NAME¶
qemu-img - QEMU disk image utility
SYNOPSIS¶
usage: qemu-img 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¶
The following commands are supported:
- check [-f fmt] filename
- create [-f fmt] [-o
options] filename [size]
- commit [-f fmt] [-t
cache] filename
- convert [-c] [-p] [-f fmt] [-t
cache ] [-O output_fmt] [-o options]
[-s snapshot_name] [-S sparse_size]
filename [filename2 [...]]
output_filename
- info [-f fmt] filename
- snapshot [-l | -a snapshot | -c
snapshot | -d snapshot] filename
- rebase [-f fmt] [-t cache]
[-p] [-u] -b backing_file [-F backing_fmt]
filename
- resize 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 ?" 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)
- -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
Command description:
- check [-f fmt] filename
- Perform a consistency check on the disk image
filename.
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.
- 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] 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.
- 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.
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.
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:
- "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.
- qed
- Image format with support for backing files and compact
image files (when your filesystem or transport medium does not support
holes). Good performance due to less metadata than the more featureful
qcow2 format, especially with cache=writethrough or cache=directsync.
Consider using qcow2 which will soon have a similar optimization and is
most actively developed.
Supported options:
- "backing_file"
- File name of a base image (see create
subcommand).
- "backing_fmt"
- Image file format of backing file (optional). Useful if the
format cannot be autodetected because it has no header, like some vhd/vpc
files.
- "cluster_size"
- Changes the cluster size (must be power-of-2 between 4K and
64K). Smaller cluster sizes can improve the image file size whereas larger
cluster sizes generally provide better performance.
- "table_size"
- Changes the number of clusters per L1/L2 table (must be
power-of-2 between 1 and 16). There is normally no need to change this
value but this option can be used for performance benchmarking.
- qcow
- Old QEMU image format. Left for compatibility.
Supported options:
- "backing_file"
- File name of a base image (see create
subcommand)
- "encryption"
- If this option is set to "on", the image is
encrypted.
- cow
- User Mode Linux Copy On Write image format. Used to be the
only growable image format in QEMU. It is supported only for compatibility
with previous versions. It does not work on win32.
- vdi
- VirtualBox 1.1 compatible image format.
- vmdk
- VMware 3 and 4 compatible image format.
Supported options:
- "backing_fmt"
- Image format of the base image
- "compat6"
- Create a VMDK version 6 image (instead of version 4)
- vpc
- VirtualPC compatible image format (VHD).
- cloop
- Linux Compressed Loop image, useful only to reuse directly
compressed CD-ROM images present for example in the Knoppix CD-ROMs.
SEE ALSO¶
The HTML documentation of QEMU for more precise information and Linux user mode
emulator invocation.
AUTHOR¶
Fabrice Bellard