Provided by: qemu-utils_8.2.2+ds-0ubuntu1.2_amd64 
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 virtual machine or any
other process; this may destroy the image. Also, be aware that querying an image that is
being modified by another process may encounter inconsistent state.
OPTIONS
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 available 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 backend.
The following commands are supported:
amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE] [--force] -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
bitmap (--merge SOURCE | --add | --remove | --clear | --enable | --disable)... [-b
SOURCE_FILE [-F SOURCE_FMT]] [-g GRANULARITY] [--object OBJECTDEF] [--image-opts | -f FMT]
FILENAME BITMAP
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] [-r
RATE_LIMIT] [-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]
[--bitmaps] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT] [-t CACHE] [-T SRC_CACHE] [-O
OUTPUT_FMT] [-B BACKING_FILE [-F BACKING_FMT]] [-o OPTIONS] [-l SNAPSHOT_PARAM] [-S
SPARSE_SIZE] [-r RATE_LIMIT] [-m NUM_COROUTINES] [-W] [--salvage] FILENAME [FILENAME2
[...]] OUTPUT_FILENAME
create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE [-F BACKING_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] [--start-offset=OFFSET]
[--max-length=LEN] [--output=OFMT] [-U] FILENAME
measure [--output=OFMT] [-O OUTPUT_FMT] [-o OPTIONS] [--size N | [--object OBJECTDEF]
[--image-opts] [-f FMT] [-l SNAPSHOT_PARAM] FILENAME]
snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAPSHOT | -c SNAPSHOT |
-d SNAPSHOT] FILENAME
rebase [--object OBJECTDEF] [--image-opts] [-U] [-q] [-f FMT] [-t CACHE] [-T SRC_CACHE]
[-p] [-u] [-c] -b BACKING_FILE [-F BACKING_FMT] FILENAME
resize [--object OBJECTDEF] [--image-opts] [-f FMT] [--preallocation=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 (kilobyte, 1024) M
(megabyte, 1024k) and G (gigabyte, 1024M) and T (terabyte, 1024G) are supported. b is
ignored.
OUTPUT_FILENAME is the destination disk image filename.
OUTPUT_FMT is the destination format.
OPTIONS is a comma separated list of format specific options in a name=value format. Use
-o help for an overview of the options supported by the used format or see the format
descriptions below for details.
SNAPSHOT_PARAM is param used for internal snapshot, format is
'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'.
--object OBJECTDEF
is a QEMU user creatable object definition. See the 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.
--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.
--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.
--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.
--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/qcow2 and vmdk with
streamOptimized subformat only).
For qcow2, the compression algorithm can be specified with the -o
compression_type=... option (see below).
-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 (destination) 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 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:
--bitmaps
Additionally copy all persistent bitmaps from the top layer of the source
-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 improves performance, but
is only recommended for preallocated devices 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 sparsify zero sectors, and may result in a fully allocated
target image depending on the host support for getting allocation information.
-r Rate limit for the convert process
--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 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:
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
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
Command description:
amend [--object OBJECTDEF] [--image-opts] [-p] [-q] [-f FMT] [-t CACHE] [--force] -o
OPTIONS FILENAME
Amends the image format specific OPTIONS for the image file FILENAME. Not all file
formats support this operation.
The set of options that can be amended are dependent on the image format, but note
that amending the backing chain relationship should instead be performed with
qemu-img rebase.
--force allows some unsafe operations. Currently for -f luks, it allows to erase
the last encryption key, and to overwrite an active encryption key.
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.
bitmap (--merge SOURCE | --add | --remove | --clear | --enable | --disable)... [-b
SOURCE_FILE [-F SOURCE_FMT]] [-g GRANULARITY] [--object OBJECTDEF] [--image-opts | -f FMT]
FILENAME BITMAP
Perform one or more modifications of the persistent bitmap BITMAP in the disk image
FILENAME. The various modifications are:
--add to create BITMAP, enabled to record future edits.
--remove to remove BITMAP.
--clear to clear BITMAP.
--enable to change BITMAP to start recording future edits.
--disable to change BITMAP to stop recording future edits.
--merge to merge the contents of the SOURCE bitmap into BITMAP.
Additional options include -g which sets a non-default GRANULARITY for --add, and
-b and -F which select an alternative source file for all SOURCE bitmaps used by
--merge.
To see what bitmaps are present in an image, use qemu-img info.
check [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [--output=OFMT] [-r [leaks | all]]
[-T SRC_CACHE] [-U] FILENAME
Perform a consistency check on the disk image FILENAME. The command can output in
the format OFMT which is either human or json. The JSON output is an object of
QAPI type ImageCheck.
If -r is specified, qemu-img tries to repair any inconsistencies found during the
check. -r leaks repairs only cluster leaks, whereas -r all fixes all kinds of
errors, with a higher risk of choosing the wrong fix or hiding corruption that has
already occurred.
Only the formats qcow2, qed, parallels, vhdx, vmdk and vdi support consistency
checks.
In case the image does not have any inconsistencies, check exits with 0. Other
exit codes indicate the kind of inconsistency found or if another error occurred.
The following table summarizes all exit codes of the check subcommand:
0 Check completed, the image is (now) consistent
1 Check not completed because of internal errors
2 Check completed, image is corrupted
3 Check completed, image has leaked clusters, but is not corrupted
63 Checks are not supported by the image format
If -r is specified, exit codes representing the image state refer to the state
after (the attempt at) repairing it. That is, a successful -r all will yield the
exit code 0, independently of the image state before.
commit [--object OBJECTDEF] [--image-opts] [-q] [-f FMT] [-t CACHE] [-b BASE] [-r
RATE_LIMIT] [-d] [-p] FILENAME
Commit the changes recorded in FILENAME in its base image or backing file. If the
backing file is smaller than the snapshot, then the backing file will be resized to
be the same size as the snapshot. If the snapshot is smaller than the backing
file, the backing file will not be truncated. If you want the backing file to
match the size of the smaller snapshot, you can safely truncate it yourself once
the commit operation successfully completes.
The image FILENAME is emptied after the operation has succeeded. If you do not need
FILENAME afterwards and intend to drop it, you may skip emptying FILENAME by
specifying the -d flag.
If the backing chain of the given image file FILENAME has more than one layer, the
backing file into which the changes will be committed may be specified as BASE
(which has to be part of FILENAME's backing chain). If BASE is not specified, the
immediate backing file of the top image (which is FILENAME) will be used. Note that
after a commit operation all images between BASE and the top image will be invalid
and may return garbage data when read. For this reason, -b implies -d (so that the
top image stays valid).
The rate limit for the commit process is specified by -r.
compare [--object OBJECTDEF] [--image-opts] [-f FMT] [-F FMT] [-T SRC_CACHE] [-p] [-q]
[-s] [-U] FILENAME1 FILENAME2
Check if two images have the same content. You can compare images with different
format or settings.
The format is probed unless you specify it by -f (used for FILENAME1) and/or -F
(used for FILENAME2) option.
By default, images with different size are considered identical if the larger image
contains only unallocated and/or zeroed sectors in the area after the end of the
other image. In addition, if any sector is not allocated in one image and contains
only zero bytes in the second one, it is evaluated as equal. You can use Strict
mode by specifying the -s option. When compare runs in Strict mode, it fails in
case image size differs or a sector is allocated in one image and is not allocated
in the second one.
By default, compare prints out a result message. This message displays information
that both images are same or the position of the first different byte. In addition,
result message can report different image size in case Strict mode is used.
Compare exits with 0 in case the images are equal and with 1 in case the images
differ. Other exit codes mean an error occurred during execution and standard error
output should contain an error message. The following table sumarizes all exit
codes of the compare subcommand:
0 Images are identical (or requested help was printed)
1 Images differ
2 Error on opening an image
3 Error on checking a sector allocation
4 Error on reading data
convert [--object OBJECTDEF] [--image-opts] [--target-image-opts] [--target-is-zero]
[--bitmaps [--skip-broken-bitmaps]] [-U] [-C] [-c] [-p] [-q] [-n] [-f FMT] [-t CACHE] [-T
SRC_CACHE] [-O OUTPUT_FMT] [-B BACKING_FILE [-F BACKING_FMT]] [-o OPTIONS] [-l
SNAPSHOT_PARAM] [-S SPARSE_SIZE] [-r RATE_LIMIT] [-m NUM_COROUTINES] [-W] FILENAME
[FILENAME2 [...]] OUTPUT_FILENAME
Convert the disk image FILENAME or a snapshot SNAPSHOT_PARAM to disk image
OUTPUT_FILENAME using format OUTPUT_FMT. It can be optionally compressed (-c
option) or use any format specific options like encryption (-o option).
Only the formats qcow and qcow2 support compression. The compression is read-only.
It means that if a compressed sector is rewritten, then it is rewritten as
uncompressed data.
Image conversion is also useful to get smaller image when using a growable format
such as qcow: the empty sectors are detected and suppressed from the destination
image.
SPARSE_SIZE indicates the consecutive number of bytes (defaults to 4k) that must
contain only zeros for qemu-img to create a sparse image during conversion. If
SPARSE_SIZE is 0, the source will not be scanned for unallocated or zero sectors,
and the destination image will always be fully allocated.
You can use the BACKING_FILE option to force the output image to be created as a
copy on write image of the specified base image; the BACKING_FILE should have the
same content as the input's base image, however the path, image format (as given by
BACKING_FMT), etc may differ.
If a relative path name is given, the backing file is looked up relative to the
directory containing OUTPUT_FILENAME.
If the -n option is specified, the target volume creation will be skipped. This is
useful for formats such as rbd if the target volume has already been created with
site specific options that cannot be supplied through qemu-img.
Out of order writes can be enabled with -W to improve performance. This is only
recommended for preallocated devices like host devices or other raw block devices.
Out of order write does not work in combination with creating compressed images.
NUM_COROUTINES specifies how many coroutines work in parallel during the convert
process (defaults to 8).
Use of --bitmaps requests that any persistent bitmaps present in the original are
also copied to the destination. If any bitmap is inconsistent in the source, the
conversion will fail unless --skip-broken-bitmaps is also specified to copy only
the consistent bitmaps.
create [--object OBJECTDEF] [-q] [-f FMT] [-b BACKING_FILE [-F BACKING_FMT]] [-u] [-o
OPTIONS] FILENAME [SIZE]
Create the new disk image FILENAME of size SIZE and format FMT. Depending on the
file format, you can add one or more OPTIONS that enable additional features of
this format.
If the option BACKING_FILE is specified, then the image will record only the
differences from BACKING_FILE. No size needs to be specified in this case.
BACKING_FILE will never be modified unless you use the commit monitor command (or
qemu-img commit).
If a relative path name is given, the backing file is looked up relative to the
directory containing FILENAME.
Note that a given backing file will be opened to check that it is valid. Use the -u
option to enable unsafe backing file mode, which means that the image will be
created even if the associated backing file cannot be opened. A matching backing
file must be created or additional options be used to make the backing file
specification valid when you want to use an image created this way.
The size can also be specified using the SIZE option with -o, it doesn't need to be
specified separately in this case.
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 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 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).
map [--object OBJECTDEF] [--image-opts] [-f FMT] [--start-offset=OFFSET]
[--max-length=LEN] [--output=OFMT] [-U] FILENAME
Dump the metadata of image FILENAME and its backing file chain. In particular,
this commands dumps the allocation state of every sector of FILENAME, together with
the topmost file that allocates it in the backing file chain.
Two option formats are possible. The default format (human) only dumps
known-nonzero areas of the file. Known-zero parts of the file are omitted
altogether, and likewise for parts that are not allocated throughout the chain.
qemu-img output will identify a file from where the data can be read, and the
offset in the file. Each line will include four fields, the first three of which
are hexadecimal numbers. For example the first line of:
Offset Length Mapped to File
0 0x20000 0x50000 /tmp/overlay.qcow2
0x100000 0x10000 0x95380000 /tmp/backing.qcow2
means that 0x20000 (131072) bytes starting at offset 0 in the image are available
in /tmp/overlay.qcow2 (opened in raw format) starting at offset 0x50000 (327680).
Data that is compressed, encrypted, or otherwise not available in raw format will
cause an error if human format is in use. Note that file names can include
newlines, thus it is not safe to parse this output format in scripts.
The alternative format json will return an array of dictionaries in JSON format.
It will include similar information in the start, length, offset fields; it will
also include other more specific information:
• boolean field data: true if the sectors contain actual data, false if the sectors
are either unallocated or stored as optimized all-zero clusters
• boolean field zero: true if the data is known to read as zero
• boolean field present: true if the data belongs to the backing chain, false if
rebasing the backing chain onto a deeper file would pick up data from the deeper
file;
• integer field depth: the depth within the backing chain at which the data was
resolved; for example, a depth of 2 refers to the backing file of the backing
file of FILENAME.
In JSON format, the offset field is optional; it is absent in cases where human
format would omit the entry or exit with an error. If data is false and the offset
field is present, the corresponding sectors in the file are not yet in use, but
they are preallocated.
For more information, consult include/block/block.h in QEMU's source code.
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.
snapshot [--object OBJECTDEF] [--image-opts] [-U] [-q] [-l | -a SNAPSHOT | -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] [-c] -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. With the -c option specified, the clusters which
are being merged (but not the entire FILENAME image) are compressed when
written.
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.
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 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). 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 or zstd 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
compression_type
This option configures which compression algorithm will be used for compressed
clusters on the image. Note that setting this option doesn't yet cause the image
to actually receive compressed writes. It is most commonly used with the -c
option of qemu-img convert, but can also be used with the compress filter driver
or backup block jobs with compression enabled.
Valid values are zlib and zstd. For images that use compat=0.10, only zlib
compression is available.
encryption
If this option is set to on, the image is encrypted with 128-bit AES-CBC.
The use of encryption in qcow and qcow2 images is considered to be flawed by
modern cryptography standards, suffering from a number of design problems:
• The AES-CBC cipher is used with predictable initialization vectors based on
the sector number. This makes it vulnerable to chosen plaintext attacks which
can reveal the existence of encrypted data.
• The user passphrase is directly used as the encryption key. A poorly chosen or
short passphrase will compromise the security of the encryption.
• In the event of the passphrase being compromised there is no way to change the
passphrase to protect data in any qcow images. The files must be cloned, using
a different encryption passphrase in the new file. The original file must then
be securely erased using a program like shred, though even this is ineffective
with many modern storage technologies.
• Initialization vectors used to encrypt sectors are based on the guest virtual
sector number, instead of the host physical sector. When a disk image has
multiple internal snapshots this means that data in multiple physical sectors
is encrypted with the same initialization vector. With the CBC mode, this
opens the possibility of watermarking attacks if the attack can collect
multiple sectors encrypted with the same IV and some predictable data. Having
multiple qcow2 images with the same passphrase also exposes this weakness
since the passphrase is directly used as the key.
Use of qcow / qcow2 encryption is thus strongly discouraged. Users are
recommended to use an alternative encryption technology such as the Linux
dm-crypt / LUKS system.
cluster_size
Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster
sizes can improve the image file size whereas larger cluster sizes generally
provide better performance.
preallocation
Preallocation mode (allowed values: off, metadata, falloc, full). An image with
preallocated metadata is initially larger but can improve performance when the
image needs to grow. falloc and full preallocations are like the same options of
raw format, but sets up metadata also.
lazy_refcounts
If this option is set to on, reference count updates are postponed with the goal
of avoiding metadata I/O and improving performance. This is particularly
interesting with cache=writethrough which doesn't batch metadata updates. The
tradeoff is that after a host crash, the reference count tables must be rebuilt,
i.e. on the next open an (automatic) qemu-img check -r all is required, which
may take some time.
This option can only be enabled if compat=1.1 is specified.
nocow If this option is set to on, it will turn off COW of the file. It's only valid
on btrfs, no effect on other file systems.
Btrfs has low performance when hosting a VM image file, even more when the guest
on the VM also using btrfs as file system. Turning off COW is a way to mitigate
this bad performance. Generally there are two ways to turn off COW on btrfs:
• Disable it by mounting with nodatacow, then all newly created files will be
NOCOW
• For an empty file, add the NOCOW file attribute. That's what this option does.
Note: this option is only valid to new or empty files. If there is an existing
file which is COW and has data blocks already, it couldn't be changed to NOCOW
by setting nocow=on. One can issue lsattr filename to check if the NOCOW flag is
set or not (Capital 'C' is NOCOW flag).
data_file
Filename where all guest data will be stored. If this option is used, the qcow2
file will only contain the image's metadata.
Note: Data loss will occur if the given filename already exists when using this
option with qemu-img create since qemu-img will create the data file anew,
overwriting the file's original contents. To simply update the reference to
point to the given pre-existing file, use qemu-img amend.
data_file_raw
If this option is set to on, QEMU will always keep the external data file
consistent as a standalone read-only raw image.
It does this by forwarding all write accesses to the qcow2 file through to the
raw data file, including their offsets. Therefore, data that is visible on the
qcow2 node (i.e., to the guest) at some offset is visible at the same offset in
the raw data file. This results in a read-only raw image. Writes that bypass the
qcow2 metadata may corrupt the qcow2 metadata because the out-of-band writes may
result in the metadata falling out of sync with the raw image.
If this option is off, QEMU will use the data file to store data in an arbitrary
manner. The file’s content will not make sense without the accompanying qcow2
metadata. Where data is written will have no relation to its offset as seen by
the guest, and some writes (specifically zero writes) may not be forwarded to
the data file at all, but will only be handled by modifying qcow2 metadata.
This option can only be enabled if data_file is set.
Other
QEMU also supports various other image file formats for compatibility with older QEMU
versions or other hypervisors, including VMDK, VDI, VHD (vpc), VHDX, qcow1 and QED. For
a full list of supported formats see qemu-img --help. For a more detailed description
of these formats, see the QEMU block drivers reference documentation.
The main purpose of the block drivers for these formats is image conversion. For
running VMs, it is recommended to convert the disk images to either raw or qcow2 in
order to achieve good performance.
AUTHOR
Fabrice Bellard
COPYRIGHT
2024, The QEMU Project Developers