qemu/qemu-img.texi
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   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