qemu

qemu-img.rst (37169B)

---

 =======================
 QEMU disk image utility
 =======================
 
 Synopsis
 --------
 
 **qemu-img** [*standard options*] *command* [*command options*]
 
 Description
 -----------
 
 qemu-img allows you to create, convert and modify images offline. It can handle
 all image formats supported by QEMU.
 
 **Warning:** Never use qemu-img to modify images in use by a running virtual
 machine or any other process; this may destroy the image. Also, be aware that
 querying an image that is being modified by another process may encounter
 inconsistent state.
 
 Options
 -------
 
 .. program:: qemu-img
 
 Standard options:
 
 .. option:: -h, --help
 
   Display this help and exit
 
 .. option:: -V, --version
 
   Display version information and exit
 
 .. option:: -T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE]
 
   .. include:: ../qemu-option-trace.rst.inc
 
 The following commands are supported:
 
 .. hxtool-doc:: qemu-img-cmds.hx
 
 Command parameters:
 
 *FILENAME* is a disk image filename.
 
 *FMT* is the disk image format. It is guessed automatically in most
 cases. See below for a description of the supported disk formats.
 
 *SIZE* is the disk image size in bytes. Optional suffixes ``k`` or
 ``K`` (kilobyte, 1024) ``M`` (megabyte, 1024k) and ``G`` (gigabyte,
 1024M) and T (terabyte, 1024G) are supported.  ``b`` is ignored.
 
 *OUTPUT_FILENAME* is the destination disk image filename.
 
 *OUTPUT_FMT* is the destination format.
 
 *OPTIONS* is a comma separated list of format specific options in a
 name=value format. Use ``-o help`` for an overview of the options supported
 by the used format or see the format descriptions below for details.
 
 *SNAPSHOT_PARAM* is param used for internal snapshot, format is
 'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'.
 
 ..
   Note the use of a new 'program'; otherwise Sphinx complains about
   the -h option appearing both in the above option list and this one.
 
 .. program:: qemu-img-common-opts
 
 .. option:: --object OBJECTDEF
 
   is a QEMU user creatable object definition. See the :manpage:`qemu(1)`
   manual page for a description of the object properties. The most common
   object type is a ``secret``, which is used to supply passwords and/or
   encryption keys.
 
 .. option:: --image-opts
 
   Indicates that the source *FILENAME* parameter is to be interpreted as a
   full option string, not a plain filename. This parameter is mutually
   exclusive with the *-f* parameter.
 
 .. option:: --target-image-opts
 
   Indicates that the OUTPUT_FILENAME parameter(s) are to be interpreted as
   a full option string, not a plain filename. This parameter is mutually
   exclusive with the *-O* parameters. It is currently required to also use
   the *-n* parameter to skip image creation. This restriction may be relaxed
   in a future release.
 
 .. option:: --force-share (-U)
 
   If specified, ``qemu-img`` will open the image in shared mode, allowing
   other QEMU processes to open it in write mode. For example, this can be used to
   get the image information (with 'info' subcommand) when the image is used by a
   running guest.  Note that this could produce inconsistent results because of
   concurrent metadata changes, etc. This option is only allowed when opening
   images in read-only mode.
 
 .. option:: --backing-chain
 
   Will enumerate information about backing files in a disk image chain. Refer
   below for further description.
 
 .. option:: -c
 
   Indicates that target image must be compressed (qcow format only).
 
 .. option:: -h
 
   With or without a command, shows help and lists the supported formats.
 
 .. option:: -p
 
   Display progress bar (compare, convert and rebase commands only).
   If the *-p* option is not used for a command that supports it, the
   progress is reported when the process receives a ``SIGUSR1`` or
   ``SIGINFO`` signal.
 
 .. option:: -q
 
   Quiet mode - do not print any output (except errors). There's no progress bar
   in case both *-q* and *-p* options are used.
 
 .. option:: -S SIZE
 
   Indicates the consecutive number of bytes that must contain only zeros
   for ``qemu-img`` to create a sparse image during conversion. This value is
   rounded down to the nearest 512 bytes. You may use the common size suffixes
   like ``k`` for kilobytes.
 
 .. option:: -t CACHE
 
   Specifies the cache mode that should be used with the (destination) file. See
   the documentation of the emulator's ``-drive cache=...`` option for allowed
   values.
 
 .. option:: -T SRC_CACHE
 
   Specifies the cache mode that should be used with the source file(s). See
   the documentation of the emulator's ``-drive cache=...`` option for allowed
   values.
 
 Parameters to compare subcommand:
 
 .. program:: qemu-img-compare
 
 .. option:: -f
 
   First image format
 
 .. option:: -F
 
   Second image format
 
 .. option:: -s
 
   Strict mode - fail on different image size or sector allocation
 
 Parameters to convert subcommand:
 
 .. program:: qemu-img-convert
 
 .. option:: --bitmaps
 
   Additionally copy all persistent bitmaps from the top layer of the source
 
 .. option:: -n
 
   Skip the creation of the target volume
 
 .. option:: -m
 
   Number of parallel coroutines for the convert process
 
 .. option:: -W
 
   Allow out-of-order writes to the destination. This option improves performance,
   but is only recommended for preallocated devices like host devices or other
   raw block devices.
 
 .. option:: -C
 
   Try to use copy offloading to move data from source image to target. This may
   improve performance if the data is remote, such as with NFS or iSCSI backends,
   but will not automatically sparsify zero sectors, and may result in a fully
   allocated target image depending on the host support for getting allocation
   information.
 
 .. option:: -r
 
    Rate limit for the convert process
 
 .. option:: --salvage
 
   Try to ignore I/O errors when reading.  Unless in quiet mode (``-q``), errors
   will still be printed.  Areas that cannot be read from the source will be
   treated as containing only zeroes.
 
 .. option:: --target-is-zero
 
   Assume that reading the destination image will always return
   zeros. This parameter is mutually exclusive with a destination image
   that has a backing file. It is required to also use the ``-n``
   parameter to skip image creation.
 
 Parameters to dd subcommand:
 
 .. program:: qemu-img-dd
 
 .. option:: bs=BLOCK_SIZE
 
   Defines the block size
 
 .. option:: count=BLOCKS
 
   Sets the number of input blocks to copy
 
 .. option:: if=INPUT
 
   Sets the input file
 
 .. option:: of=OUTPUT
 
   Sets the output file
 
 .. option:: skip=BLOCKS
 
   Sets the number of input blocks to skip
 
 Parameters to snapshot subcommand:
 
 .. program:: qemu-img-snapshot
 
 .. option:: snapshot
 
   Is the name of the snapshot to create, apply or delete
 
 .. option:: -a
 
   Applies a snapshot (revert disk to saved state)
 
 .. option:: -c
 
   Creates a snapshot
 
 .. option:: -d
 
   Deletes a snapshot
 
 .. option:: -l
 
   Lists all snapshots in the given image
 
 Command description:
 
 .. program:: qemu-img-commands
 
 .. option:: amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE] [--force] -o OPTIONS FILENAME
 
   Amends the image format specific *OPTIONS* for the image file
   *FILENAME*. Not all file formats support this operation.
 
   The set of options that can be amended are dependent on the image
   format, but note that amending the backing chain relationship should
   instead be performed with ``qemu-img rebase``.
 
   --force allows some unsafe operations. Currently for -f luks, it allows to
   erase the last encryption key, and to overwrite an active encryption key.
 
 .. option:: bench [-c COUNT] [-d DEPTH] [-f FMT] [--flush-interval=FLUSH_INTERVAL] [-i AIO] [-n] [--no-drain] [-o OFFSET] [--pattern=PATTERN] [-q] [-s BUFFER_SIZE] [-S STEP_SIZE] [-t CACHE] [-w] [-U] FILENAME
 
   Run a simple sequential I/O benchmark on the specified image. If ``-w`` is
   specified, a write test is performed, otherwise a read test is performed.
 
   A total number of *COUNT* I/O requests is performed, each *BUFFER_SIZE*
   bytes in size, and with *DEPTH* requests in parallel. The first request
   starts at the position given by *OFFSET*, each following request increases
   the current position by *STEP_SIZE*. If *STEP_SIZE* is not given,
   *BUFFER_SIZE* is used for its value.
 
   If *FLUSH_INTERVAL* is specified for a write test, the request queue is
   drained and a flush is issued before new writes are made whenever the number of
   remaining requests is a multiple of *FLUSH_INTERVAL*. If additionally
   ``--no-drain`` is specified, a flush is issued without draining the request
   queue first.
 
   if ``-i`` is specified, *AIO* option can be used to specify different
   AIO backends: ``threads``, ``native`` or ``io_uring``.
 
   If ``-n`` is specified, the native AIO backend is used if possible. On
   Linux, this option only works if ``-t none`` or ``-t directsync`` is
   specified as well.
 
   For write tests, by default a buffer filled with zeros is written. This can be
   overridden with a pattern byte specified by *PATTERN*.
 
 .. option:: bitmap (--merge SOURCE | --add | --remove | --clear | --enable | --disable)... [-b SOURCE_FILE [-F SOURCE_FMT]] [-g GRANULARITY] [--object OBJECTDEF] [--image-opts | -f FMT] FILENAME BITMAP
 
   Perform one or more modifications of the persistent bitmap *BITMAP*
   in the disk image *FILENAME*.  The various modifications are:
 
   ``--add`` to create *BITMAP*, enabled to record future edits.
 
   ``--remove`` to remove *BITMAP*.
 
   ``--clear`` to clear *BITMAP*.
 
   ``--enable`` to change *BITMAP* to start recording future edits.
 
   ``--disable`` to change *BITMAP* to stop recording future edits.
 
   ``--merge`` to merge the contents of the *SOURCE* bitmap into *BITMAP*.
 
   Additional options include ``-g`` which sets a non-default
   *GRANULARITY* for ``--add``, and ``-b`` and ``-F`` which select an
   alternative source file for all *SOURCE* bitmaps used by
   ``--merge``.
 
   To see what bitmaps are present in an image, use ``qemu-img info``.
 
 .. option:: check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT] [-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME
 
   Perform a consistency check on the disk image *FILENAME*. The command can
   output in the format *OFMT* which is either ``human`` or ``json``.
   The JSON output is an object of QAPI type ``ImageCheck``.
 
   If ``-r`` is specified, qemu-img tries to repair any inconsistencies found
   during the check. ``-r leaks`` repairs only cluster leaks, whereas
   ``-r all`` fixes all kinds of errors, with a higher risk of choosing the
   wrong fix or hiding corruption that has already occurred.
 
   Only the formats ``qcow2``, ``qed``, ``parallels``, ``vhdx``, ``vmdk`` and
   ``vdi`` support consistency checks.
 
   In case the image does not have any inconsistencies, check exits with ``0``.
   Other exit codes indicate the kind of inconsistency found or if another error
   occurred. The following table summarizes all exit codes of the check subcommand:
 
   0
     Check completed, the image is (now) consistent
   1
     Check not completed because of internal errors
   2
     Check completed, image is corrupted
   3
     Check completed, image has leaked clusters, but is not corrupted
   63
     Checks are not supported by the image format
 
   If ``-r`` is specified, exit codes representing the image state refer to the
   state after (the attempt at) repairing it. That is, a successful ``-r all``
   will yield the exit code 0, independently of the image state before.
 
 .. option:: commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b BASE] [-r RATE_LIMIT] [-d] [-p] FILENAME
 
   Commit the changes recorded in *FILENAME* in its base image or backing file.
   If the backing file is smaller than the snapshot, then the backing file will be
   resized to be the same size as the snapshot.  If the snapshot is smaller than
   the backing file, the backing file will not be truncated.  If you want the
   backing file to match the size of the smaller snapshot, you can safely truncate
   it yourself once the commit operation successfully completes.
 
   The image *FILENAME* is emptied after the operation has succeeded. If you do
   not need *FILENAME* afterwards and intend to drop it, you may skip emptying
   *FILENAME* by specifying the ``-d`` flag.
 
   If the backing chain of the given image file *FILENAME* has more than one
   layer, the backing file into which the changes will be committed may be
   specified as *BASE* (which has to be part of *FILENAME*'s backing
   chain). If *BASE* is not specified, the immediate backing file of the top
   image (which is *FILENAME*) will be used. Note that after a commit operation
   all images between *BASE* and the top image will be invalid and may return
   garbage data when read. For this reason, ``-b`` implies ``-d`` (so that
   the top image stays valid).
 
   The rate limit for the commit process is specified by ``-r``.
 
 .. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 FILENAME2
 
   Check if two images have the same content. You can compare images with
   different format or settings.
 
   The format is probed unless you specify it by ``-f`` (used for
   *FILENAME1*) and/or ``-F`` (used for *FILENAME2*) option.
 
   By default, images with different size are considered identical if the larger
   image contains only unallocated and/or zeroed sectors in the area after the end
   of the other image. In addition, if any sector is not allocated in one image
   and contains only zero bytes in the second one, it is evaluated as equal. You
   can use Strict mode by specifying the ``-s`` option. When compare runs in
   Strict mode, it fails in case image size differs or a sector is allocated in
   one image and is not allocated in the second one.
 
   By default, compare prints out a result message. This message displays
   information that both images are same or the position of the first different
   byte. In addition, result message can report different image size in case
   Strict mode is used.
 
   Compare exits with ``0`` in case the images are equal and with ``1``
   in case the images differ. Other exit codes mean an error occurred during
   execution and standard error output should contain an error message.
   The following table sumarizes all exit codes of the compare subcommand:
 
   0
     Images are identical (or requested help was printed)
   1
     Images differ
   2
     Error on opening an image
   3
     Error on checking a sector allocation
   4
     Error on reading data
 
 .. option:: convert [--object OBJECTDEF] [--image-opts] [--target-image-opts] [--target-is-zero] [--bitmaps [--skip-broken-bitmaps]] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-O OUTPUT_FMT] [-B BACKING_FILE [-F BACKING_FMT]] [-o OPTIONS] [-l SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-r RATE_LIMIT] [-m NUM_COROUTINES] [-W] FILENAME [FILENAME2 [...]] OUTPUT_FILENAME
 
   Convert the disk image *FILENAME* or a snapshot *SNAPSHOT_PARAM*
   to disk image *OUTPUT_FILENAME* using format *OUTPUT_FMT*. It can
   be optionally compressed (``-c`` option) or use any format specific
   options like encryption (``-o`` option).
 
   Only the formats ``qcow`` and ``qcow2`` support compression. The
   compression is read-only. It means that if a compressed sector is
   rewritten, then it is rewritten as uncompressed data.
 
   Image conversion is also useful to get smaller image when using a
   growable format such as ``qcow``: the empty sectors are detected and
   suppressed from the destination image.
 
   *SPARSE_SIZE* indicates the consecutive number of bytes (defaults to 4k)
   that must contain only zeros for ``qemu-img`` to create a sparse image during
   conversion. If *SPARSE_SIZE* is 0, the source will not be scanned for
   unallocated or zero sectors, and the destination image will always be
   fully allocated.
 
   You can use the *BACKING_FILE* option to force the output image to be
   created as a copy on write image of the specified base image; the
   *BACKING_FILE* should have the same content as the input's base image,
   however the path, image format (as given by *BACKING_FMT*), etc may differ.
 
   If a relative path name is given, the backing file is looked up relative to
   the directory containing *OUTPUT_FILENAME*.
 
   If the ``-n`` option is specified, the target volume creation will be
   skipped. This is useful for formats such as ``rbd`` if the target
   volume has already been created with site specific options that cannot
   be supplied through ``qemu-img``.
 
   Out of order writes can be enabled with ``-W`` to improve performance.
   This is only recommended for preallocated devices like host devices or other
   raw block devices. Out of order write does not work in combination with
   creating compressed images.
 
   *NUM_COROUTINES* specifies how many coroutines work in parallel during
   the convert process (defaults to 8).
 
   Use of ``--bitmaps`` requests that any persistent bitmaps present in
   the original are also copied to the destination.  If any bitmap is
   inconsistent in the source, the conversion will fail unless
   ``--skip-broken-bitmaps`` is also specified to copy only the
   consistent bitmaps.
 
 .. option:: create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE [-F BACKING_FMT]] [-u] [-o OPTIONS] FILENAME [SIZE]
 
   Create the new disk image *FILENAME* of size *SIZE* and format
   *FMT*. Depending on the file format, you can add one or more *OPTIONS*
   that enable additional features of this format.
 
   If the option *BACKING_FILE* is specified, then the image will record
   only the differences from *BACKING_FILE*. No size needs to be specified in
   this case. *BACKING_FILE* will never be modified unless you use the
   ``commit`` monitor command (or ``qemu-img commit``).
 
   If a relative path name is given, the backing file is looked up relative to
   the directory containing *FILENAME*.
 
   Note that a given backing file will be opened to check that it is valid. Use
   the ``-u`` option to enable unsafe backing file mode, which means that the
   image will be created even if the associated backing file cannot be opened. A
   matching backing file must be created or additional options be used to make the
   backing file specification valid when you want to use an image created this
   way.
 
   The size can also be specified using the *SIZE* option with ``-o``,
   it doesn't need to be specified separately in this case.
 
 
 .. option:: dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE] [count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT
 
   dd copies from *INPUT* file to *OUTPUT* file converting it from
   *FMT* format to *OUTPUT_FMT* format.
 
   The data is by default read and written using blocks of 512 bytes but can be
   modified by specifying *BLOCK_SIZE*. If count=\ *BLOCKS* is specified
   dd will stop reading input after reading *BLOCKS* input blocks.
 
   The size syntax is similar to :manpage:`dd(1)`'s size syntax.
 
 .. option:: info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [--backing-chain] [-U] FILENAME
 
   Give information about the disk image *FILENAME*. Use it in
   particular to know the size reserved on disk which can be different
   from the displayed size. If VM snapshots are stored in the disk image,
   they are displayed too.
 
   If a disk image has a backing file chain, information about each disk image in
   the chain can be recursively enumerated by using the option ``--backing-chain``.
 
   For instance, if you have an image chain like:
 
   ::
 
     base.qcow2 <- snap1.qcow2 <- snap2.qcow2
 
   To enumerate information about each disk image in the above chain, starting from top to base, do:
 
   ::
 
     qemu-img info --backing-chain snap2.qcow2
 
   The command can output in the format *OFMT* which is either ``human`` or
   ``json``.  The JSON output is an object of QAPI type ``ImageInfo``; with
   ``--backing-chain``, it is an array of ``ImageInfo`` objects.
 
   ``--output=human`` reports the following information (for every image in the
   chain):
 
   *image*
     The image file name
 
   *file format*
     The image format
 
   *virtual size*
     The size of the guest disk
 
   *disk size*
     How much space the image file occupies on the host file system (may be
     shown as 0 if this information is unavailable, e.g. because there is no
     file system)
 
   *cluster_size*
     Cluster size of the image format, if applicable
 
   *encrypted*
     Whether the image is encrypted (only present if so)
 
   *cleanly shut down*
     This is shown as ``no`` if the image is dirty and will have to be
     auto-repaired the next time it is opened in qemu.
 
   *backing file*
     The backing file name, if present
 
   *backing file format*
     The format of the backing file, if the image enforces it
 
   *Snapshot list*
     A list of all internal snapshots
 
   *Format specific information*
     Further information whose structure depends on the image format.  This
     section is a textual representation of the respective
     ``ImageInfoSpecific*`` QAPI object (e.g. ``ImageInfoSpecificQCow2``
     for qcow2 images).
 
 .. option:: map [--object OBJECTDEF] [--image-opts] [-f FMT] [--start-offset=OFFSET] [--max-length=LEN] [--output=OFMT] [-U] FILENAME
 
   Dump the metadata of image *FILENAME* and its backing file chain.
   In particular, this commands dumps the allocation state of every sector
   of *FILENAME*, together with the topmost file that allocates it in
   the backing file chain.
 
   Two option formats are possible.  The default format (``human``)
   only dumps known-nonzero areas of the file.  Known-zero parts of the
   file are omitted altogether, and likewise for parts that are not allocated
   throughout the chain.  ``qemu-img`` output will identify a file
   from where the data can be read, and the offset in the file.  Each line
   will include four fields, the first three of which are hexadecimal
   numbers.  For example the first line of:
 
   ::
 
     Offset          Length          Mapped to       File
     0               0x20000         0x50000         /tmp/overlay.qcow2
     0x100000        0x10000         0x95380000      /tmp/backing.qcow2
 
   means that 0x20000 (131072) bytes starting at offset 0 in the image are
   available in /tmp/overlay.qcow2 (opened in ``raw`` format) starting
   at offset 0x50000 (327680).  Data that is compressed, encrypted, or
   otherwise not available in raw format will cause an error if ``human``
   format is in use.  Note that file names can include newlines, thus it is
   not safe to parse this output format in scripts.
 
   The alternative format ``json`` will return an array of dictionaries
   in JSON format.  It will include similar information in
   the ``start``, ``length``, ``offset`` fields;
   it will also include other more specific information:
 
   - boolean field ``data``: true if the sectors contain actual data,
     false if the sectors are either unallocated or stored as optimized
     all-zero clusters
   - boolean field ``zero``: true if the data is known to read as zero
   - boolean field ``present``: true if the data belongs to the backing
     chain, false if rebasing the backing chain onto a deeper file
     would pick up data from the deeper file;
   - integer field ``depth``: the depth within the backing chain at
     which the data was resolved; for example, a depth of 2 refers to
     the backing file of the backing file of *FILENAME*.
 
   In JSON format, the ``offset`` field is optional; it is absent in
   cases where ``human`` format would omit the entry or exit with an error.
   If ``data`` is false and the ``offset`` field is present, the
   corresponding sectors in the file are not yet in use, but they are
   preallocated.
 
   For more information, consult ``include/block/block.h`` in QEMU's
   source code.
 
 .. option:: measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--object OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME]
 
   Calculate the file size required for a new image.  This information
   can be used to size logical volumes or SAN LUNs appropriately for
   the image that will be placed in them.  The values reported are
   guaranteed to be large enough to fit the image.  The command can
   output in the format *OFMT* which is either ``human`` or ``json``.
   The JSON output is an object of QAPI type ``BlockMeasureInfo``.
 
   If the size *N* is given then act as if creating a new empty image file
   using ``qemu-img create``.  If *FILENAME* is given then act as if
   converting an existing image file using ``qemu-img convert``.  The format
   of the new file is given by *OUTPUT_FMT* while the format of an existing
   file is given by *FMT*.
 
   A snapshot in an existing image can be specified using *SNAPSHOT_PARAM*.
 
   The following fields are reported:
 
   ::
 
     required size: 524288
     fully allocated size: 1074069504
     bitmaps size: 0
 
   The ``required size`` is the file size of the new image.  It may be smaller
   than the virtual disk size if the image format supports compact representation.
 
   The ``fully allocated size`` is the file size of the new image once data has
   been written to all sectors.  This is the maximum size that the image file can
   occupy with the exception of internal snapshots, dirty bitmaps, vmstate data,
   and other advanced image format features.
 
   The ``bitmaps size`` is the additional size required in order to
   copy bitmaps from a source image in addition to the guest-visible
   data; the line is omitted if either source or destination lacks
   bitmap support, or 0 if bitmaps are supported but there is nothing
   to copy.
 
 .. option:: snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAPSHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME
 
   List, apply, create or delete snapshots in image *FILENAME*.
 
 .. option:: rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-p] [-u] -b BACKING_FILE [-F BACKING_FMT] FILENAME
 
   Changes the backing file of an image. Only the formats ``qcow2`` and
   ``qed`` support changing the backing file.
 
   The backing file is changed to *BACKING_FILE* and (if the image format of
   *FILENAME* supports this) the backing file format is changed to
   *BACKING_FMT*. If *BACKING_FILE* is specified as "" (the empty
   string), then the image is rebased onto no backing file (i.e. it will exist
   independently of any backing file).
 
   If a relative path name is given, the backing file is looked up relative to
   the directory containing *FILENAME*.
 
   *CACHE* specifies the cache mode to be used for *FILENAME*, whereas
   *SRC_CACHE* specifies the cache mode for reading backing files.
 
   There are two different modes in which ``rebase`` can operate:
 
   Safe mode
     This is the default mode and performs a real rebase operation. The
     new backing file may differ from the old one and ``qemu-img rebase``
     will take care of keeping the guest-visible content of *FILENAME*
     unchanged.
 
     In order to achieve this, any clusters that differ between
     *BACKING_FILE* and the old backing file of *FILENAME* are merged
     into *FILENAME* before actually changing the backing file.
 
     Note that the safe mode is an expensive operation, comparable to
     converting an image. It only works if the old backing file still
     exists.
 
   Unsafe mode
     ``qemu-img`` uses the unsafe mode if ``-u`` is specified. In this
     mode, only the backing file name and format of *FILENAME* is changed
     without any checks on the file contents. The user must take care of
     specifying the correct new backing file, or the guest-visible
     content of the image will be corrupted.
 
     This mode is useful for renaming or moving the backing file to
     somewhere else.  It can be used without an accessible old backing
     file, i.e. you can use it to fix an image whose backing file has
     already been moved/renamed.
 
   You can use ``rebase`` to perform a "diff" operation on two
   disk images.  This can be useful when you have copied or cloned
   a guest, and you want to get back to a thin image on top of a
   template or base image.
 
   Say that ``base.img`` has been cloned as ``modified.img`` by
   copying it, and that the ``modified.img`` guest has run so there
   are now some changes compared to ``base.img``.  To construct a thin
   image called ``diff.qcow2`` that contains just the differences, do:
 
   ::
 
     qemu-img create -f qcow2 -b modified.img diff.qcow2
     qemu-img rebase -b base.img diff.qcow2
 
   At this point, ``modified.img`` can be discarded, since
   ``base.img + diff.qcow2`` contains the same information.
 
 .. option:: resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--preallocation=PREALLOC] [-q] [--shrink] FILENAME [+ | -]SIZE
 
   Change the disk image as if it had been created with *SIZE*.
 
   Before using this command to shrink a disk image, you MUST use file system and
   partitioning tools inside the VM to reduce allocated file systems and partition
   sizes accordingly.  Failure to do so will result in data loss!
 
   When shrinking images, the ``--shrink`` option must be given. This informs
   ``qemu-img`` that the user acknowledges all loss of data beyond the truncated
   image's end.
 
   After using this command to grow a disk image, you must use file system and
   partitioning tools inside the VM to actually begin using the new space on the
   device.
 
   When growing an image, the ``--preallocation`` option may be used to specify
   how the additional image area should be allocated on the host.  See the format
   description in the :ref:`notes` section which values are allowed.  Using this
   option may result in slightly more data being allocated than necessary.
 
 .. _notes:
 
 Notes
 -----
 
 Supported image file formats:
 
 ``raw``
 
   Raw disk image format (default). This format has the advantage of
   being simple and easily exportable to all other emulators. If your
   file system supports *holes* (for example in ext2 or ext3 on
   Linux or NTFS on Windows), then only the written sectors will reserve
   space. Use ``qemu-img info`` to know the real size used by the
   image or ``ls -ls`` on Unix/Linux.
 
   Supported options:
 
   ``preallocation``
     Preallocation mode (allowed values: ``off``, ``falloc``,
     ``full``).  ``falloc`` mode preallocates space for image by
     calling ``posix_fallocate()``.  ``full`` mode preallocates space
     for image by writing data to underlying storage.  This data may or
     may not be zero, depending on the storage location.
 
 ``qcow2``
 
   QEMU image format, the most versatile format. Use it to have smaller
   images (useful if your filesystem does not supports holes, for example
   on Windows), optional AES encryption, zlib based compression and
   support of multiple VM snapshots.
 
   Supported options:
 
   ``compat``
     Determines the qcow2 version to use. ``compat=0.10`` uses the
     traditional image format that can be read by any QEMU since 0.10.
     ``compat=1.1`` enables image format extensions that only QEMU 1.1 and
     newer understand (this is the default). Amongst others, this includes zero
     clusters, which allow efficient copy-on-read for sparse images.
 
   ``backing_file``
     File name of a base image (see ``create`` subcommand)
 
   ``backing_fmt``
     Image format of the base image
 
   ``encryption``
     If this option is set to ``on``, the image is encrypted with
     128-bit AES-CBC.
 
     The use of encryption in qcow and qcow2 images is considered to be
     flawed by modern cryptography standards, suffering from a number
     of design problems:
 
     - The AES-CBC cipher is used with predictable initialization
       vectors based on the sector number. This makes it vulnerable to
       chosen plaintext attacks which can reveal the existence of
       encrypted data.
 
     - The user passphrase is directly used as the encryption key. A
       poorly chosen or short passphrase will compromise the security
       of the encryption.
 
     - In the event of the passphrase being compromised there is no way
       to change the passphrase to protect data in any qcow images. The
       files must be cloned, using a different encryption passphrase in
       the new file. The original file must then be securely erased
       using a program like shred, though even this is ineffective with
       many modern storage technologies.
 
     - Initialization vectors used to encrypt sectors are based on the
       guest virtual sector number, instead of the host physical
       sector. When a disk image has multiple internal snapshots this
       means that data in multiple physical sectors is encrypted with
       the same initialization vector. With the CBC mode, this opens
       the possibility of watermarking attacks if the attack can
       collect multiple sectors encrypted with the same IV and some
       predictable data. Having multiple qcow2 images with the same
       passphrase also exposes this weakness since the passphrase is
       directly used as the key.
 
     Use of qcow / qcow2 encryption is thus strongly discouraged. Users are
     recommended to use an alternative encryption technology such as the
     Linux dm-crypt / LUKS system.
 
   ``cluster_size``
     Changes the qcow2 cluster size (must be between 512 and
     2M). Smaller cluster sizes can improve the image file size whereas
     larger cluster sizes generally provide better performance.
 
   ``preallocation``
     Preallocation mode (allowed values: ``off``, ``metadata``,
     ``falloc``, ``full``). An image with preallocated metadata is
     initially larger but can improve performance when the image needs
     to grow. ``falloc`` and ``full`` preallocations are like the same
     options of ``raw`` format, but sets up metadata also.
 
   ``lazy_refcounts``
     If this option is set to ``on``, reference count updates are
     postponed with the goal of avoiding metadata I/O and improving
     performance. This is particularly interesting with
     ``cache=writethrough`` which doesn't batch metadata
     updates. The tradeoff is that after a host crash, the reference
     count tables must be rebuilt, i.e. on the next open an (automatic)
     ``qemu-img check -r all`` is required, which may take some time.
 
     This option can only be enabled if ``compat=1.1`` is specified.
 
   ``nocow``
     If this option is set to ``on``, it will turn off COW of the file. It's
     only valid on btrfs, no effect on other file systems.
 
     Btrfs has low performance when hosting a VM image file, even more
     when the guest on the VM also using btrfs as file system. Turning
     off COW is a way to mitigate this bad performance. Generally there
     are two ways to turn off COW on btrfs:
 
     - Disable it by mounting with nodatacow, then all newly created files
       will be NOCOW
     - For an empty file, add the NOCOW file attribute. That's what this
       option does.
 
     Note: this option is only valid to new or empty files. If there is
     an existing file which is COW and has data blocks already, it
     couldn't be changed to NOCOW by setting ``nocow=on``. One can
     issue ``lsattr filename`` to check if the NOCOW flag is set or not
     (Capital 'C' is NOCOW flag).
 
   ``data_file``
     Filename where all guest data will be stored. If this option is used,
     the qcow2 file will only contain the image's metadata.
 
     Note: Data loss will occur if the given filename already exists when
     using this option with ``qemu-img create`` since ``qemu-img`` will create
     the data file anew, overwriting the file's original contents. To simply
     update the reference to point to the given pre-existing file, use
     ``qemu-img amend``.
 
   ``data_file_raw``
     If this option is set to ``on``, QEMU will always keep the external data
     file consistent as a standalone read-only raw image.
 
     It does this by forwarding all write accesses to the qcow2 file through to
     the raw data file, including their offsets. Therefore, data that is visible
     on the qcow2 node (i.e., to the guest) at some offset is visible at the same
     offset in the raw data file. This results in a read-only raw image. Writes
     that bypass the qcow2 metadata may corrupt the qcow2 metadata because the
     out-of-band writes may result in the metadata falling out of sync with the
     raw image.
 
     If this option is ``off``, QEMU will use the data file to store data in an
     arbitrary manner. The file’s content will not make sense without the
     accompanying qcow2 metadata. Where data is written will have no relation to
     its offset as seen by the guest, and some writes (specifically zero writes)
     may not be forwarded to the data file at all, but will only be handled by
     modifying qcow2 metadata.
 
     This option can only be enabled if ``data_file`` is set.
 
 ``Other``
 
   QEMU also supports various other image file formats for
   compatibility with older QEMU versions or other hypervisors,
   including VMDK, VDI, VHD (vpc), VHDX, qcow1 and QED. For a full list
   of supported formats see ``qemu-img --help``.  For a more detailed
   description of these formats, see the QEMU block drivers reference
   documentation.
 
   The main purpose of the block drivers for these formats is image
   conversion.  For running VMs, it is recommended to convert the disk
   images to either raw or qcow2 in order to achieve good performance.