QEMU-IMG(1) QEMU QEMU-IMG(1)
NAME
qemu-img - 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 vir-
tual 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
Standard options:
-h, --help
Display this help and exit
-V, --version
Display version information and exit
-T, --trace [[enable=]PATTERN][,events=FILE][,file=FILE]
Specify tracing options.
[enable=]PATTERN
Immediately enable events matching PATTERN (either event
name or a globbing pattern). This option is only avail-
able if QEMU has been compiled with the simple, log or
ftrace tracing backend. To specify multiple events or
patterns, specify the -trace option multiple times.
Use -trace help to print a list of names of trace points.
events=FILE
Immediately enable events listed in FILE. The file must
contain one event name (as listed in the trace-events-all
file) per line; globbing patterns are accepted too. This
option is only available if QEMU has been compiled with
the simple, log or ftrace tracing backend.
file=FILE
Log output traces to FILE. This option is only available
if QEMU has been compiled with the simple tracing back-
end.
The following commands are supported:
amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE]
-o OPTIONS FILENAME
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
check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT]
[-r [leaks | all]] [-T SRC_CACHE] [-U] FILENAME
commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b
BASE] [-d] [-p] FILENAME
compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T
SRC_CACHE] [-p] [-q] [-s] [-U] FILENAME1 FILENAME2
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 SNAP-
SHOT_PARAM] [-S SPARSE_SIZE] [-m NUM_COROUTINES] [-W] [--salvage] FILE-
NAME [FILENAME2 [...]] OUTPUT_FILENAME
create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE] [-F BACK-
ING_FMT] [-u] [-o OPTIONS] FILENAME [SIZE]
dd [--image-opts] [-U] [-f FMT] [-O OUTPUT_FMT] [bs=BLOCK_SIZE]
[count=BLOCKS] [skip=BLOCKS] if=INPUT of=OUTPUT
info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT]
[--backing-chain] [-U] FILENAME
map [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT] [-U]
FILENAME
measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--ob-
ject OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME]
snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAP-
SHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME
rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t
CACHE] [-T SRC_CACHE] [-p] [-u] -b BACKING_FILE [-F BACKING_FMT] FILE-
NAME
resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--prealloca-
tion=PREALLOC] [-q] [--shrink] FILENAME [+ | -]SIZE
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 (kilo-
byte, 1024) M (megabyte, 1024k) and G (gigabyte, 1024M) and T (ter-
abyte, 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 'snap-
shot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'.
--object OBJECTDEF
is a QEMU user creatable object definition. See the qemu(1) man-
ual page for a description of the object properties. The most
common object type is a secret, which is used to supply pass-
words and/or encryption keys.
--image-opts
Indicates that the source FILENAME parameter is to be inter-
preted as a full option string, not a plain filename. This pa-
rameter is mutually exclusive with the -f parameter.
--target-image-opts
Indicates that the OUTPUT_FILENAME parameter(s) are to be inter-
preted as a full option string, not a plain filename. This pa-
rameter is mutually exclusive with the -O parameters. It is cur-
rently required to also use the -n parameter to skip image cre-
ation. This restriction may be relaxed in a future release.
--force-share (-U)
If specified, qemu-img will open the image in shared mode, al-
lowing other QEMU processes to open it in write mode. For exam-
ple, 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 concur-
rent metadata changes, etc. This option is only allowed when
opening images in read-only mode.
--backing-chain
Will enumerate information about backing files in a disk image
chain. Refer below for further description.
-c Indicates that target image must be compressed (qcow format
only).
-h With or without a command, shows help and lists the supported
formats.
-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.
-q Quiet mode - do not print any output (except errors). There's no
progress bar in case both -q and -p options are used.
-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.
-t CACHE
Specifies the cache mode that should be used with the (destina-
tion) file. See the documentation of the emulator's -drive
cache=... option for allowed values.
-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 snapshot subcommand:
snapshot
Is the name of the snapshot to create, apply or delete
-a Applies a snapshot (revert disk to saved state)
-c Creates a snapshot
-d Deletes a snapshot
-l Lists all snapshots in the given image
Parameters to compare subcommand:
-f First image format
-F Second image format
-s Strict mode - fail on different image size or sector allocation
Parameters to convert subcommand:
-n Skip the creation of the target volume
-m Number of parallel coroutines for the convert process
-W Allow out-of-order writes to the destination. This option im-
proves performance, but is only recommended for preallocated de-
vices like host devices or other raw block devices.
-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 spar-
sify zero sectors, and may result in a fully allocated target
image depending on the host support for getting allocation in-
formation.
--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.
--target-is-zero
Assume that reading the destination image will always return ze-
ros. This parameter is mutually exclusive with a destination im-
age that has a backing file. It is required to also use the -n
parameter to skip image creation.
Parameters to dd subcommand:
bs=BLOCK_SIZE
Defines the block size
count=BLOCKS
Sets the number of input blocks to copy
if=INPUT
Sets the input file
of=OUTPUT
Sets the output file
skip=BLOCKS
Sets the number of input blocks to skip
Command description:
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.
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 BUF-
FER_SIZE bytes in size, and with DEPTH requests in parallel. The
first request starts at the position given by OFFSET, each fol-
lowing 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 writ-
ten. This can be overridden with a pattern byte specified by
PATTERN.
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 com-
mand 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 oc-
curred.
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 summa-
rizes 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 re-
fer 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.
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 com-
pletes.
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 immedi-
ate 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).
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 im-
ages with different format or settings.
The format is probed unless you specify it by -f (used for FILE-
NAME1) 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 sec-
tors 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 re-
port 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 er-
ror message. The following table sumarizes all exit codes of
the compare subcommand:
0 Images are identical
1 Images differ
2 Error on opening an image
3 Error on checking a sector allocation
4 Error on reading data
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 SNAP-
SHOT_PARAM] [-S SPARSE_SIZE] [-m NUM_COROUTINES] [-W] FILENAME [FILE-
NAME2 [...]] 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 op-
tions like encryption (-o option).
Only the formats qcow and qcow2 support compression. The com-
pression 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 perfor-
mance. 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).
create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE] [-F BACK-
ING_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 associ-
ated 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.
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 read-
ing BLOCKS input blocks.
The size syntax is similar to dd(1)'s size syntax.
info [--object OBJECTDEF] [--image-opts] [-f FMT] [--output=OFMT]
[--backing-chain] [-U] FILENAME
Give information about the disk image FILENAME. Use it in par-
ticular 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 im-
age 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 unavail-
able, 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. ImageIn-
foSpecificQCow2 for qcow2 images).
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 allo-
cates 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 iden-
tify 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 for-
mat 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:
o 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);
o whether the data is known to read as zero (boolean field
zero);
o in order to make the output shorter, the target file is ex-
pressed 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.
measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--ob-
ject OBJECTDEF] [--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME]
Calculate the file size required for a new image. This informa-
tion can be used to size logical volumes or SAN LUNs appropri-
ately for the image that will be placed in them. The values re-
ported 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 BlockMeasure-
Info.
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 SNAP-
SHOT_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.
snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAP-
SHOT | -c SNAPSHOT | -d SNAPSHOT] FILENAME
List, apply, create or delete snapshots in image FILENAME.
rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t
CACHE] [-T SRC_CACHE] [-p] [-u] -b BACKING_FILE [-F BACKING_FMT] FILE-
NAME
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 oper-
ation. 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 be-
tween 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, compa-
rable 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 acces-
sible old backing file, i.e. you can use it to fix an im-
age whose backing file has already been moved/renamed.
You can use rebase to perform a "diff" operation on two disk im-
ages. 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 tem-
plate 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.
resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--prealloca-
tion=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 allo-
cated 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 be-
yond 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 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
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). fal-
loc mode preallocates space for image by calling posix_fallo-
cate(). 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 exam-
ple 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:
o The AES-CBC cipher is used with predictable initialization
vectors based on the sector number. This makes it vulnera-
ble to chosen plaintext attacks which can reveal the exis-
tence of encrypted data.
o The user passphrase is directly used as the encryption key.
A poorly chosen or short passphrase will compromise the se-
curity of the encryption.
o 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 encryp-
tion 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 technolo-
gies.
o Initialization vectors used to encrypt sectors are based on
the guest virtual sector number, instead of the host physi-
cal sector. When a disk image has multiple internal snap-
shots 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 tech-
nology 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 op-
tions 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 improv-
ing 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 ta-
bles 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 sys-
tems.
Btrfs has low performance when hosting a VM image file, even
more when the guest on the VM also using btrfs as file sys-
tem. Turning off COW is a way to mitigate this bad perfor-
mance. Generally there are two ways to turn off COW on btrfs:
o Disable it by mounting with nodatacow, then all newly cre-
ated files will be NOCOW
o 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 compatibil-
ity 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.
AUTHOR
Fabrice Bellard
COPYRIGHT
2020, The QEMU Project Developers
5.0.0 Jul 22, 2020 QEMU-IMG(1)