99b1e64688
This switches qemu-img from a QemuOpts-based parser for --object to
user_creatable_process_cmdline() which uses a keyval parser and enforces
the QAPI schema.
Apart from being a cleanup, this makes non-scalar properties accessible.
As a side effect, fix wrong exit codes in the object parsing error path
of 'qemu-img compare'. This was broken in commit 334c43e2c3 because
&error_fatal exits with an exit code of 1, while it should have been 2.
Document that exit code 0 is also returned when just requested help was
printed instead of comparing images. This is preexisting behaviour that
isn't changed by this patch, though another instance of it is added with
'--object help'.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Acked-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
881 lines
34 KiB
ReStructuredText
881 lines
34 KiB
ReStructuredText
QEMU disk image utility
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=======================
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Synopsis
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--------
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**qemu-img** [*standard options*] *command* [*command options*]
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Description
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-----------
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qemu-img allows you to create, convert and modify images offline. It can handle
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all image formats supported by QEMU.
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**Warning:** Never use qemu-img to modify images in use by a running virtual
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machine or any other process; this may destroy the image. Also, be aware that
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querying an image that is being modified by another process may encounter
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inconsistent state.
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Options
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-------
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.. program:: qemu-img
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Standard options:
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.. option:: -h, --help
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Display this help and exit
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.. option:: -V, --version
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Display version information and exit
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.. option:: -T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE]
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.. include:: ../qemu-option-trace.rst.inc
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The following commands are supported:
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.. hxtool-doc:: qemu-img-cmds.hx
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Command parameters:
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*FILENAME* is a disk image filename.
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*FMT* is the disk image format. It is guessed automatically in most
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cases. See below for a description of the supported disk formats.
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*SIZE* is the disk image size in bytes. Optional suffixes ``k`` or
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``K`` (kilobyte, 1024) ``M`` (megabyte, 1024k) and ``G`` (gigabyte,
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1024M) and T (terabyte, 1024G) are supported. ``b`` is ignored.
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*OUTPUT_FILENAME* is the destination disk image filename.
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*OUTPUT_FMT* is the destination format.
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*OPTIONS* is a comma separated list of format specific options in a
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name=value format. Use ``-o ?`` for an overview of the options supported
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by the used format or see the format descriptions below for details.
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*SNAPSHOT_PARAM* is param used for internal snapshot, format is
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'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'.
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..
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Note the use of a new 'program'; otherwise Sphinx complains about
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the -h option appearing both in the above option list and this one.
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.. program:: qemu-img-common-opts
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.. option:: --object OBJECTDEF
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is a QEMU user creatable object definition. See the :manpage:`qemu(1)`
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manual page for a description of the object properties. The most common
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object type is a ``secret``, which is used to supply passwords and/or
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encryption keys.
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.. option:: --image-opts
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Indicates that the source *FILENAME* parameter is to be interpreted as a
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full option string, not a plain filename. This parameter is mutually
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exclusive with the *-f* parameter.
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.. option:: --target-image-opts
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Indicates that the OUTPUT_FILENAME parameter(s) are to be interpreted as
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a full option string, not a plain filename. This parameter is mutually
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exclusive with the *-O* parameters. It is currently required to also use
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the *-n* parameter to skip image creation. This restriction may be relaxed
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in a future release.
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.. option:: --force-share (-U)
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If specified, ``qemu-img`` will open the image in shared mode, allowing
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other QEMU processes to open it in write mode. For example, this can be used to
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get the image information (with 'info' subcommand) when the image is used by a
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running guest. Note that this could produce inconsistent results because of
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concurrent metadata changes, etc. This option is only allowed when opening
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images in read-only mode.
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.. option:: --backing-chain
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Will enumerate information about backing files in a disk image chain. Refer
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below for further description.
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.. option:: -c
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Indicates that target image must be compressed (qcow format only).
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.. option:: -h
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With or without a command, shows help and lists the supported formats.
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.. option:: -p
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Display progress bar (compare, convert and rebase commands only).
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If the *-p* option is not used for a command that supports it, the
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progress is reported when the process receives a ``SIGUSR1`` or
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``SIGINFO`` signal.
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.. option:: -q
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Quiet mode - do not print any output (except errors). There's no progress bar
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in case both *-q* and *-p* options are used.
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.. option:: -S SIZE
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Indicates the consecutive number of bytes that must contain only zeros
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for qemu-img to create a sparse image during conversion. This value is rounded
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down to the nearest 512 bytes. You may use the common size suffixes like
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``k`` for kilobytes.
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.. option:: -t CACHE
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Specifies the cache mode that should be used with the (destination) file. See
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the documentation of the emulator's ``-drive cache=...`` option for allowed
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values.
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.. option:: -T SRC_CACHE
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Specifies the cache mode that should be used with the source file(s). See
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the documentation of the emulator's ``-drive cache=...`` option for allowed
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values.
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Parameters to compare subcommand:
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.. program:: qemu-img-compare
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.. option:: -f
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First image format
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.. option:: -F
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Second image format
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.. option:: -s
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Strict mode - fail on different image size or sector allocation
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Parameters to convert subcommand:
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.. program:: qemu-img-convert
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.. option:: --bitmaps
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Additionally copy all persistent bitmaps from the top layer of the source
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.. option:: -n
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Skip the creation of the target volume
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.. option:: -m
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Number of parallel coroutines for the convert process
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.. option:: -W
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Allow out-of-order writes to the destination. This option improves performance,
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but is only recommended for preallocated devices like host devices or other
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raw block devices.
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.. option:: -C
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Try to use copy offloading to move data from source image to target. This may
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improve performance if the data is remote, such as with NFS or iSCSI backends,
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but will not automatically sparsify zero sectors, and may result in a fully
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allocated target image depending on the host support for getting allocation
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information.
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.. option:: -r
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Rate limit for the convert process
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.. option:: --salvage
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Try to ignore I/O errors when reading. Unless in quiet mode (``-q``), errors
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will still be printed. Areas that cannot be read from the source will be
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treated as containing only zeroes.
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.. option:: --target-is-zero
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Assume that reading the destination image will always return
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zeros. This parameter is mutually exclusive with a destination image
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that has a backing file. It is required to also use the ``-n``
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parameter to skip image creation.
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Parameters to dd subcommand:
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.. program:: qemu-img-dd
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.. option:: bs=BLOCK_SIZE
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Defines the block size
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.. option:: count=BLOCKS
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Sets the number of input blocks to copy
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.. option:: if=INPUT
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Sets the input file
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.. option:: of=OUTPUT
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Sets the output file
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.. option:: skip=BLOCKS
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Sets the number of input blocks to skip
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Parameters to snapshot subcommand:
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.. program:: qemu-img-snapshot
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.. option:: snapshot
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Is the name of the snapshot to create, apply or delete
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.. option:: -a
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Applies a snapshot (revert disk to saved state)
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.. option:: -c
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Creates a snapshot
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.. option:: -d
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Deletes a snapshot
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.. option:: -l
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Lists all snapshots in the given image
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Command description:
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.. program:: qemu-img-commands
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.. option:: amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE] [--force] -o OPTIONS FILENAME
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Amends the image format specific *OPTIONS* for the image file
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*FILENAME*. Not all file formats support this operation.
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The set of options that can be amended are dependent on the image
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format, but note that amending the backing chain relationship should
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instead be performed with ``qemu-img rebase``.
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--force allows some unsafe operations. Currently for -f luks, it allows to
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erase the last encryption key, and to overwrite an active encryption key.
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.. 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
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Run a simple sequential I/O benchmark on the specified image. If ``-w`` is
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specified, a write test is performed, otherwise a read test is performed.
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A total number of *COUNT* I/O requests is performed, each *BUFFER_SIZE*
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bytes in size, and with *DEPTH* requests in parallel. The first request
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starts at the position given by *OFFSET*, each following request increases
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the current position by *STEP_SIZE*. If *STEP_SIZE* is not given,
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*BUFFER_SIZE* is used for its value.
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If *FLUSH_INTERVAL* is specified for a write test, the request queue is
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drained and a flush is issued before new writes are made whenever the number of
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remaining requests is a multiple of *FLUSH_INTERVAL*. If additionally
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``--no-drain`` is specified, a flush is issued without draining the request
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queue first.
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if ``-i`` is specified, *AIO* option can be used to specify different
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AIO backends: ``threads``, ``native`` or ``io_uring``.
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If ``-n`` is specified, the native AIO backend is used if possible. On
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Linux, this option only works if ``-t none`` or ``-t directsync`` is
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specified as well.
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For write tests, by default a buffer filled with zeros is written. This can be
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overridden with a pattern byte specified by *PATTERN*.
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.. 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
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Perform one or more modifications of the persistent bitmap *BITMAP*
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in the disk image *FILENAME*. The various modifications are:
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``--add`` to create *BITMAP*, enabled to record future edits.
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``--remove`` to remove *BITMAP*.
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``--clear`` to clear *BITMAP*.
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``--enable`` to change *BITMAP* to start recording future edits.
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``--disable`` to change *BITMAP* to stop recording future edits.
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``--merge`` to merge the contents of the *SOURCE* bitmap into *BITMAP*.
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Additional options include ``-g`` which sets a non-default
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*GRANULARITY* for ``--add``, and ``-b`` and ``-F`` which select an
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alternative source file for all *SOURCE* bitmaps used by
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``--merge``.
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To see what bitmaps are present in an image, use ``qemu-img info``.
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.. option:: check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT] [-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME
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Perform a consistency check on the disk image *FILENAME*. The command can
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output in the format *OFMT* which is either ``human`` or ``json``.
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The JSON output is an object of QAPI type ``ImageCheck``.
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If ``-r`` is specified, qemu-img tries to repair any inconsistencies found
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during the check. ``-r leaks`` repairs only cluster leaks, whereas
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``-r all`` fixes all kinds of errors, with a higher risk of choosing the
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wrong fix or hiding corruption that has already occurred.
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Only the formats ``qcow2``, ``qed`` and ``vdi`` support
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consistency checks.
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In case the image does not have any inconsistencies, check exits with ``0``.
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Other exit codes indicate the kind of inconsistency found or if another error
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occurred. The following table summarizes all exit codes of the check subcommand:
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0
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Check completed, the image is (now) consistent
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1
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Check not completed because of internal errors
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2
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Check completed, image is corrupted
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3
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Check completed, image has leaked clusters, but is not corrupted
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63
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Checks are not supported by the image format
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If ``-r`` is specified, exit codes representing the image state refer to the
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state after (the attempt at) repairing it. That is, a successful ``-r all``
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will yield the exit code 0, independently of the image state before.
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.. option:: commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b BASE] [-r RATE_LIMIT] [-d] [-p] FILENAME
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Commit the changes recorded in *FILENAME* in its base image or backing file.
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If the backing file is smaller than the snapshot, then the backing file will be
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resized to be the same size as the snapshot. If the snapshot is smaller than
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the backing file, the backing file will not be truncated. If you want the
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backing file to match the size of the smaller snapshot, you can safely truncate
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it yourself once the commit operation successfully completes.
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The image *FILENAME* is emptied after the operation has succeeded. If you do
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not need *FILENAME* afterwards and intend to drop it, you may skip emptying
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*FILENAME* by specifying the ``-d`` flag.
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If the backing chain of the given image file *FILENAME* has more than one
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layer, the backing file into which the changes will be committed may be
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specified as *BASE* (which has to be part of *FILENAME*'s backing
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chain). If *BASE* is not specified, the immediate backing file of the top
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image (which is *FILENAME*) will be used. Note that after a commit operation
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all images between *BASE* and the top image will be invalid and may return
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garbage data when read. For this reason, ``-b`` implies ``-d`` (so that
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the top image stays valid).
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The rate limit for the commit process is specified by ``-r``.
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.. option:: compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 FILENAME2
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Check if two images have the same content. You can compare images with
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different format or settings.
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The format is probed unless you specify it by ``-f`` (used for
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*FILENAME1*) and/or ``-F`` (used for *FILENAME2*) option.
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By default, images with different size are considered identical if the larger
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image contains only unallocated and/or zeroed sectors in the area after the end
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of the other image. In addition, if any sector is not allocated in one image
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and contains only zero bytes in the second one, it is evaluated as equal. You
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can use Strict mode by specifying the ``-s`` option. When compare runs in
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Strict mode, it fails in case image size differs or a sector is allocated in
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one image and is not allocated in the second one.
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By default, compare prints out a result message. This message displays
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information that both images are same or the position of the first different
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byte. In addition, result message can report different image size in case
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Strict mode is used.
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Compare exits with ``0`` in case the images are equal and with ``1``
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in case the images differ. Other exit codes mean an error occurred during
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execution and standard error output should contain an error message.
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The following table sumarizes all exit codes of the compare subcommand:
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0
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Images are identical (or requested help was printed)
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1
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Images differ
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2
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Error on opening an image
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3
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Error on checking a sector allocation
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4
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Error on reading data
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.. option:: convert [--object OBJECTDEF] [--image-opts] [--target-image-opts] [--target-is-zero] [--bitmaps] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-O OUTPUT_FMT] [-B BACKING_FILE] [-o OPTIONS] [-l SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-r RATE_LIMIT] [-m NUM_COROUTINES] [-W] FILENAME [FILENAME2 [...]] OUTPUT_FILENAME
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Convert the disk image *FILENAME* or a snapshot *SNAPSHOT_PARAM*
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to disk image *OUTPUT_FILENAME* using format *OUTPUT_FMT*. It can
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be optionally compressed (``-c`` option) or use any format specific
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options like encryption (``-o`` option).
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Only the formats ``qcow`` and ``qcow2`` support compression. The
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compression is read-only. It means that if a compressed sector is
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rewritten, then it is rewritten as uncompressed data.
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Image conversion is also useful to get smaller image when using a
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growable format such as ``qcow``: the empty sectors are detected and
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suppressed from the destination image.
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*SPARSE_SIZE* indicates the consecutive number of bytes (defaults to 4k)
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that must contain only zeros for qemu-img to create a sparse image during
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conversion. If *SPARSE_SIZE* is 0, the source will not be scanned for
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unallocated or zero sectors, and the destination image will always be
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fully allocated.
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You can use the *BACKING_FILE* option to force the output image to be
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created as a copy on write image of the specified base image; the
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*BACKING_FILE* should have the same content as the input's base image,
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however the path, image format, etc may differ.
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If a relative path name is given, the backing file is looked up relative to
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the directory containing *OUTPUT_FILENAME*.
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If the ``-n`` option is specified, the target volume creation will be
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skipped. This is useful for formats such as ``rbd`` if the target
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volume has already been created with site specific options that cannot
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be supplied through qemu-img.
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Out of order writes can be enabled with ``-W`` to improve performance.
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This is only recommended for preallocated devices like host devices or other
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raw block devices. Out of order write does not work in combination with
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creating compressed images.
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*NUM_COROUTINES* specifies how many coroutines work in parallel during
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the convert process (defaults to 8).
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.. option:: create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE] [-F BACKING_FMT] [-u] [-o OPTIONS] FILENAME [SIZE]
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Create the new disk image *FILENAME* of size *SIZE* and format
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*FMT*. Depending on the file format, you can add one or more *OPTIONS*
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that enable additional features of this format.
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If the option *BACKING_FILE* is specified, then the image will record
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only the differences from *BACKING_FILE*. No size needs to be specified in
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this case. *BACKING_FILE* will never be modified unless you use the
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``commit`` monitor command (or qemu-img commit).
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If a relative path name is given, the backing file is looked up relative to
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the directory containing *FILENAME*.
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Note that a given backing file will be opened to check that it is valid. Use
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the ``-u`` option to enable unsafe backing file mode, which means that the
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image will be created even if the associated backing file cannot be opened. A
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matching backing file must be created or additional options be used to make the
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backing file specification valid when you want to use an image created this
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way.
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The size can also be specified using the *SIZE* option with ``-o``,
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it doesn't need to be specified separately in this case.
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.. option:: dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE] [count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT
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dd copies from *INPUT* file to *OUTPUT* file converting it from
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*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:
|
|
|
|
- whether the sectors contain actual data or not (boolean field ``data``;
|
|
if false, the sectors are either unallocated or stored as optimized
|
|
all-zero clusters);
|
|
- whether the data is known to read as zero (boolean field ``zero``);
|
|
- in order to make the output shorter, the target file is expressed as
|
|
a ``depth``; 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).
|
|
|
|
``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.
|