Name
qemu-img - QEMU disk image utility
Synopsis
usage: qemu-img command [command options]
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 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)，仅用于qcow镜像，进行压缩操作
-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 sparse_size] filename [filename2 [...]] output_filename
Convert the disk image filename 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.
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.
host_device
Host device format. This format should be used instead of raw when converting to block devices or other devices where "holes" are not supported.
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, full). An image with preallocated metadata is initially larger but can improve performance when the image needs to grow. Full preallocation additionally writes zeros to the whole image in order to preallocate lower layers (e.g. the file system containing the image file) as well. Note that full preallocation writes to every byte of the virtual disk, so it can take a long time for large images.
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