QEMU-IMG(1)QEMU-IMG(1)NAMEqemu-img - QEMU disk image utility
SYNOPSIS
usage: qemu-img command [command options]
DESCRIPTIONqemu-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
The following commands are supported:
check [-q] [-f fmt] [--output=ofmt] [-r [leaks | all]] filename
create [-q] [-f fmt] [-o options] filename [size]
commit [-q] [-f fmt] [-t cache] filename
compare [-f fmt] [-F fmt] [-p] [-q] [-s] filename1 filename2
convert [-c] [-p] [-q] [-f fmt] [-t cache] [-O output_fmt] [-o options]
[-s snapshot_name] [-S sparse_size] filename [filename2 [...]]
output_filename
info [-f fmt] [--output=ofmt] [--backing-chain] filename
snapshot [-q] [-l | -a snapshot | -c snapshot | -d snapshot] filename
rebase [-q] [-f fmt] [-t cache] [-p] [-u] -b backing_file [-F
backing_fmt] filename
resize [-q] 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.
--backing-chain
will enumerate information about backing files in a disk image
chain. Refer below for further description.
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.
-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 (convert and rebase commands only)
-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.
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 on different image size or sector allocation
Command description:
check [-f fmt] [--output=ofmt] [-r [leaks | all]] filename
Perform a consistency check on the disk image filename. The command
can output in the format ofmt which is either "human" or "json".
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.
create [-f fmt] [-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).
The size can also be specified using the size option with "-o", it
doesn't need to be specified separately in this case.
commit [-f fmt] [-t cache] filename
Commit the changes recorded in filename in its base image.
compare [-f fmt] [-F fmt] [-p] [-s] [-q] 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
1 Images differ
2 Error on opening an image
3 Error on checking a sector allocation
4 Error on reading data
convert [-c] [-p] [-f fmt] [-t cache] [-O output_fmt] [-o options] [-s
snapshot_name] [-S sparse_size] filename [filename2 [...]]
output_filename
Convert the disk image filename or a snapshot snapshot_name 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" or "cow": the empty sectors are
detected and suppressed from the destination image.
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.
info [-f fmt] [--output=ofmt] [--backing-chain] 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. The command can output in the format
ofmt which is either "human" or "json".
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
snapshot [-l | -a snapshot | -c snapshot | -d snapshot ] filename
List, apply, create or delete snapshots in image filename.
rebase [-f fmt] [-t 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).
There are two different modes in which "rebase" can operate:
Safe mode
This is the default mode and performs a real rebase operation.
The new backing file may differ from the old one and qemu-img
rebase will take care of keeping the guest-visible content of
filename unchanged.
In order to achieve this, any clusters that differ between
backing_file and the old backing file of filename are merged
into filename before actually changing the backing file.
Note that the safe mode is an expensive operation, comparable
to converting an image. It only works if the old backing file
still exists.
Unsafe mode
qemu-img uses the unsafe mode if "-u" is specified. In this
mode, only the backing file name and format of filename is
changed without any checks on the file contents. The user must
take care of specifying the correct new backing file, or the
guest-visible content of the image will be corrupted.
This mode is useful for renaming or moving the backing file to
somewhere else. It can be used without an accessible old
backing file, i.e. you can use it to fix an image whose backing
file has already been moved/renamed.
You can use "rebase" to perform a "diff" operation on two disk
images. This can be useful when you have copied or cloned a guest,
and you want to get back to a thin image on top of a template or
base image.
Say that "base.img" has been cloned as "modified.img" by copying
it, and that the "modified.img" guest has run so there are now some
changes compared to "base.img". To construct a thin image called
"diff.qcow2" that contains just the differences, do:
qemu-img create -f qcow2 -b modified.img diff.qcow2
qemu-img rebase -b base.img diff.qcow2
At this point, "modified.img" can be discarded, since "base.img +
diff.qcow2" contains the same information.
resize 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!
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.
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.
qcow2
QEMU image format, the most versatile format. Use it to have
smaller images (useful if your filesystem does not supports holes,
for example on Windows), optional AES encryption, zlib based
compression and support of multiple VM snapshots.
Supported options:
"compat"
Determines the qcow2 version to use. "compat=0.10" uses the
traditional image format that can be read by any QEMU since
0.10 (this is the default). "compat=1.1" enables image format
extensions that only QEMU 1.1 and newer understand. 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.
Encryption uses the AES format which is very secure (128 bit
keys). Use a long password (16 characters) to get maximum
protection.
"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). An image
with preallocated metadata is initially larger but can improve
performance when the image needs to grow.
"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.
Other
QEMU also supports various other image file formats for
compatibility with older QEMU versions or other hypervisors,
including VMDK, VDI, VHD (vpc), 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 Emulation User
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.
SEE ALSO
The HTML documentation of QEMU for more precise information and Linux
user mode emulator invocation.
AUTHOR
Fabrice Bellard
2013-06-05 QEMU-IMG(1)
