DragonFly On-Line Manual Pages

QEMU-BLOCK-DRIVERS(7) QEMU QEMU-BLOCK-DRIVERS(7)

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:

o 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.

o The user passphrase is directly used as the encryption
key. A poorly chosen or short passphrase will
compromise the security of the encryption.

o 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:

o Disable it by mounting with nodatacow, then all newly
created files will be NOCOW.

o 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:

o use non-ASCII filenames

o use "-snapshot" together with ":rw:"

o expect it to work when loadvm'ing

o 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.

SEE ALSO
The HTML documentation of QEMU for more precise information and Linux
user mode emulator invocation.

AUTHOR
Fabrice Bellard and the QEMU Project developers

8.0.2 October 1, 2023 QEMU-BLOCK-DRIVERS(7)

DragonFly On-Line Manual Pages

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