Ubuntu Manpage: qemu-block-drivers - QEMU block drivers reference

Provided by: qemu-utils_8.0.4+dfsg-1ubuntu3.23.10.5_amd64

NAME

       qemu-block-drivers - QEMU block drivers reference

SYNOPSIS

       QEMU block driver reference manual

DESCRIPTION

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 or ls -ls
on Unix/Linux.

Supported options:

preallocation
Preallocation mode (allowed values: off, falloc, full). falloc mode
preallocates space for image by calling posix_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 the encrypt.key-secret parameter. LUKS
encryption parameters can be tuned with the other encrypt.* parameters.

If this is set to aes, the image is encrypted with 128-bit AES-CBC. The
encryption key is given by the encrypt.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 when encrypt.format=luks.

encrypt.cipher-mode
Name of the encryption mode to use. Currently defaults to xts. Only used
when encrypt.format=luks.

encrypt.ivgen-alg
Name of the initialization vector generator algorithm. Currently defaults to
plain64. Only used when encrypt.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 when encrypt.format=luks.

encrypt.hash-alg
Name of the hash algorithm to use for PBKDF algorithm Defaults to sha256.
Only used when encrypt.format=luks.

encrypt.iter-time
Amount of time, in milliseconds, to use for PBKDF algorithm per key slot.
Defaults to 2000. Only used when encrypt.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 and full preallocations are like the
same options of raw 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 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.

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 issue lsattr 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 the encrypt.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 the qcow2 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
with encrypt.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 and
streamOptimized.

vpc VirtualPC compatible image format (VHD).

Supported options:

subformat
Specifies which VHD subformat to use. Valid options are dynamic (default)
and fixed.

VHDX Hyper-V compatible image format (VHDX).

Supported options:

subformat
Specifies which VHDX subformat to use. Valid options are dynamic (default)
and fixed.

block_state_zero
Force use of payload blocks of type 'ZERO'. Can be set to on
(default) or off. When set to off, new blocks will be created as
PAYLOAD_BLOCK_NOT_PRESENT, which means parsers are free to return
arbitrary data for those blocks. Do not set to off when using
qemu-img convert with subformat=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 or eject 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.

AUTHOR

       Fabrice Bellard and the QEMU Project developers

COPYRIGHT

       2024, The QEMU Project Developers

NAME

SYNOPSIS

DESCRIPTION

SEE ALSO

AUTHOR

COPYRIGHT