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@item filename 37 is a disk image filename 38 39@item --object @var{objectdef} 40 41is a QEMU user creatable object definition. See the @code{qemu(1)} manual 42page for a description of the object properties. The most common object 43type is a @code{secret}, which is used to supply passwords and/or encryption 44keys. 45 46@item --image-opts 47 48Indicates that the source @var{filename} parameter is to be interpreted as a 49full option string, not a plain filename. This parameter is mutually 50exclusive with the @var{-f} parameter. 51 52@item --target-image-opts 53 54Indicates that the @var{output_filename} parameter(s) are to be interpreted as 55a full option string, not a plain filename. This parameter is mutually 56exclusive with the @var{-O} parameters. It is currently required to also use 57the @var{-n} parameter to skip image creation. This restriction may be relaxed 58in a future release. 59 60@item fmt 61is the disk image format. It is guessed automatically in most cases. See below 62for a description of the supported disk formats. 63 64@item --backing-chain 65will enumerate information about backing files in a disk image chain. Refer 66below for further description. 67 68@item size 69is the disk image size in bytes. Optional suffixes @code{k} or @code{K} 70(kilobyte, 1024) @code{M} (megabyte, 1024k) and @code{G} (gigabyte, 1024M) 71and T (terabyte, 1024G) are supported. @code{b} is ignored. 72 73@item output_filename 74is the destination disk image filename 75 76@item output_fmt 77 is the destination format 78@item options 79is a comma separated list of format specific options in a 80name=value format. Use @code{-o ?} for an overview of the options supported 81by the used format or see the format descriptions below for details. 82@item snapshot_param 83is param used for internal snapshot, format is 84'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]' 85@item snapshot_id_or_name 86is deprecated, use snapshot_param instead 87 88@item -c 89indicates that target image must be compressed (qcow format only) 90@item -h 91with or without a command shows help and lists the supported formats 92@item -p 93display progress bar (compare, convert and rebase commands only). 94If the @var{-p} option is not used for a command that supports it, the 95progress is reported when the process receives a @code{SIGUSR1} or 96@code{SIGINFO} signal. 97@item -q 98Quiet mode - do not print any output (except errors). There's no progress bar 99in case both @var{-q} and @var{-p} options are used. 100@item -S @var{size} 101indicates the consecutive number of bytes that must contain only zeros 102for qemu-img to create a sparse image during conversion. This value is rounded 103down to the nearest 512 bytes. You may use the common size suffixes like 104@code{k} for kilobytes. 105@item -t @var{cache} 106specifies the cache mode that should be used with the (destination) file. See 107the documentation of the emulator's @code{-drive cache=...} option for allowed 108values. 109@item -T @var{src_cache} 110specifies the cache mode that should be used with the source file(s). See 111the documentation of the emulator's @code{-drive cache=...} option for allowed 112values. 113@end table 114 115Parameters to snapshot subcommand: 116 117@table @option 118 119@item snapshot 120is the name of the snapshot to create, apply or delete 121@item -a 122applies a snapshot (revert disk to saved state) 123@item -c 124creates a snapshot 125@item -d 126deletes a snapshot 127@item -l 128lists all snapshots in the given image 129@end table 130 131Parameters to compare subcommand: 132 133@table @option 134 135@item -f 136First image format 137@item -F 138Second image format 139@item -s 140Strict mode - fail on different image size or sector allocation 141@end table 142 143Parameters to convert subcommand: 144 145@table @option 146 147@item -n 148Skip the creation of the target volume 149@item -m 150Number of parallel coroutines for the convert process 151@item -W 152Allow out-of-order writes to the destination. This option improves performance, 153but is only recommended for preallocated devices like host devices or other 154raw block devices. 155@end table 156 157Parameters to dd subcommand: 158 159@table @option 160 161@item bs=@var{block_size} 162defines the block size 163@item count=@var{blocks} 164sets the number of input blocks to copy 165@item if=@var{input} 166sets the input file 167@item of=@var{output} 168sets the output file 169@item skip=@var{blocks} 170sets the number of input blocks to skip 171@end table 172 173Command description: 174 175@table @option 176@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} 177 178Run a simple sequential I/O benchmark on the specified image. If @code{-w} is 179specified, a write test is performed, otherwise a read test is performed. 180 181A total number of @var{count} I/O requests is performed, each @var{buffer_size} 182bytes in size, and with @var{depth} requests in parallel. The first request 183starts at the position given by @var{offset}, each following request increases 184the current position by @var{step_size}. If @var{step_size} is not given, 185@var{buffer_size} is used for its value. 186 187If @var{flush_interval} is specified for a write test, the request queue is 188drained and a flush is issued before new writes are made whenever the number of 189remaining requests is a multiple of @var{flush_interval}. If additionally 190@code{--no-drain} is specified, a flush is issued without draining the request 191queue first. 192 193If @code{-n} is specified, the native AIO backend is used if possible. On 194Linux, this option only works if @code{-t none} or @code{-t directsync} is 195specified as well. 196 197For write tests, by default a buffer filled with zeros is written. This can be 198overridden with a pattern byte specified by @var{pattern}. 199 200@item check [-f @var{fmt}] [--output=@var{ofmt}] [-r [leaks | all]] [-T @var{src_cache}] @var{filename} 201 202Perform a consistency check on the disk image @var{filename}. The command can 203output in the format @var{ofmt} which is either @code{human} or @code{json}. 204 205If @code{-r} is specified, qemu-img tries to repair any inconsistencies found 206during the check. @code{-r leaks} repairs only cluster leaks, whereas 207@code{-r all} fixes all kinds of errors, with a higher risk of choosing the 208wrong fix or hiding corruption that has already occurred. 209 210Only the formats @code{qcow2}, @code{qed} and @code{vdi} support 211consistency checks. 212 213In case the image does not have any inconsistencies, check exits with @code{0}. 214Other exit codes indicate the kind of inconsistency found or if another error 215occurred. The following table summarizes all exit codes of the check subcommand: 216 217@table @option 218 219@item 0 220Check completed, the image is (now) consistent 221@item 1 222Check not completed because of internal errors 223@item 2 224Check completed, image is corrupted 225@item 3 226Check completed, image has leaked clusters, but is not corrupted 227@item 63 228Checks are not supported by the image format 229 230@end table 231 232If @code{-r} is specified, exit codes representing the image state refer to the 233state after (the attempt at) repairing it. That is, a successful @code{-r all} 234will yield the exit code 0, independently of the image state before. 235 236@item create [-f @var{fmt}] [-b @var{backing_file}] [-F @var{backing_fmt}] [-u] [-o @var{options}] @var{filename} [@var{size}] 237 238Create the new disk image @var{filename} of size @var{size} and format 239@var{fmt}. Depending on the file format, you can add one or more @var{options} 240that enable additional features of this format. 241 242If the option @var{backing_file} is specified, then the image will record 243only the differences from @var{backing_file}. No size needs to be specified in 244this case. @var{backing_file} will never be modified unless you use the 245@code{commit} monitor command (or qemu-img commit). 246 247If a relative path name is given, the backing file is looked up relative to 248the directory containing @var{filename}. 249 250Note that a given backing file will be opened to check that it is valid. Use 251the @code{-u} option to enable unsafe backing file mode, which means that the 252image will be created even if the associated backing file cannot be opened. A 253matching backing file must be created or additional options be used to make the 254backing file specification valid when you want to use an image created this 255way. 256 257The size can also be specified using the @var{size} option with @code{-o}, 258it doesn't need to be specified separately in this case. 259 260@item commit [-q] [-f @var{fmt}] [-t @var{cache}] [-b @var{base}] [-d] [-p] @var{filename} 261 262Commit the changes recorded in @var{filename} in its base image or backing file. 263If the backing file is smaller than the snapshot, then the backing file will be 264resized to be the same size as the snapshot. If the snapshot is smaller than 265the backing file, the backing file will not be truncated. If you want the 266backing file to match the size of the smaller snapshot, you can safely truncate 267it yourself once the commit operation successfully completes. 268 269The image @var{filename} is emptied after the operation has succeeded. If you do 270not need @var{filename} afterwards and intend to drop it, you may skip emptying 271@var{filename} by specifying the @code{-d} flag. 272 273If the backing chain of the given image file @var{filename} has more than one 274layer, the backing file into which the changes will be committed may be 275specified as @var{base} (which has to be part of @var{filename}'s backing 276chain). If @var{base} is not specified, the immediate backing file of the top 277image (which is @var{filename}) will be used. Note that after a commit operation 278all images between @var{base} and the top image will be invalid and may return 279garbage data when read. For this reason, @code{-b} implies @code{-d} (so that 280the top image stays valid). 281 282@item compare [-f @var{fmt}] [-F @var{fmt}] [-T @var{src_cache}] [-p] [-s] [-q] @var{filename1} @var{filename2} 283 284Check if two images have the same content. You can compare images with 285different format or settings. 286 287The format is probed unless you specify it by @var{-f} (used for 288@var{filename1}) and/or @var{-F} (used for @var{filename2}) option. 289 290By default, images with different size are considered identical if the larger 291image contains only unallocated and/or zeroed sectors in the area after the end 292of the other image. In addition, if any sector is not allocated in one image 293and contains only zero bytes in the second one, it is evaluated as equal. You 294can use Strict mode by specifying the @var{-s} option. When compare runs in 295Strict mode, it fails in case image size differs or a sector is allocated in 296one image and is not allocated in the second one. 297 298By default, compare prints out a result message. This message displays 299information that both images are same or the position of the first different 300byte. In addition, result message can report different image size in case 301Strict mode is used. 302 303Compare exits with @code{0} in case the images are equal and with @code{1} 304in case the images differ. Other exit codes mean an error occurred during 305execution and standard error output should contain an error message. 306The following table sumarizes all exit codes of the compare subcommand: 307 308@table @option 309 310@item 0 311Images are identical 312@item 1 313Images differ 314@item 2 315Error on opening an image 316@item 3 317Error on checking a sector allocation 318@item 4 319Error on reading data 320 321@end table 322 323@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} 324 325Convert the disk image @var{filename} or a snapshot @var{snapshot_param}(@var{snapshot_id_or_name} is deprecated) 326to disk image @var{output_filename} using format @var{output_fmt}. It can be optionally compressed (@code{-c} 327option) or use any format specific options like encryption (@code{-o} option). 328 329Only the formats @code{qcow} and @code{qcow2} support compression. The 330compression is read-only. It means that if a compressed sector is 331rewritten, then it is rewritten as uncompressed data. 332 333Image conversion is also useful to get smaller image when using a 334growable format such as @code{qcow}: the empty sectors are detected and 335suppressed from the destination image. 336 337@var{sparse_size} indicates the consecutive number of bytes (defaults to 4k) 338that must contain only zeros for qemu-img to create a sparse image during 339conversion. If @var{sparse_size} is 0, the source will not be scanned for 340unallocated or zero sectors, and the destination image will always be 341fully allocated. 342 343You can use the @var{backing_file} option to force the output image to be 344created as a copy on write image of the specified base image; the 345@var{backing_file} should have the same content as the input's base image, 346however the path, image format, etc may differ. 347 348If a relative path name is given, the backing file is looked up relative to 349the directory containing @var{output_filename}. 350 351If the @code{-n} option is specified, the target volume creation will be 352skipped. This is useful for formats such as @code{rbd} if the target 353volume has already been created with site specific options that cannot 354be supplied through qemu-img. 355 356Out of order writes can be enabled with @code{-W} to improve performance. 357This is only recommended for preallocated devices like host devices or other 358raw block devices. Out of order write does not work in combination with 359creating compressed images. 360 361@var{num_coroutines} specifies how many coroutines work in parallel during 362the convert process (defaults to 8). 363 364@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} 365 366Dd copies from @var{input} file to @var{output} file converting it from 367@var{fmt} format to @var{output_fmt} format. 368 369The data is by default read and written using blocks of 512 bytes but can be 370modified by specifying @var{block_size}. If count=@var{blocks} is specified 371dd will stop reading input after reading @var{blocks} input blocks. 372 373The size syntax is similar to dd(1)'s size syntax. 374 375@item info [-f @var{fmt}] [--output=@var{ofmt}] [--backing-chain] @var{filename} 376 377Give information about the disk image @var{filename}. Use it in 378particular to know the size reserved on disk which can be different 379from the displayed size. If VM snapshots are stored in the disk image, 380they are displayed too. The command can output in the format @var{ofmt} 381which is either @code{human} or @code{json}. 382 383If a disk image has a backing file chain, information about each disk image in 384the chain can be recursively enumerated by using the option @code{--backing-chain}. 385 386For instance, if you have an image chain like: 387 388@example 389base.qcow2 <- snap1.qcow2 <- snap2.qcow2 390@end example 391 392To enumerate information about each disk image in the above chain, starting from top to base, do: 393 394@example 395qemu-img info --backing-chain snap2.qcow2 396@end example 397 398@item map [-f @var{fmt}] [--output=@var{ofmt}] @var{filename} 399 400Dump the metadata of image @var{filename} and its backing file chain. 401In particular, this commands dumps the allocation state of every sector 402of @var{filename}, together with the topmost file that allocates it in 403the backing file chain. 404 405Two option formats are possible. The default format (@code{human}) 406only dumps known-nonzero areas of the file. Known-zero parts of the 407file are omitted altogether, and likewise for parts that are not allocated 408throughout the chain. @command{qemu-img} output will identify a file 409from where the data can be read, and the offset in the file. Each line 410will include four fields, the first three of which are hexadecimal 411numbers. For example the first line of: 412@example 413Offset Length Mapped to File 4140 0x20000 0x50000 /tmp/overlay.qcow2 4150x100000 0x10000 0x95380000 /tmp/backing.qcow2 416@end example 417@noindent 418means that 0x20000 (131072) bytes starting at offset 0 in the image are 419available in /tmp/overlay.qcow2 (opened in @code{raw} format) starting 420at offset 0x50000 (327680). Data that is compressed, encrypted, or 421otherwise not available in raw format will cause an error if @code{human} 422format is in use. Note that file names can include newlines, thus it is 423not safe to parse this output format in scripts. 424 425The alternative format @code{json} will return an array of dictionaries 426in JSON format. It will include similar information in 427the @code{start}, @code{length}, @code{offset} fields; 428it will also include other more specific information: 429@itemize @minus 430@item 431whether the sectors contain actual data or not (boolean field @code{data}; 432if false, the sectors are either unallocated or stored as optimized 433all-zero clusters); 434 435@item 436whether the data is known to read as zero (boolean field @code{zero}); 437 438@item 439in order to make the output shorter, the target file is expressed as 440a @code{depth}; for example, a depth of 2 refers to the backing file 441of the backing file of @var{filename}. 442@end itemize 443 444In JSON format, the @code{offset} field is optional; it is absent in 445cases where @code{human} format would omit the entry or exit with an error. 446If @code{data} is false and the @code{offset} field is present, the 447corresponding sectors in the file are not yet in use, but they are 448preallocated. 449 450For more information, consult @file{include/block/block.h} in QEMU's 451source code. 452 453@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}] 454 455Calculate the file size required for a new image. This information can be used 456to size logical volumes or SAN LUNs appropriately for the image that will be 457placed in them. The values reported are guaranteed to be large enough to fit 458the image. The command can output in the format @var{ofmt} which is either 459@code{human} or @code{json}. 460 461If the size @var{N} is given then act as if creating a new empty image file 462using @command{qemu-img create}. If @var{filename} is given then act as if 463converting an existing image file using @command{qemu-img convert}. The format 464of the new file is given by @var{output_fmt} while the format of an existing 465file is given by @var{fmt}. 466 467A snapshot in an existing image can be specified using @var{snapshot_param}. 468 469The following fields are reported: 470@example 471required size: 524288 472fully allocated size: 1074069504 473@end example 474 475The @code{required size} is the file size of the new image. It may be smaller 476than the virtual disk size if the image format supports compact representation. 477 478The @code{fully allocated size} is the file size of the new image once data has 479been written to all sectors. This is the maximum size that the image file can 480occupy with the exception of internal snapshots, dirty bitmaps, vmstate data, 481and other advanced image format features. 482 483@item snapshot [-l | -a @var{snapshot} | -c @var{snapshot} | -d @var{snapshot} ] @var{filename} 484 485List, apply, create or delete snapshots in image @var{filename}. 486 487@item rebase [-f @var{fmt}] [-t @var{cache}] [-T @var{src_cache}] [-p] [-u] -b @var{backing_file} [-F @var{backing_fmt}] @var{filename} 488 489Changes the backing file of an image. Only the formats @code{qcow2} and 490@code{qed} support changing the backing file. 491 492The backing file is changed to @var{backing_file} and (if the image format of 493@var{filename} supports this) the backing file format is changed to 494@var{backing_fmt}. If @var{backing_file} is specified as ``'' (the empty 495string), then the image is rebased onto no backing file (i.e. it will exist 496independently of any backing file). 497 498If a relative path name is given, the backing file is looked up relative to 499the directory containing @var{filename}. 500 501@var{cache} specifies the cache mode to be used for @var{filename}, whereas 502@var{src_cache} specifies the cache mode for reading backing files. 503 504There are two different modes in which @code{rebase} can operate: 505@table @option 506@item Safe mode 507This is the default mode and performs a real rebase operation. The new backing 508file may differ from the old one and qemu-img rebase will take care of keeping 509the guest-visible content of @var{filename} unchanged. 510 511In order to achieve this, any clusters that differ between @var{backing_file} 512and the old backing file of @var{filename} are merged into @var{filename} 513before actually changing the backing file. 514 515Note that the safe mode is an expensive operation, comparable to converting 516an image. It only works if the old backing file still exists. 517 518@item Unsafe mode 519qemu-img uses the unsafe mode if @code{-u} is specified. In this mode, only the 520backing file name and format of @var{filename} is changed without any checks 521on the file contents. The user must take care of specifying the correct new 522backing file, or the guest-visible content of the image will be corrupted. 523 524This mode is useful for renaming or moving the backing file to somewhere else. 525It can be used without an accessible old backing file, i.e. you can use it to 526fix an image whose backing file has already been moved/renamed. 527@end table 528 529You can use @code{rebase} to perform a ``diff'' operation on two 530disk images. This can be useful when you have copied or cloned 531a guest, and you want to get back to a thin image on top of a 532template or base image. 533 534Say that @code{base.img} has been cloned as @code{modified.img} by 535copying it, and that the @code{modified.img} guest has run so there 536are now some changes compared to @code{base.img}. To construct a thin 537image called @code{diff.qcow2} that contains just the differences, do: 538 539@example 540qemu-img create -f qcow2 -b modified.img diff.qcow2 541qemu-img rebase -b base.img diff.qcow2 542@end example 543 544At this point, @code{modified.img} can be discarded, since 545@code{base.img + diff.qcow2} contains the same information. 546 547@item resize [--shrink] [--preallocation=@var{prealloc}] @var{filename} [+ | -]@var{size} 548 549Change the disk image as if it had been created with @var{size}. 550 551Before using this command to shrink a disk image, you MUST use file system and 552partitioning tools inside the VM to reduce allocated file systems and partition 553sizes accordingly. Failure to do so will result in data loss! 554 555When shrinking images, the @code{--shrink} option must be given. This informs 556qemu-img that the user acknowledges all loss of data beyond the truncated 557image's end. 558 559After using this command to grow a disk image, you must use file system and 560partitioning tools inside the VM to actually begin using the new space on the 561device. 562 563When growing an image, the @code{--preallocation} option may be used to specify 564how the additional image area should be allocated on the host. See the format 565description in the @code{NOTES} section which values are allowed. Using this 566option may result in slightly more data being allocated than necessary. 567 568@item amend [-p] [-f @var{fmt}] [-t @var{cache}] -o @var{options} @var{filename} 569 570Amends the image format specific @var{options} for the image file 571@var{filename}. Not all file formats support this operation. 572@end table 573@c man end 574 575@ignore 576@c man begin NOTES 577Supported image file formats: 578 579@table @option 580@item raw 581 582Raw disk image format (default). This format has the advantage of 583being simple and easily exportable to all other emulators. If your 584file system supports @emph{holes} (for example in ext2 or ext3 on 585Linux or NTFS on Windows), then only the written sectors will reserve 586space. Use @code{qemu-img info} to know the real size used by the 587image or @code{ls -ls} on Unix/Linux. 588 589Supported options: 590@table @code 591@item preallocation 592Preallocation mode (allowed values: @code{off}, @code{falloc}, @code{full}). 593@code{falloc} mode preallocates space for image by calling posix_fallocate(). 594@code{full} mode preallocates space for image by writing zeros to underlying 595storage. 596@end table 597 598@item qcow2 599QEMU image format, the most versatile format. Use it to have smaller 600images (useful if your filesystem does not supports holes, for example 601on Windows), optional AES encryption, zlib based compression and 602support of multiple VM snapshots. 603 604Supported options: 605@table @code 606@item compat 607Determines the qcow2 version to use. @code{compat=0.10} uses the 608traditional image format that can be read by any QEMU since 0.10. 609@code{compat=1.1} enables image format extensions that only QEMU 1.1 and 610newer understand (this is the default). Amongst others, this includes zero 611clusters, which allow efficient copy-on-read for sparse images. 612 613@item backing_file 614File name of a base image (see @option{create} subcommand) 615@item backing_fmt 616Image format of the base image 617@item encryption 618If this option is set to @code{on}, the image is encrypted with 128-bit AES-CBC. 619 620The use of encryption in qcow and qcow2 images is considered to be flawed by 621modern cryptography standards, suffering from a number of design problems: 622 623@itemize @minus 624@item 625The AES-CBC cipher is used with predictable initialization vectors based 626on the sector number. This makes it vulnerable to chosen plaintext attacks 627which can reveal the existence of encrypted data. 628@item 629The user passphrase is directly used as the encryption key. A poorly 630chosen or short passphrase will compromise the security of the encryption. 631@item 632In the event of the passphrase being compromised there is no way to 633change the passphrase to protect data in any qcow images. The files must 634be cloned, using a different encryption passphrase in the new file. The 635original file must then be securely erased using a program like shred, 636though even this is ineffective with many modern storage technologies. 637@item 638Initialization vectors used to encrypt sectors are based on the 639guest virtual sector number, instead of the host physical sector. When 640a disk image has multiple internal snapshots this means that data in 641multiple physical sectors is encrypted with the same initialization 642vector. With the CBC mode, this opens the possibility of watermarking 643attacks if the attack can collect multiple sectors encrypted with the 644same IV and some predictable data. Having multiple qcow2 images with 645the same passphrase also exposes this weakness since the passphrase 646is directly used as the key. 647@end itemize 648 649Use of qcow / qcow2 encryption is thus strongly discouraged. Users are 650recommended to use an alternative encryption technology such as the 651Linux dm-crypt / LUKS system. 652 653@item cluster_size 654Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster 655sizes can improve the image file size whereas larger cluster sizes generally 656provide better performance. 657 658@item preallocation 659Preallocation mode (allowed values: @code{off}, @code{metadata}, @code{falloc}, 660@code{full}). An image with preallocated metadata is initially larger but can 661improve performance when the image needs to grow. @code{falloc} and @code{full} 662preallocations are like the same options of @code{raw} format, but sets up 663metadata also. 664 665@item lazy_refcounts 666If this option is set to @code{on}, reference count updates are postponed with 667the goal of avoiding metadata I/O and improving performance. This is 668particularly interesting with @option{cache=writethrough} which doesn't batch 669metadata updates. The tradeoff is that after a host crash, the reference count 670tables must be rebuilt, i.e. on the next open an (automatic) @code{qemu-img 671check -r all} is required, which may take some time. 672 673This option can only be enabled if @code{compat=1.1} is specified. 674 675@item nocow 676If this option is set to @code{on}, it will turn off COW of the file. It's only 677valid on btrfs, no effect on other file systems. 678 679Btrfs has low performance when hosting a VM image file, even more when the guest 680on the VM also using btrfs as file system. Turning off COW is a way to mitigate 681this bad performance. Generally there are two ways to turn off COW on btrfs: 682a) Disable it by mounting with nodatacow, then all newly created files will be 683NOCOW. b) For an empty file, add the NOCOW file attribute. That's what this option 684does. 685 686Note: this option is only valid to new or empty files. If there is an existing 687file which is COW and has data blocks already, it couldn't be changed to NOCOW 688by setting @code{nocow=on}. One can issue @code{lsattr filename} to check if 689the NOCOW flag is set or not (Capital 'C' is NOCOW flag). 690 691@end table 692 693@item Other 694QEMU also supports various other image file formats for compatibility with 695older QEMU versions or other hypervisors, including VMDK, VDI, VHD (vpc), VHDX, 696qcow1 and QED. For a full list of supported formats see @code{qemu-img --help}. 697For a more detailed description of these formats, see the QEMU Emulation User 698Documentation. 699 700The main purpose of the block drivers for these formats is image conversion. 701For running VMs, it is recommended to convert the disk images to either raw or 702qcow2 in order to achieve good performance. 703@end table 704 705 706@c man end 707 708@setfilename qemu-img 709@settitle QEMU disk image utility 710 711@c man begin SEEALSO 712The HTML documentation of QEMU for more precise information and Linux 713user mode emulator invocation. 714@c man end 715 716@c man begin AUTHOR 717Fabrice Bellard 718@c man end 719 720@end ignore 721