QEMU-IMG(1) QEMU-IMG(1)
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
2014-10-04 QEMU-IMG(1)