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QEMU disk image utility
**qemu-img** [*standard options*] *command* [*command options*]
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.
.. 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:: ../
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 ?`` 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
'[ID],[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
.. 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
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
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:: -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
.. 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
Command description:
.. program:: qemu-img-commands
.. option:: amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE] -o OPTIONS FILENAME
Amends the image format specific *OPTIONS* for the image file
*FILENAME*. Not all file formats support this operation.
.. 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:: 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`` 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:
Check completed, the image is (now) consistent
Check not completed because of internal errors
Check completed, image is corrupted
Check completed, image has leaked clusters, but is not corrupted
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] [-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).
.. 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:
Images are identical
Images differ
Error on opening an image
Error on checking a sector allocation
Error on reading data
.. option:: convert [--object OBJECTDEF] [--image-opts] [--target-image-opts] [--target-is-zero] [-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] [-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, 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).
.. 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
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
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 of the image format, if applicable
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] [--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
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
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.
.. 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*
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
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
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:
Supported image file formats:
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 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.
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:
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.
File name of a base image (see ``create`` subcommand)
Image format of the base image
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.
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 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.
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.
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).
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
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.