2@c man begin SYNOPSIS
   3@command{qemu-img} @var{command} [@var{command} @var{options}]
   4@c man end
   5@end example
   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.
  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
  17@c man begin OPTIONS
  19The following commands are supported:
  21@include qemu-img-cmds.texi
  23Command parameters:
  24@table @var
  25@item filename
  26 is a disk image filename
  28@item --object @var{objectdef}
  30is a QEMU user creatable object definition. See the @code{qemu(1)} manual
  31page for a description of the object properties. The most common object
  32type is a @code{secret}, which is used to supply passwords and/or encryption
  35@item --image-opts
  37Indicates that the @var{filename} parameter is to be interpreted as a
  38full option string, not a plain filename. This parameter is mutually
  39exclusive with the @var{-f} and @var{-F} parameters.
  41@item fmt
  42is the disk image format. It is guessed automatically in most cases. See below
  43for a description of the supported disk formats.
  45@item --backing-chain
  46will enumerate information about backing files in a disk image chain. Refer
  47below for further description.
  49@item size
  50is the disk image size in bytes. Optional suffixes @code{k} or @code{K}
  51(kilobyte, 1024) @code{M} (megabyte, 1024k) and @code{G} (gigabyte, 1024M)
  52and T (terabyte, 1024G) are supported.  @code{b} is ignored.
  54@item output_filename
  55is the destination disk image filename
  57@item output_fmt
  58 is the destination format
  59@item options
  60is a comma separated list of format specific options in a
  61name=value format. Use @code{-o ?} for an overview of the options supported
  62by the used format or see the format descriptions below for details.
  63@item snapshot_param
  64is param used for internal snapshot, format is
  65'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'
  66@item snapshot_id_or_name
  67is deprecated, use snapshot_param instead
  69@item -c
  70indicates that target image must be compressed (qcow format only)
  71@item -h
  72with or without a command shows help and lists the supported formats
  73@item -p
  74display progress bar (compare, convert and rebase commands only).
  75If the @var{-p} option is not used for a command that supports it, the
  76progress is reported when the process receives a @code{SIGUSR1} signal.
  77@item -q
  78Quiet mode - do not print any output (except errors). There's no progress bar
  79in case both @var{-q} and @var{-p} options are used.
  80@item -S @var{size}
  81indicates the consecutive number of bytes that must contain only zeros
  82for qemu-img to create a sparse image during conversion. This value is rounded
  83down to the nearest 512 bytes. You may use the common size suffixes like
  84@code{k} for kilobytes.
  85@item -t @var{cache}
  86specifies the cache mode that should be used with the (destination) file. See
  87the documentation of the emulator's @code{-drive cache=...} option for allowed
  89@item -T @var{src_cache}
  90specifies the cache mode that should be used with the source file(s). See
  91the documentation of the emulator's @code{-drive cache=...} option for allowed
  93@end table
  95Parameters to snapshot subcommand:
  97@table @option
  99@item snapshot
 100is the name of the snapshot to create, apply or delete
 101@item -a
 102applies a snapshot (revert disk to saved state)
 103@item -c
 104creates a snapshot
 105@item -d
 106deletes a snapshot
 107@item -l
 108lists all snapshots in the given image
 109@end table
 111Parameters to compare subcommand:
 113@table @option
 115@item -f
 116First image format
 117@item -F
 118Second image format
 119@item -s
 120Strict mode - fail on different image size or sector allocation
 121@end table
 123Parameters to convert subcommand:
 125@table @option
 127@item -n
 128Skip the creation of the target volume
 129@end table
 131Command description:
 133@table @option
 134@item check [-f @var{fmt}] [--output=@var{ofmt}] [-r [leaks | all]] [-T @var{src_cache}] @var{filename}
 136Perform a consistency check on the disk image @var{filename}. The command can
 137output in the format @var{ofmt} which is either @code{human} or @code{json}.
 139If @code{-r} is specified, qemu-img tries to repair any inconsistencies found
 140during the check. @code{-r leaks} repairs only cluster leaks, whereas
 141@code{-r all} fixes all kinds of errors, with a higher risk of choosing the
 142wrong fix or hiding corruption that has already occurred.
 144Only the formats @code{qcow2}, @code{qed} and @code{vdi} support
 145consistency checks.
 147In case the image does not have any inconsistencies, check exits with @code{0}.
 148Other exit codes indicate the kind of inconsistency found or if another error
 149occurred. The following table summarizes all exit codes of the check subcommand:
 151@table @option
 153@item 0
 154Check completed, the image is (now) consistent
 155@item 1
 156Check not completed because of internal errors
 157@item 2
 158Check completed, image is corrupted
 159@item 3
 160Check completed, image has leaked clusters, but is not corrupted
 161@item 63
 162Checks are not supported by the image format
 164@end table
 166If @code{-r} is specified, exit codes representing the image state refer to the
 167state after (the attempt at) repairing it. That is, a successful @code{-r all}
 168will yield the exit code 0, independently of the image state before.
 170@item create [-f @var{fmt}] [-o @var{options}] @var{filename} [@var{size}]
 172Create the new disk image @var{filename} of size @var{size} and format
 173@var{fmt}. Depending on the file format, you can add one or more @var{options}
 174that enable additional features of this format.
 176If the option @var{backing_file} is specified, then the image will record
 177only the differences from @var{backing_file}. No size needs to be specified in
 178this case. @var{backing_file} will never be modified unless you use the
 179@code{commit} monitor command (or qemu-img commit).
 181The size can also be specified using the @var{size} option with @code{-o},
 182it doesn't need to be specified separately in this case.
 184@item commit [-q] [-f @var{fmt}] [-t @var{cache}] [-b @var{base}] [-d] [-p] @var{filename}
 186Commit the changes recorded in @var{filename} in its base image or backing file.
 187If the backing file is smaller than the snapshot, then the backing file will be
 188resized to be the same size as the snapshot.  If the snapshot is smaller than
 189the backing file, the backing file will not be truncated.  If you want the
 190backing file to match the size of the smaller snapshot, you can safely truncate
 191it yourself once the commit operation successfully completes.
 193The image @var{filename} is emptied after the operation has succeeded. If you do
 194not need @var{filename} afterwards and intend to drop it, you may skip emptying
 195@var{filename} by specifying the @code{-d} flag.
 197If the backing chain of the given image file @var{filename} has more than one
 198layer, the backing file into which the changes will be committed may be
 199specified as @var{base} (which has to be part of @var{filename}'s backing
 200chain). If @var{base} is not specified, the immediate backing file of the top
 201image (which is @var{filename}) will be used. For reasons of consistency,
 202explicitly specifying @var{base} will always imply @code{-d} (since emptying an
 203image after committing to an indirect backing file would lead to different data
 204being read from the image due to content in the intermediate backing chain
 205overruling the commit target).
 207@item compare [-f @var{fmt}] [-F @var{fmt}] [-T @var{src_cache}] [-p] [-s] [-q] @var{filename1} @var{filename2}
 209Check if two images have the same content. You can compare images with
 210different format or settings.
 212The format is probed unless you specify it by @var{-f} (used for
 213@var{filename1}) and/or @var{-F} (used for @var{filename2}) option.
 215By default, images with different size are considered identical if the larger
 216image contains only unallocated and/or zeroed sectors in the area after the end
 217of the other image. In addition, if any sector is not allocated in one image
 218and contains only zero bytes in the second one, it is evaluated as equal. You
 219can use Strict mode by specifying the @var{-s} option. When compare runs in
 220Strict mode, it fails in case image size differs or a sector is allocated in
 221one image and is not allocated in the second one.
 223By default, compare prints out a result message. This message displays
 224information that both images are same or the position of the first different
 225byte. In addition, result message can report different image size in case
 226Strict mode is used.
 228Compare exits with @code{0} in case the images are equal and with @code{1}
 229in case the images differ. Other exit codes mean an error occurred during
 230execution and standard error output should contain an error message.
 231The following table sumarizes all exit codes of the compare subcommand:
 233@table @option
 235@item 0
 236Images are identical
 237@item 1
 238Images differ
 239@item 2
 240Error on opening an image
 241@item 3
 242Error on checking a sector allocation
 243@item 4
 244Error on reading data
 246@end table
 248@item convert [-c] [-p] [-n] [-f @var{fmt}] [-t @var{cache}] [-T @var{src_cache}] [-O @var{output_fmt}] [-o @var{options}] [-s @var{snapshot_id_or_name}] [-l @var{snapshot_param}] [-S @var{sparse_size}] @var{filename} [@var{filename2} [...]] @var{output_filename}
 250Convert the disk image @var{filename} or a snapshot @var{snapshot_param}(@var{snapshot_id_or_name} is deprecated)
 251to disk image @var{output_filename} using format @var{output_fmt}. It can be optionally compressed (@code{-c}
 252option) or use any format specific options like encryption (@code{-o} option).
 254Only the formats @code{qcow} and @code{qcow2} support compression. The
 255compression is read-only. It means that if a compressed sector is
 256rewritten, then it is rewritten as uncompressed data.
 258Image conversion is also useful to get smaller image when using a
 259growable format such as @code{qcow}: the empty sectors are detected and
 260suppressed from the destination image.
 262@var{sparse_size} indicates the consecutive number of bytes (defaults to 4k)
 263that must contain only zeros for qemu-img to create a sparse image during
 264conversion. If @var{sparse_size} is 0, the source will not be scanned for
 265unallocated or zero sectors, and the destination image will always be
 266fully allocated.
 268You can use the @var{backing_file} option to force the output image to be
 269created as a copy on write image of the specified base image; the
 270@var{backing_file} should have the same content as the input's base image,
 271however the path, image format, etc may differ.
 273If the @code{-n} option is specified, the target volume creation will be
 274skipped. This is useful for formats such as @code{rbd} if the target
 275volume has already been created with site specific options that cannot
 276be supplied through qemu-img.
 278@item info [-f @var{fmt}] [--output=@var{ofmt}] [--backing-chain] @var{filename}
 280Give information about the disk image @var{filename}. Use it in
 281particular to know the size reserved on disk which can be different
 282from the displayed size. If VM snapshots are stored in the disk image,
 283they are displayed too. The command can output in the format @var{ofmt}
 284which is either @code{human} or @code{json}.
 286If a disk image has a backing file chain, information about each disk image in
 287the chain can be recursively enumerated by using the option @code{--backing-chain}.
 289For instance, if you have an image chain like:
 292base.qcow2 <- snap1.qcow2 <- snap2.qcow2
 293@end example
 295To enumerate information about each disk image in the above chain, starting from top to base, do:
 298qemu-img info --backing-chain snap2.qcow2
 299@end example
 301@item map [-f @var{fmt}] [--output=@var{ofmt}] @var{filename}
 303Dump the metadata of image @var{filename} and its backing file chain.
 304In particular, this commands dumps the allocation state of every sector
 305of @var{filename}, together with the topmost file that allocates it in
 306the backing file chain.
 308Two option formats are possible.  The default format (@code{human})
 309only dumps known-nonzero areas of the file.  Known-zero parts of the
 310file are omitted altogether, and likewise for parts that are not allocated
 311throughout the chain.  @command{qemu-img} output will identify a file
 312from where the data can be read, and the offset in the file.  Each line
 313will include four fields, the first three of which are hexadecimal
 314numbers.  For example the first line of:
 316Offset          Length          Mapped to       File
 3170               0x20000         0x50000         /tmp/overlay.qcow2
 3180x100000        0x10000         0x95380000      /tmp/backing.qcow2
 319@end example
 321means that 0x20000 (131072) bytes starting at offset 0 in the image are
 322available in /tmp/overlay.qcow2 (opened in @code{raw} format) starting
 323at offset 0x50000 (327680).  Data that is compressed, encrypted, or
 324otherwise not available in raw format will cause an error if @code{human}
 325format is in use.  Note that file names can include newlines, thus it is
 326not safe to parse this output format in scripts.
 328The alternative format @code{json} will return an array of dictionaries
 329in JSON format.  It will include similar information in
 330the @code{start}, @code{length}, @code{offset} fields;
 331it will also include other more specific information:
 332@itemize @minus
 334whether the sectors contain actual data or not (boolean field @code{data};
 335if false, the sectors are either unallocated or stored as optimized
 336all-zero clusters);
 339whether the data is known to read as zero (boolean field @code{zero});
 342in order to make the output shorter, the target file is expressed as
 343a @code{depth}; for example, a depth of 2 refers to the backing file
 344of the backing file of @var{filename}.
 345@end itemize
 347In JSON format, the @code{offset} field is optional; it is absent in
 348cases where @code{human} format would omit the entry or exit with an error.
 349If @code{data} is false and the @code{offset} field is present, the
 350corresponding sectors in the file are not yet in use, but they are
 353For more information, consult @file{include/block/block.h} in QEMU's
 354source code.
 356@item snapshot [-l | -a @var{snapshot} | -c @var{snapshot} | -d @var{snapshot} ] @var{filename}
 358List, apply, create or delete snapshots in image @var{filename}.
 360@item rebase [-f @var{fmt}] [-t @var{cache}] [-T @var{src_cache}] [-p] [-u] -b @var{backing_file} [-F @var{backing_fmt}] @var{filename}
 362Changes the backing file of an image. Only the formats @code{qcow2} and
 363@code{qed} support changing the backing file.
 365The backing file is changed to @var{backing_file} and (if the image format of
 366@var{filename} supports this) the backing file format is changed to
 367@var{backing_fmt}. If @var{backing_file} is specified as ``'' (the empty
 368string), then the image is rebased onto no backing file (i.e. it will exist
 369independently of any backing file).
 371@var{cache} specifies the cache mode to be used for @var{filename}, whereas
 372@var{src_cache} specifies the cache mode for reading backing files.
 374There are two different modes in which @code{rebase} can operate:
 375@table @option
 376@item Safe mode
 377This is the default mode and performs a real rebase operation. The new backing
 378file may differ from the old one and qemu-img rebase will take care of keeping
 379the guest-visible content of @var{filename} unchanged.
 381In order to achieve this, any clusters that differ between @var{backing_file}
 382and the old backing file of @var{filename} are merged into @var{filename}
 383before actually changing the backing file.
 385Note that the safe mode is an expensive operation, comparable to converting
 386an image. It only works if the old backing file still exists.
 388@item Unsafe mode
 389qemu-img uses the unsafe mode if @code{-u} is specified. In this mode, only the
 390backing file name and format of @var{filename} is changed without any checks
 391on the file contents. The user must take care of specifying the correct new
 392backing file, or the guest-visible content of the image will be corrupted.
 394This mode is useful for renaming or moving the backing file to somewhere else.
 395It can be used without an accessible old backing file, i.e. you can use it to
 396fix an image whose backing file has already been moved/renamed.
 397@end table
 399You can use @code{rebase} to perform a ``diff'' operation on two
 400disk images.  This can be useful when you have copied or cloned
 401a guest, and you want to get back to a thin image on top of a
 402template or base image.
 404Say that @code{base.img} has been cloned as @code{modified.img} by
 405copying it, and that the @code{modified.img} guest has run so there
 406are now some changes compared to @code{base.img}.  To construct a thin
 407image called @code{diff.qcow2} that contains just the differences, do:
 410qemu-img create -f qcow2 -b modified.img diff.qcow2
 411qemu-img rebase -b base.img diff.qcow2
 412@end example
 414At this point, @code{modified.img} can be discarded, since
 415@code{base.img + diff.qcow2} contains the same information.
 417@item resize @var{filename} [+ | -]@var{size}
 419Change the disk image as if it had been created with @var{size}.
 421Before using this command to shrink a disk image, you MUST use file system and
 422partitioning tools inside the VM to reduce allocated file systems and partition
 423sizes accordingly.  Failure to do so will result in data loss!
 425After using this command to grow a disk image, you must use file system and
 426partitioning tools inside the VM to actually begin using the new space on the
 429@item amend [-p] [-f @var{fmt}] [-t @var{cache}] -o @var{options} @var{filename}
 431Amends the image format specific @var{options} for the image file
 432@var{filename}. Not all file formats support this operation.
 433@end table
 434@c man end
 437@c man begin NOTES
 438Supported image file formats:
 440@table @option
 441@item raw
 443Raw disk image format (default). This format has the advantage of
 444being simple and easily exportable to all other emulators. If your
 445file system supports @emph{holes} (for example in ext2 or ext3 on
 446Linux or NTFS on Windows), then only the written sectors will reserve
 447space. Use @code{qemu-img info} to know the real size used by the
 448image or @code{ls -ls} on Unix/Linux.
 450Supported options:
 451@table @code
 452@item preallocation
 453Preallocation mode (allowed values: @code{off}, @code{falloc}, @code{full}).
 454@code{falloc} mode preallocates space for image by calling posix_fallocate().
 455@code{full} mode preallocates space for image by writing zeros to underlying
 457@end table
 459@item qcow2
 460QEMU image format, the most versatile format. Use it to have smaller
 461images (useful if your filesystem does not supports holes, for example
 462on Windows), optional AES encryption, zlib based compression and
 463support of multiple VM snapshots.
 465Supported options:
 466@table @code
 467@item compat
 468Determines the qcow2 version to use. @code{compat=0.10} uses the
 469traditional image format that can be read by any QEMU since 0.10.
 470@code{compat=1.1} enables image format extensions that only QEMU 1.1 and
 471newer understand (this is the default). Amongst others, this includes zero
 472clusters, which allow efficient copy-on-read for sparse images.
 474@item backing_file
 475File name of a base image (see @option{create} subcommand)
 476@item backing_fmt
 477Image format of the base image
 478@item encryption
 479If this option is set to @code{on}, the image is encrypted with 128-bit AES-CBC.
 481The use of encryption in qcow and qcow2 images is considered to be flawed by
 482modern cryptography standards, suffering from a number of design problems:
 484@itemize @minus
 485@item The AES-CBC cipher is used with predictable initialization vectors based
 486on the sector number. This makes it vulnerable to chosen plaintext attacks
 487which can reveal the existence of encrypted data.
 488@item The user passphrase is directly used as the encryption key. A poorly
 489chosen or short passphrase will compromise the security of the encryption.
 490@item In the event of the passphrase being compromised there is no way to
 491change the passphrase to protect data in any qcow images. The files must
 492be cloned, using a different encryption passphrase in the new file. The
 493original file must then be securely erased using a program like shred,
 494though even this is ineffective with many modern storage technologies.
 495@end itemize
 497Use of qcow / qcow2 encryption is thus strongly discouraged. Users are
 498recommended to use an alternative encryption technology such as the
 499Linux dm-crypt / LUKS system.
 501@item cluster_size
 502Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster
 503sizes can improve the image file size whereas larger cluster sizes generally
 504provide better performance.
 506@item preallocation
 507Preallocation mode (allowed values: @code{off}, @code{metadata}, @code{falloc},
 508@code{full}). An image with preallocated metadata is initially larger but can
 509improve performance when the image needs to grow. @code{falloc} and @code{full}
 510preallocations are like the same options of @code{raw} format, but sets up
 511metadata also.
 513@item lazy_refcounts
 514If this option is set to @code{on}, reference count updates are postponed with
 515the goal of avoiding metadata I/O and improving performance. This is
 516particularly interesting with @option{cache=writethrough} which doesn't batch
 517metadata updates. The tradeoff is that after a host crash, the reference count
 518tables must be rebuilt, i.e. on the next open an (automatic) @code{qemu-img
 519check -r all} is required, which may take some time.
 521This option can only be enabled if @code{compat=1.1} is specified.
 523@item nocow
 524If this option is set to @code{on}, it will turn off COW of the file. It's only
 525valid on btrfs, no effect on other file systems.
 527Btrfs has low performance when hosting a VM image file, even more when the guest
 528on the VM also using btrfs as file system. Turning off COW is a way to mitigate
 529this bad performance. Generally there are two ways to turn off COW on btrfs:
 530a) Disable it by mounting with nodatacow, then all newly created files will be
 531NOCOW. b) For an empty file, add the NOCOW file attribute. That's what this option
 534Note: this option is only valid to new or empty files. If there is an existing
 535file which is COW and has data blocks already, it couldn't be changed to NOCOW
 536by setting @code{nocow=on}. One can issue @code{lsattr filename} to check if
 537the NOCOW flag is set or not (Capital 'C' is NOCOW flag).
 539@end table
 541@item Other
 542QEMU also supports various other image file formats for compatibility with
 543older QEMU versions or other hypervisors, including VMDK, VDI, VHD (vpc), VHDX,
 544qcow1 and QED. For a full list of supported formats see @code{qemu-img --help}.
 545For a more detailed description of these formats, see the QEMU Emulation User
 548The main purpose of the block drivers for these formats is image conversion.
 549For running VMs, it is recommended to convert the disk images to either raw or
 550qcow2 in order to achieve good performance.
 551@end table
 554@c man end
 556@setfilename qemu-img
 557@settitle QEMU disk image utility
 559@c man begin SEEALSO
 560The HTML documentation of QEMU for more precise information and Linux
 561user mode emulator invocation.
 562@c man end
 564@c man begin AUTHOR
 565Fabrice Bellard
 566@c man end
 568@end ignore