only dumps known-nonzero areas of the file. Known-zero parts of the 436file are omitted altogether, and likewise for parts that are not allocated 437throughout the chain. @command{qemu-img} output will identify a file 438from where the data can be read, and the offset in the file. Each line 439will include four fields, the first three of which are hexadecimal 440numbers. For example the first line of: 441@example 442Offset Length Mapped to File 4430 0x20000 0x50000 /tmp/overlay.qcow2 4440x100000 0x10000 0x95380000 /tmp/backing.qcow2 445@end example 446@noindent 447means that 0x20000 (131072) bytes starting at offset 0 in the image are 448available in /tmp/overlay.qcow2 (opened in @code{raw} format) starting 449at offset 0x50000 (327680). Data that is compressed, encrypted, or 450otherwise not available in raw format will cause an error if @code{human} 451format is in use. Note that file names can include newlines, thus it is 452not safe to parse this output format in scripts. 453 454The alternative format @code{json} will return an array of dictionaries 455in JSON format. It will include similar information in 456the @code{start}, @code{length}, @code{offset} fields; 457it will also include other more specific information: 458@itemize @minus 459@item 460whether the sectors contain actual data or not (boolean field @code{data}; 461if false, the sectors are either unallocated or stored as optimized 462all-zero clusters); 463 464@item 465whether the data is known to read as zero (boolean field @code{zero}); 466 467@item 468in order to make the output shorter, the target file is expressed as 469a @code{depth}; for example, a depth of 2 refers to the backing file 470of the backing file of @var{filename}. 471@end itemize 472 473In JSON format, the @code{offset} field is optional; it is absent in 474cases where @code{human} format would omit the entry or exit with an error. 475If @code{data} is false and the @code{offset} field is present, the 476corresponding sectors in the file are not yet in use, but they are 477preallocated. 478 479For more information, consult @file{include/block/block.h} in QEMU's 480source code. 481 482@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}] 483 484Calculate the file size required for a new image. This information can be used 485to size logical volumes or SAN LUNs appropriately for the image that will be 486placed in them. The values reported are guaranteed to be large enough to fit 487the image. The command can output in the format @var{ofmt} which is either 488@code{human} or @code{json}. 489 490If the size @var{N} is given then act as if creating a new empty image file 491using @command{qemu-img create}. If @var{filename} is given then act as if 492converting an existing image file using @command{qemu-img convert}. The format 493of the new file is given by @var{output_fmt} while the format of an existing 494file is given by @var{fmt}. 495 496A snapshot in an existing image can be specified using @var{snapshot_param}. 497 498The following fields are reported: 499@example 500required size: 524288 501fully allocated size: 1074069504 502@end example 503 504The @code{required size} is the file size of the new image. It may be smaller 505than the virtual disk size if the image format supports compact representation. 506 507The @code{fully allocated size} is the file size of the new image once data has 508been written to all sectors. This is the maximum size that the image file can 509occupy with the exception of internal snapshots, dirty bitmaps, vmstate data, 510and other advanced image format features. 511 512@item snapshot [--object @var{objectdef}] [--image-opts] [-U] [-q] [-l | -a @var{snapshot} | -c @var{snapshot} | -d @var{snapshot}] @var{filename} 513 514List, apply, create or delete snapshots in image @var{filename}. 515 516@item rebase [--object @var{objectdef}] [--image-opts] [-U] [-q] [-f @var{fmt}] [-t @var{cache}] [-T @var{src_cache}] [-p] [-u] -b @var{backing_file} [-F @var{backing_fmt}] @var{filename} 517 518Changes the backing file of an image. Only the formats @code{qcow2} and 519@code{qed} support changing the backing file. 520 521The backing file is changed to @var{backing_file} and (if the image format of 522@var{filename} supports this) the backing file format is changed to 523@var{backing_fmt}. If @var{backing_file} is specified as ``'' (the empty 524string), then the image is rebased onto no backing file (i.e. it will exist 525independently of any backing file). 526 527If a relative path name is given, the backing file is looked up relative to 528the directory containing @var{filename}. 529 530@var{cache} specifies the cache mode to be used for @var{filename}, whereas 531@var{src_cache} specifies the cache mode for reading backing files. 532 533There are two different modes in which @code{rebase} can operate: 534@table @option 535@item Safe mode 536This is the default mode and performs a real rebase operation. The new backing 537file may differ from the old one and qemu-img rebase will take care of keeping 538the guest-visible content of @var{filename} unchanged. 539 540In order to achieve this, any clusters that differ between @var{backing_file} 541and the old backing file of @var{filename} are merged into @var{filename} 542before actually changing the backing file. 543 544Note that the safe mode is an expensive operation, comparable to converting 545an image. It only works if the old backing file still exists. 546 547@item Unsafe mode 548qemu-img uses the unsafe mode if @code{-u} is specified. In this mode, only the 549backing file name and format of @var{filename} is changed without any checks 550on the file contents. The user must take care of specifying the correct new 551backing file, or the guest-visible content of the image will be corrupted. 552 553This mode is useful for renaming or moving the backing file to somewhere else. 554It can be used without an accessible old backing file, i.e. you can use it to 555fix an image whose backing file has already been moved/renamed. 556@end table 557 558You can use @code{rebase} to perform a ``diff'' operation on two 559disk images. This can be useful when you have copied or cloned 560a guest, and you want to get back to a thin image on top of a 561template or base image. 562 563Say that @code{base.img} has been cloned as @code{modified.img} by 564copying it, and that the @code{modified.img} guest has run so there 565are now some changes compared to @code{base.img}. To construct a thin 566image called @code{diff.qcow2} that contains just the differences, do: 567 568@example 569qemu-img create -f qcow2 -b modified.img diff.qcow2 570qemu-img rebase -b base.img diff.qcow2 571@end example 572 573At this point, @code{modified.img} can be discarded, since 574@code{base.img + diff.qcow2} contains the same information. 575 576@item resize [--object @var{objectdef}] [--image-opts] [-f @var{fmt}] [--preallocation=@var{prealloc}] [-q] [--shrink] @var{filename} [+ | -]@var{size} 577 578Change the disk image as if it had been created with @var{size}. 579 580Before using this command to shrink a disk image, you MUST use file system and 581partitioning tools inside the VM to reduce allocated file systems and partition 582sizes accordingly. Failure to do so will result in data loss! 583 584When shrinking images, the @code{--shrink} option must be given. This informs 585qemu-img that the user acknowledges all loss of data beyond the truncated 586image's end. 587 588After using this command to grow a disk image, you must use file system and 589partitioning tools inside the VM to actually begin using the new space on the 590device. 591 592When growing an image, the @code{--preallocation} option may be used to specify 593how the additional image area should be allocated on the host. See the format 594description in the @code{NOTES} section which values are allowed. Using this 595option may result in slightly more data being allocated than necessary. 596 597@end table 598@c man end 599 600@ignore 601@c man begin NOTES 602Supported image file formats: 603 604@table @option 605@item raw 606 607Raw disk image format (default). This format has the advantage of 608being simple and easily exportable to all other emulators. If your 609file system supports @emph{holes} (for example in ext2 or ext3 on 610Linux or NTFS on Windows), then only the written sectors will reserve 611space. Use @code{qemu-img info} to know the real size used by the 612image or @code{ls -ls} on Unix/Linux. 613 614Supported options: 615@table @code 616@item preallocation 617Preallocation mode (allowed values: @code{off}, @code{falloc}, @code{full}). 618@code{falloc} mode preallocates space for image by calling posix_fallocate(). 619@code{full} mode preallocates space for image by writing zeros to underlying 620storage. 621@end table 622 623@item qcow2 624QEMU image format, the most versatile format. Use it to have smaller 625images (useful if your filesystem does not supports holes, for example 626on Windows), optional AES encryption, zlib based compression and 627support of multiple VM snapshots. 628 629Supported options: 630@table @code 631@item compat 632Determines the qcow2 version to use. @code{compat=0.10} uses the 633traditional image format that can be read by any QEMU since 0.10. 634@code{compat=1.1} enables image format extensions that only QEMU 1.1 and 635newer understand (this is the default). Amongst others, this includes zero 636clusters, which allow efficient copy-on-read for sparse images. 637 638@item backing_file 639File name of a base image (see @option{create} subcommand) 640@item backing_fmt 641Image format of the base image 642@item encryption 643If this option is set to @code{on}, the image is encrypted with 128-bit AES-CBC. 644 645The use of encryption in qcow and qcow2 images is considered to be flawed by 646modern cryptography standards, suffering from a number of design problems: 647 648@itemize @minus 649@item 650The AES-CBC cipher is used with predictable initialization vectors based 651on the sector number. This makes it vulnerable to chosen plaintext attacks 652which can reveal the existence of encrypted data. 653@item 654The user passphrase is directly used as the encryption key. A poorly 655chosen or short passphrase will compromise the security of the encryption. 656@item 657In the event of the passphrase being compromised there is no way to 658change the passphrase to protect data in any qcow images. The files must 659be cloned, using a different encryption passphrase in the new file. The 660original file must then be securely erased using a program like shred, 661though even this is ineffective with many modern storage technologies. 662@item 663Initialization vectors used to encrypt sectors are based on the 664guest virtual sector number, instead of the host physical sector. When 665a disk image has multiple internal snapshots this means that data in 666multiple physical sectors is encrypted with the same initialization 667vector. With the CBC mode, this opens the possibility of watermarking 668attacks if the attack can collect multiple sectors encrypted with the 669same IV and some predictable data. Having multiple qcow2 images with 670the same passphrase also exposes this weakness since the passphrase 671is directly used as the key. 672@end itemize 673 674Use of qcow / qcow2 encryption is thus strongly discouraged. Users are 675recommended to use an alternative encryption technology such as the 676Linux dm-crypt / LUKS system. 677 678@item cluster_size 679Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster 680sizes can improve the image file size whereas larger cluster sizes generally 681provide better performance. 682 683@item preallocation 684Preallocation mode (allowed values: @code{off}, @code{metadata}, @code{falloc}, 685@code{full}). An image with preallocated metadata is initially larger but can 686improve performance when the image needs to grow. @code{falloc} and @code{full} 687preallocations are like the same options of @code{raw} format, but sets up 688metadata also. 689 690@item lazy_refcounts 691If this option is set to @code{on}, reference count updates are postponed with 692the goal of avoiding metadata I/O and improving performance. This is 693particularly interesting with @option{cache=writethrough} which doesn't batch 694metadata updates. The tradeoff is that after a host crash, the reference count 695tables must be rebuilt, i.e. on the next open an (automatic) @code{qemu-img 696check -r all} is required, which may take some time. 697 698This option can only be enabled if @code{compat=1.1} is specified. 699 700@item nocow 701If this option is set to @code{on}, it will turn off COW of the file. It's only 702valid on btrfs, no effect on other file systems. 703 704Btrfs has low performance when hosting a VM image file, even more when the guest 705on the VM also using btrfs as file system. Turning off COW is a way to mitigate 706this bad performance. Generally there are two ways to turn off COW on btrfs: 707a) Disable it by mounting with nodatacow, then all newly created files will be 708NOCOW. b) For an empty file, add the NOCOW file attribute. That's what this option 709does. 710 711Note: this option is only valid to new or empty files. If there is an existing 712file which is COW and has data blocks already, it couldn't be changed to NOCOW 713by setting @code{nocow=on}. One can issue @code{lsattr filename} to check if 714the NOCOW flag is set or not (Capital 'C' is NOCOW flag). 715 716@end table 717 718@item Other 719QEMU also supports various other image file formats for compatibility with 720older QEMU versions or other hypervisors, including VMDK, VDI, VHD (vpc), VHDX, 721qcow1 and QED. For a full list of supported formats see @code{qemu-img --help}. 722For a more detailed description of these formats, see the QEMU Emulation User 723Documentation. 724 725The main purpose of the block drivers for these formats is image conversion. 726For running VMs, it is recommended to convert the disk images to either raw or 727qcow2 in order to achieve good performance. 728@end table 729 730 731@c man end 732 733@setfilename qemu-img 734@settitle QEMU disk image utility 735 736@c man begin SEEALSO 737The HTML documentation of QEMU for more precise information and Linux 738user mode emulator invocation. 739@c man end 740 741@c man begin AUTHOR 742Fabrice Bellard 743@c man end 744 745@end ignore 746