qemu-nbd - QEMU Disk Network Block Device Server
usage: qemu-nbd [OPTION]... filename
Export QEMU disk image using NBD protocol.
is a disk image filename
port to listen on (default B<1024>)
offset into the image
interface to bind to (default B<0.0.0.0>)
Use a unix socket with path I<path>
Set image format as I<format>
only expose partition I<num>
use I<filename> as an external snapshot, create a temporary
file with backing_file=I<filename>, redirect the write to
the temporary one
load an internal snapshot inside I<filename> and export it
as an read-only device, I<snapshot_param> format is
'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'
set cache mode to be used with the file. See the documentation of
the emulator's C<-drive cache=...> option for allowed values.
choose asynchronous I/O mode between B<threads> (the default)
and B<native> (Linux only).
toggles whether I<discard> (also known as I<trim> or I<unmap>)
requests are ignored or passed to the filesystem. The default is no
connect I<filename> to NBD device I<dev>
disconnect the specified device
device can be shared by I<num> clients (default B<1>)
force block driver for format I<fmt> instead of auto-detecting
don't exit on the last connection
display extra debugging information
display this help and exit
output version information and exit
Copyright (C) 2006 Anthony Liguori <firstname.lastname@example.org>. This is
free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
QEMU-IO(1) General Commands Manual QEMU-IO(1)
qemu-io - QEMU Disk exerciser
qemu-io [-h] [-V] [-rsnm] [-c cmd] ... [file]
qemu-io is a command line utility to exercise the QEMU I/O path.
-c, --cmd <cmd>
Command to execute
Use snapshot file
Disable host cache
Allow file to grow (only applies to protocols)
Misalign allocations for O_DIRECT
Use kernel AIO implementation (on Linux only)
Display help and exit
Output version information and exit
This manual page was written by Asias He <email@example.com>, for
the Debian project (and may be used by others). Permission is granted
to copy, distribute and/or modify this document under the terms of the
Creative Commons Attribution-Share Alike 3.0 United States License.
December 18, 2011 QEMU-IO(1)
qemu-img - QEMU disk image utility
usage: qemu-img command [command options]
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.
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] [-n] [-f fmt] [-t cache] [-O output_fmt] [-o
options] [-s snapshot_id_or_name] [-l snapshot_param] [-S sparse_size]
filename [filename2 [...]] output_filename
info [-f fmt] [--output=ofmt] [--backing-chain] filename
map [-f fmt] [--output=ofmt] filename
snapshot [-q] [-l | -a snapshot | -c snapshot | -d snapshot] filename
rebase [-q] [-f fmt] [-t cache] [-p] [-u] -b backing_file [-F
resize [-q] filename [+ | -]size
amend [-q] [-f fmt] -o options 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.
will enumerate information about backing files in a disk image
chain. Refer below for further description.
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.
is the destination disk image filename
is the destination format
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
is param used for internal snapshot, format is
'snapshot.id=[ID],snapshot.name=[NAME]' or '[ID_OR_NAME]'
is deprecated, use snapshot_param instead
-c indicates that target image must be compressed (qcow format only)
-h with or without a command shows help and lists the supported
-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" 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.
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.
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:
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
Parameters to convert subcommand:
-n Skip the creation of the target volume
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
Only the formats "qcow2", "qed" and "vdi" support consistency
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 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.
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
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] [-n] [-f fmt] [-t cache] [-O output_fmt] [-o options]
[-s snapshot_id_or_name] [-l snapshot_param] [-S sparse_size] filename
[filename2 [...]] output_filename
Convert the disk image filename or a snapshot
snapshot_param(snapshot_id_or_name is deprecated) 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.
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, etc may differ.
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.
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
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
map [-f fmt] [--output=ofmt] 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
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
- whether the sectors contain actual data or not (boolean field
"data"; if false, the sectors are either unallocated or stored
as optimized all-zero clusters);
- whether the data is known to read as zero (boolean field
- in order to make the output shorter, the target file is
expressed as a "depth"; 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
For more information, consult include/block/block.h in QEMU's
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]
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:
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
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
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
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.
amend [-f fmt] -o options filename
Amends the image format specific options for the image file
filename. Not all file formats support this operation.
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.
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.
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.
File name of a base image (see create subcommand)
Image format of the base image
If this option is set to "on", the image is encrypted with
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
on the sector number. This makes it vulnerable to chosen
plaintext attacks which can reveal the existence of
-<The user passphrase is directly used as the encryption key. A
chosen or short passphrase will compromise the security of
-<In the event of the passphrase being compromised there is no
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.
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.
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 mode (allowed values: off, metadata). An image
with preallocated metadata is initially larger but can improve
performance when the image needs to grow.
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.
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 Emulation User
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.
The HTML documentation of QEMU for more precise information and Linux
user mode emulator invocation.