Disk Images
QEMU supports many disk image formats, including growable disk images (their size increase as non empty sectors are written), compressed and encrypted disk images.
Quick start for disk image creation
You can create a disk image with the command:
qemu-img create myimage.img mysize
where myimage.img is the disk image filename and mysize is its size in
kilobytes. You can add an M
suffix to give the size in megabytes and
a G
suffix for gigabytes.
See the qemu-img
invocation documentation for more information.
Snapshot mode
If you use the option -snapshot
, all disk images are considered as
read only. When sectors in written, they are written in a temporary file
created in /tmp
. You can however force the write back to the raw
disk images by using the commit
monitor command (or C-a s in the
serial console).
VM snapshots
VM snapshots are snapshots of the complete virtual machine including CPU
state, RAM, device state and the content of all the writable disks. In
order to use VM snapshots, you must have at least one non removable and
writable block device using the qcow2
disk image format. Normally
this device is the first virtual hard drive.
Use the monitor command savevm
to create a new VM snapshot or
replace an existing one. A human readable name can be assigned to each
snapshot in addition to its numerical ID.
Use loadvm
to restore a VM snapshot and delvm
to remove a VM
snapshot. info snapshots
lists the available snapshots with their
associated information:
(qemu) info snapshots
Snapshot devices: hda
Snapshot list (from hda):
ID TAG VM SIZE DATE VM CLOCK
1 start 41M 2006-08-06 12:38:02 00:00:14.954
2 40M 2006-08-06 12:43:29 00:00:18.633
3 msys 40M 2006-08-06 12:44:04 00:00:23.514
A VM snapshot is made of a VM state info (its size is shown in
info snapshots
) and a snapshot of every writable disk image. The VM
state info is stored in the first qcow2
non removable and writable
block device. The disk image snapshots are stored in every disk image.
The size of a snapshot in a disk image is difficult to evaluate and is
not shown by info snapshots
because the associated disk sectors are
shared among all the snapshots to save disk space (otherwise each
snapshot would need a full copy of all the disk images).
When using the (unrelated) -snapshot
option
(Snapshot mode),
you can always make VM snapshots, but they are deleted as soon as you
exit QEMU.
VM snapshots currently have the following known limitations:
They cannot cope with removable devices if they are removed or inserted after a snapshot is done.
A few device drivers still have incomplete snapshot support so their state is not saved or restored properly (in particular USB).
Disk image file formats
QEMU supports many image file formats that can be used with VMs as well as with
any of the tools (like qemu-img
). This includes the preferred formats
raw and qcow2 as well as formats that are supported for compatibility with
older QEMU versions or other hypervisors.
Depending on the image format, different options can be passed to
qemu-img create
and qemu-img convert
using the -o
option.
This section describes each format and the options that are supported for it.
- raw
Raw disk image format. 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 orls -ls
on Unix/Linux.Supported options:
- preallocation
Preallocation mode (allowed values:
off
,falloc
,full
).falloc
mode preallocates space for image by callingposix_fallocate()
.full
mode preallocates space for image by writing data to underlying storage. This data may or may not be zero, depending on the storage location.
- 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), 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.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.
- backing_file
File name of a base image (see
create
subcommand)
- backing_fmt
Image format of the base image
- encryption
This option is deprecated and equivalent to
encrypt.format=aes
- encrypt.format
If this is set to
luks
, it requests that the qcow2 payload (not qcow2 header) be encrypted using the LUKS format. The passphrase to use to unlock the LUKS key slot is given by theencrypt.key-secret
parameter. LUKS encryption parameters can be tuned with the otherencrypt.*
parameters.If this is set to
aes
, the image is encrypted with 128-bit AES-CBC. The encryption key is given by theencrypt.key-secret
parameter. This encryption format 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 vectors based on the sector number. This makes it vulnerable to chosen plaintext attacks which can reveal the existence of encrypted data.
The user passphrase is directly used as the encryption key. A poorly chosen or short passphrase will compromise the security of the encryption.
In the event of the passphrase being compromised there is no way to 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.
The use of this is no longer supported in system emulators. Support only remains in the command line utilities, for the purposes of data liberation and interoperability with old versions of QEMU. The
luks
format should be used instead.
- encrypt.key-secret
Provides the ID of a
secret
object that contains the passphrase (encrypt.format=luks
) or encryption key (encrypt.format=aes
).
- encrypt.cipher-alg
Name of the cipher algorithm and key length. Currently defaults to
aes-256
. Only used whenencrypt.format=luks
.
- encrypt.cipher-mode
Name of the encryption mode to use. Currently defaults to
xts
. Only used whenencrypt.format=luks
.
- encrypt.ivgen-alg
Name of the initialization vector generator algorithm. Currently defaults to
plain64
. Only used whenencrypt.format=luks
.
- encrypt.ivgen-hash-alg
Name of the hash algorithm to use with the initialization vector generator (if required). Defaults to
sha256
. Only used whenencrypt.format=luks
.
- encrypt.hash-alg
Name of the hash algorithm to use for PBKDF algorithm Defaults to
sha256
. Only used whenencrypt.format=luks
.
- encrypt.iter-time
Amount of time, in milliseconds, to use for PBKDF algorithm per key slot. Defaults to
2000
. Only used whenencrypt.format=luks
.
- 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
,falloc
,full
). An image with preallocated metadata is initially larger but can improve performance when the image needs to grow.falloc
andfull
preallocations are like the same options ofraw
format, but sets up metadata also.
- 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 withcache=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.
- nocow
If this option is set to
on
, it will turn off COW of the file. It’s only valid on btrfs, no effect on other file systems.Btrfs has low performance when hosting a VM image file, even more when the guest on the VM also using btrfs as file system. Turning off COW is a way to mitigate this bad performance. Generally there are two ways to turn off COW on btrfs:
Disable it by mounting with nodatacow, then all newly created files will be NOCOW.
For an empty file, add the NOCOW file attribute. That’s what this option does.
Note: this option is only valid to new or empty files. If there is an existing file which is COW and has data blocks already, it couldn’t be changed to NOCOW by setting
nocow=on
. One can issuelsattr filename
to check if the NOCOW flag is set or not (Capital ‘C’ is NOCOW flag).
- qed
Old QEMU image format with support for backing files and compact image files (when your filesystem or transport medium does not support holes).
When converting QED images to qcow2, you might want to consider using the
lazy_refcounts=on
option to get a more QED-like behaviour.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 between used for performance benchmarking.
- qcow
Old QEMU image format with support for backing files, compact image files, encryption and compression.
Supported options:
- backing_file
File name of a base image (see
create
subcommand)
- encryption
This option is deprecated and equivalent to
encrypt.format=aes
- encrypt.format
If this is set to
aes
, the image is encrypted with 128-bit AES-CBC. The encryption key is given by theencrypt.key-secret
parameter. This encryption format is considered to be flawed by modern cryptography standards, suffering from a number of design problems enumerated previously against theqcow2
image format.The use of this is no longer supported in system emulators. Support only remains in the command line utilities, for the purposes of data liberation and interoperability with old versions of QEMU.
Users requiring native encryption should use the
qcow2
format instead withencrypt.format=luks
.
- encrypt.key-secret
Provides the ID of a
secret
object that contains the encryption key (encrypt.format=aes
).
- luks
LUKS v1 encryption format, compatible with Linux dm-crypt/cryptsetup
Supported options:
- key-secret
Provides the ID of a
secret
object that contains the passphrase.
- cipher-alg
Name of the cipher algorithm and key length. Currently defaults to
aes-256
.
- cipher-mode
Name of the encryption mode to use. Currently defaults to
xts
.
- ivgen-alg
Name of the initialization vector generator algorithm. Currently defaults to
plain64
.
- ivgen-hash-alg
Name of the hash algorithm to use with the initialization vector generator (if required). Defaults to
sha256
.
- hash-alg
Name of the hash algorithm to use for PBKDF algorithm Defaults to
sha256
.
- iter-time
Amount of time, in milliseconds, to use for PBKDF algorithm per key slot. Defaults to
2000
.
- vdi
VirtualBox 1.1 compatible image format.
Supported options:
- static
If this option is set to
on
, the image is created with metadata preallocation.
- vmdk
VMware 3 and 4 compatible image format.
Supported options:
- backing_file
File name of a base image (see
create
subcommand).
- compat6
Create a VMDK version 6 image (instead of version 4)
- hwversion
Specify vmdk virtual hardware version. Compat6 flag cannot be enabled if hwversion is specified.
- subformat
Specifies which VMDK subformat to use. Valid options are
monolithicSparse
(default),monolithicFlat
,twoGbMaxExtentSparse
,twoGbMaxExtentFlat
andstreamOptimized
.
- vpc
VirtualPC compatible image format (VHD).
Supported options:
- subformat
Specifies which VHD subformat to use. Valid options are
dynamic
(default) andfixed
.
- VHDX
Hyper-V compatible image format (VHDX).
Supported options:
- subformat
Specifies which VHDX subformat to use. Valid options are
dynamic
(default) andfixed
.- block_state_zero
Force use of payload blocks of type ‘ZERO’. Can be set to
on
(default) oroff
. When set tooff
, new blocks will be created asPAYLOAD_BLOCK_NOT_PRESENT
, which means parsers are free to return arbitrary data for those blocks. Do not set tooff
when usingqemu-img convert
withsubformat=dynamic
.
- block_size
Block size; min 1 MB, max 256 MB. 0 means auto-calculate based on image size.
- log_size
Log size; min 1 MB.
Read-only formats
More disk image file formats are supported in a read-only mode.
- bochs
Bochs images of
growing
type.
- cloop
Linux Compressed Loop image, useful only to reuse directly compressed CD-ROM images present for example in the Knoppix CD-ROMs.
- dmg
Apple disk image.
- parallels
Parallels disk image format.
Using host drives
In addition to disk image files, QEMU can directly access host devices. We describe here the usage for QEMU version >= 0.8.3.
Linux
On Linux, you can directly use the host device filename instead of a
disk image filename provided you have enough privileges to access
it. For example, use /dev/cdrom
to access to the CDROM.
- CD
You can specify a CDROM device even if no CDROM is loaded. QEMU has specific code to detect CDROM insertion or removal. CDROM ejection by the guest OS is supported. Currently only data CDs are supported.
- Floppy
You can specify a floppy device even if no floppy is loaded. Floppy removal is currently not detected accurately (if you change floppy without doing floppy access while the floppy is not loaded, the guest OS will think that the same floppy is loaded). Use of the host’s floppy device is deprecated, and support for it will be removed in a future release.
- Hard disks
Hard disks can be used. Normally you must specify the whole disk (
/dev/hdb
instead of/dev/hdb1
) so that the guest OS can see it as a partitioned disk. WARNING: unless you know what you do, it is better to only make READ-ONLY accesses to the hard disk otherwise you may corrupt your host data (use the-snapshot
command line option or modify the device permissions accordingly).
Windows
- CD
The preferred syntax is the drive letter (e.g.
d:
). The alternate syntax\\.\d:
is supported./dev/cdrom
is supported as an alias to the first CDROM drive.Currently there is no specific code to handle removable media, so it is better to use the
change
oreject
monitor commands to change or eject media.- Hard disks
Hard disks can be used with the syntax:
\\.\PhysicalDriveN
where N is the drive number (0 is the first hard disk).WARNING: unless you know what you do, it is better to only make READ-ONLY accesses to the hard disk otherwise you may corrupt your host data (use the
-snapshot
command line so that the modifications are written in a temporary file).
Mac OS X
/dev/cdrom
is an alias to the first CDROM.
Currently there is no specific code to handle removable media, so it
is better to use the change
or eject
monitor commands to
change or eject media.
Virtual FAT disk images
QEMU can automatically create a virtual FAT disk image from a directory tree. In order to use it, just type:
qemu-system-x86_64 linux.img -hdb fat:/my_directory
Then you access access to all the files in the /my_directory
directory without having to copy them in a disk image or to export
them via SAMBA or NFS. The default access is read-only.
Floppies can be emulated with the :floppy:
option:
qemu-system-x86_64 linux.img -fda fat:floppy:/my_directory
A read/write support is available for testing (beta stage) with the
:rw:
option:
qemu-system-x86_64 linux.img -fda fat:floppy:rw:/my_directory
What you should never do:
use non-ASCII filenames
use “-snapshot” together with “:rw:”
expect it to work when loadvm’ing
write to the FAT directory on the host system while accessing it with the guest system
NBD access
QEMU can access directly to block device exported using the Network Block Device protocol.
qemu-system-x86_64 linux.img -hdb nbd://my_nbd_server.mydomain.org:1024/
If the NBD server is located on the same host, you can use an unix socket instead of an inet socket:
qemu-system-x86_64 linux.img -hdb nbd+unix://?socket=/tmp/my_socket
In this case, the block device must be exported using qemu-nbd
:
qemu-nbd --socket=/tmp/my_socket my_disk.qcow2
The use of qemu-nbd
allows sharing of a disk between several guests:
qemu-nbd --socket=/tmp/my_socket --share=2 my_disk.qcow2
and then you can use it with two guests:
qemu-system-x86_64 linux1.img -hdb nbd+unix://?socket=/tmp/my_socket qemu-system-x86_64 linux2.img -hdb nbd+unix://?socket=/tmp/my_socket
If the nbd-server
uses named exports (supported since NBD 2.9.18, or with QEMU’s
own embedded NBD server), you must specify an export name in the URI:
qemu-system-x86_64 -cdrom nbd://localhost/debian-500-ppc-netinst qemu-system-x86_64 -cdrom nbd://localhost/openSUSE-11.1-ppc-netinst
The URI syntax for NBD is supported since QEMU 1.3. An alternative syntax is also available. Here are some example of the older syntax:
qemu-system-x86_64 linux.img -hdb nbd:my_nbd_server.mydomain.org:1024 qemu-system-x86_64 linux2.img -hdb nbd:unix:/tmp/my_socket qemu-system-x86_64 -cdrom nbd:localhost:10809:exportname=debian-500-ppc-netinst
iSCSI LUNs
iSCSI is a popular protocol used to access SCSI devices across a computer network.
There are two different ways iSCSI devices can be used by QEMU.
The first method is to mount the iSCSI LUN on the host, and make it appear as any other ordinary SCSI device on the host and then to access this device as a /dev/sd device from QEMU. How to do this differs between host OSes.
The second method involves using the iSCSI initiator that is built into QEMU. This provides a mechanism that works the same way regardless of which host OS you are running QEMU on. This section will describe this second method of using iSCSI together with QEMU.
In QEMU, iSCSI devices are described using special iSCSI URLs. URL syntax:
iscsi://[<username>[%<password>]@]<host>[:<port>]/<target-iqn-name>/<lun>
Username and password are optional and only used if your target is set up using CHAP authentication for access control. Alternatively the username and password can also be set via environment variables to have these not show up in the process list:
export LIBISCSI_CHAP_USERNAME=<username>
export LIBISCSI_CHAP_PASSWORD=<password>
iscsi://<host>/<target-iqn-name>/<lun>
Various session related parameters can be set via special options, either in a configuration file provided via ‘-readconfig’ or directly on the command line.
If the initiator-name is not specified qemu will use a default name of ‘iqn.2008-11.org.linux-kvm[:<uuid>’] where <uuid> is the UUID of the virtual machine. If the UUID is not specified qemu will use ‘iqn.2008-11.org.linux-kvm[:<name>’] where <name> is the name of the virtual machine.
Setting a specific initiator name to use when logging in to the target:
-iscsi initiator-name=iqn.qemu.test:my-initiator
Controlling which type of header digest to negotiate with the target:
-iscsi header-digest=CRC32C|CRC32C-NONE|NONE-CRC32C|NONE
These can also be set via a configuration file:
[iscsi]
user = "CHAP username"
password = "CHAP password"
initiator-name = "iqn.qemu.test:my-initiator"
# header digest is one of CRC32C|CRC32C-NONE|NONE-CRC32C|NONE
header-digest = "CRC32C"
Setting the target name allows different options for different targets:
[iscsi "iqn.target.name"]
user = "CHAP username"
password = "CHAP password"
initiator-name = "iqn.qemu.test:my-initiator"
# header digest is one of CRC32C|CRC32C-NONE|NONE-CRC32C|NONE
header-digest = "CRC32C"
How to use a configuration file to set iSCSI configuration options:
cat >iscsi.conf <<EOF [iscsi] user = "me" password = "my password" initiator-name = "iqn.qemu.test:my-initiator" header-digest = "CRC32C" EOF qemu-system-x86_64 -drive file=iscsi://127.0.0.1/iqn.qemu.test/1 \ -readconfig iscsi.conf
How to set up a simple iSCSI target on loopback and access it via QEMU: this example shows how to set up an iSCSI target with one CDROM and one DISK using the Linux STGT software target. This target is available on Red Hat based systems as the package ‘scsi-target-utils’.
tgtd --iscsi portal=127.0.0.1:3260 tgtadm --lld iscsi --op new --mode target --tid 1 -T iqn.qemu.test tgtadm --lld iscsi --mode logicalunit --op new --tid 1 --lun 1 \ -b /IMAGES/disk.img --device-type=disk tgtadm --lld iscsi --mode logicalunit --op new --tid 1 --lun 2 \ -b /IMAGES/cd.iso --device-type=cd tgtadm --lld iscsi --op bind --mode target --tid 1 -I ALL qemu-system-x86_64 -iscsi initiator-name=iqn.qemu.test:my-initiator \ -boot d -drive file=iscsi://127.0.0.1/iqn.qemu.test/1 \ -cdrom iscsi://127.0.0.1/iqn.qemu.test/2
GlusterFS disk images
GlusterFS is a user space distributed file system.
You can boot from the GlusterFS disk image with the command:
URI:
qemu-system-x86_64 -drive file=gluster[+TYPE]://[HOST}[:PORT]]/VOLUME/PATH [?socket=...][,file.debug=9][,file.logfile=...]
JSON:
qemu-system-x86_64 'json:{"driver":"qcow2", "file":{"driver":"gluster", "volume":"testvol","path":"a.img","debug":9,"logfile":"...", "server":[{"type":"tcp","host":"...","port":"..."}, {"type":"unix","socket":"..."}]}}'
gluster is the protocol.
TYPE specifies the transport type used to connect to gluster management daemon (glusterd). Valid transport types are tcp and unix. In the URI form, if a transport type isn’t specified, then tcp type is assumed.
HOST specifies the server where the volume file specification for the given volume resides. This can be either a hostname or an ipv4 address. If transport type is unix, then HOST field should not be specified. Instead socket field needs to be populated with the path to unix domain socket.
PORT is the port number on which glusterd is listening. This is optional and if not specified, it defaults to port 24007. If the transport type is unix, then PORT should not be specified.
VOLUME is the name of the gluster volume which contains the disk image.
PATH is the path to the actual disk image that resides on gluster volume.
debug is the logging level of the gluster protocol driver. Debug levels are 0-9, with 9 being the most verbose, and 0 representing no debugging output. The default level is 4. The current logging levels defined in the gluster source are 0 - None, 1 - Emergency, 2 - Alert, 3 - Critical, 4 - Error, 5 - Warning, 6 - Notice, 7 - Info, 8 - Debug, 9 - Trace
logfile is a commandline option to mention log file path which helps in logging to the specified file and also help in persisting the gfapi logs. The default is stderr.
You can create a GlusterFS disk image with the command:
qemu-img create gluster://HOST/VOLUME/PATH SIZE
Examples
qemu-system-x86_64 -drive file=gluster://1.2.3.4/testvol/a.img qemu-system-x86_64 -drive file=gluster+tcp://1.2.3.4/testvol/a.img qemu-system-x86_64 -drive file=gluster+tcp://1.2.3.4:24007/testvol/dir/a.img qemu-system-x86_64 -drive file=gluster+tcp://[1:2:3:4:5:6:7:8]/testvol/dir/a.img qemu-system-x86_64 -drive file=gluster+tcp://[1:2:3:4:5:6:7:8]:24007/testvol/dir/a.img qemu-system-x86_64 -drive file=gluster+tcp://server.domain.com:24007/testvol/dir/a.img qemu-system-x86_64 -drive file=gluster+unix:///testvol/dir/a.img?socket=/tmp/glusterd.socket qemu-system-x86_64 -drive file=gluster+rdma://1.2.3.4:24007/testvol/a.img qemu-system-x86_64 -drive file=gluster://1.2.3.4/testvol/a.img,file.debug=9,file.logfile=/var/log/qemu-gluster.log qemu-system-x86_64 'json:{"driver":"qcow2", "file":{"driver":"gluster", "volume":"testvol","path":"a.img", "debug":9,"logfile":"/var/log/qemu-gluster.log", "server":[{"type":"tcp","host":"1.2.3.4","port":24007}, {"type":"unix","socket":"/var/run/glusterd.socket"}]}}' qemu-system-x86_64 -drive driver=qcow2,file.driver=gluster,file.volume=testvol,file.path=/path/a.img, file.debug=9,file.logfile=/var/log/qemu-gluster.log, file.server.0.type=tcp,file.server.0.host=1.2.3.4,file.server.0.port=24007, file.server.1.type=unix,file.server.1.socket=/var/run/glusterd.socket
Secure Shell (ssh) disk images
You can access disk images located on a remote ssh server by using the ssh protocol:
qemu-system-x86_64 -drive file=ssh://[USER@]SERVER[:PORT]/PATH[?host_key_check=HOST_KEY_CHECK]
Alternative syntax using properties:
qemu-system-x86_64 -drive file.driver=ssh[,file.user=USER],file.host=SERVER[,file.port=PORT],file.path=PATH[,file.host_key_check=HOST_KEY_CHECK]
ssh is the protocol.
USER is the remote user. If not specified, then the local username is tried.
SERVER specifies the remote ssh server. Any ssh server can be used, but it must implement the sftp-server protocol. Most Unix/Linux systems should work without requiring any extra configuration.
PORT is the port number on which sshd is listening. By default the standard ssh port (22) is used.
PATH is the path to the disk image.
The optional HOST_KEY_CHECK parameter controls how the remote
host’s key is checked. The default is yes
which means to use
the local .ssh/known_hosts
file. Setting this to no
turns off known-hosts checking. Or you can check that the host key
matches a specific fingerprint. The fingerprint can be provided in
md5
, sha1
, or sha256
format, however, it is strongly
recommended to only use sha256
, since the other options are
considered insecure by modern standards. The fingerprint value
must be given as a hex encoded string:
host_key_check=sha256:04ce2ae89ff4295a6b9c4111640bdcb3297858ee55cb434d9dd88796e93aa795
The key string may optionally contain “:” separators between each pair of hex digits.
The $HOME/.ssh/known_hosts
file contains the base64 encoded
host keys. These can be converted into the format needed for
QEMU using a command such as:
$ for key in `grep 10.33.8.112 known_hosts | awk '{print $3}'`
do
echo $key | base64 -d | sha256sum
done
6c3aa525beda9dc83eadfbd7e5ba7d976ecb59575d1633c87cd06ed2ed6e366f -
12214fd9ea5b408086f98ecccd9958609bd9ac7c0ea316734006bc7818b45dc8 -
d36420137bcbd101209ef70c3b15dc07362fbe0fa53c5b135eba6e6afa82f0ce -
Note that there can be multiple keys present per host, each with different key ciphers. Care is needed to pick the key fingerprint that matches the cipher QEMU will negotiate with the remote server.
Currently authentication must be done using ssh-agent. Other authentication methods may be supported in future.
Note: Many ssh servers do not support an fsync
-style operation.
The ssh driver cannot guarantee that disk flush requests are
obeyed, and this causes a risk of disk corruption if the remote
server or network goes down during writes. The driver will
print a warning when fsync
is not supported:
warning: ssh server ssh.example.com:22 does not support fsync
With sufficiently new versions of libssh and OpenSSH, fsync
is
supported.
NVMe disk images
NVM Express (NVMe) storage controllers can be accessed directly by a userspace
driver in QEMU. This bypasses the host kernel file system and block layers
while retaining QEMU block layer functionalities, such as block jobs, I/O
throttling, image formats, etc. Disk I/O performance is typically higher than
with -drive file=/dev/sda
using either thread pool or linux-aio.
The controller will be exclusively used by the QEMU process once started. To be able to share storage between multiple VMs and other applications on the host, please use the file based protocols.
Before starting QEMU, bind the host NVMe controller to the host vfio-pci driver. For example:
# modprobe vfio-pci # lspci -n -s 0000:06:0d.0 06:0d.0 0401: 1102:0002 (rev 08) # echo 0000:06:0d.0 > /sys/bus/pci/devices/0000:06:0d.0/driver/unbind # echo 1102 0002 > /sys/bus/pci/drivers/vfio-pci/new_id # qemu-system-x86_64 -drive file=nvme://HOST:BUS:SLOT.FUNC/NAMESPACE
Alternative syntax using properties:
qemu-system-x86_64 -drive file.driver=nvme,file.device=HOST:BUS:SLOT.FUNC,file.namespace=NAMESPACE
HOST:BUS:SLOT.FUNC is the NVMe controller’s PCI device address on the host.
NAMESPACE is the NVMe namespace number, starting from 1.
Disk image file locking
By default, QEMU tries to protect image files from unexpected concurrent access, as long as it’s supported by the block protocol driver and host operating system. If multiple QEMU processes (including QEMU emulators and utilities) try to open the same image with conflicting accessing modes, all but the first one will get an error.
This feature is currently supported by the file protocol on Linux with the Open File Descriptor (OFD) locking API, and can be configured to fall back to POSIX locking if the POSIX host doesn’t support Linux OFD locking.
To explicitly enable image locking, specify “locking=on” in the file protocol driver options. If OFD locking is not possible, a warning will be printed and the POSIX locking API will be used. In this case there is a risk that the lock will get silently lost when doing hot plugging and block jobs, due to the shortcomings of the POSIX locking API.
QEMU transparently handles lock handover during shared storage migration. For shared virtual disk images between multiple VMs, the “share-rw” device option should be used.
By default, the guest has exclusive write access to its disk image. If the
guest can safely share the disk image with other writers the
-device ...,share-rw=on
parameter can be used. This is only safe if
the guest is running software, such as a cluster file system, that
coordinates disk accesses to avoid corruption.
Note that share-rw=on only declares the guest’s ability to share the disk. Some QEMU features, such as image file formats, require exclusive write access to the disk image and this is unaffected by the share-rw=on option.
Alternatively, locking can be fully disabled by “locking=off” block device option. In the command line, the option is usually in the form of “file.locking=off” as the protocol driver is normally placed as a “file” child under a format driver. For example:
-blockdev driver=qcow2,file.filename=/path/to/image,file.locking=off,file.driver=file
To check if image locking is active, check the output of the “lslocks” command on host and see if there are locks held by the QEMU process on the image file. More than one byte could be locked by the QEMU instance, each byte of which reflects a particular permission that is acquired or protected by the running block driver.
Filter drivers
QEMU supports several filter drivers, which don’t store any data, but perform some additional tasks, hooking io requests.
- preallocate
The preallocate filter driver is intended to be inserted between format and protocol nodes and preallocates some additional space (expanding the protocol file) when writing past the file’s end. This can be useful for file-systems with slow allocation.
Supported options:
- prealloc-align
On preallocation, align the file length to this value (in bytes), default 1M.
- prealloc-size
How much to preallocate (in bytes), default 128M.