FreeBSD 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 compat- ibility 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 sup- port 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 de- fault). 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.for- mat=aes encrypt.format If this is set to luks, it requests that the qcow2 pay- load (not qcow2 header) be encrypted using the LUKS for- mat. The passphrase to use to unlock the LUKS key slot is given by the encrypt.key-secret parameter. LUKS encryp- tion parameters can be tuned with the other encrypt.* pa- rameters. If this is set to aes, the image is encrypted with 128-bit AES-CBC. The encryption key is given by the en- crypt.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 initializa- tion 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 compro- mise 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 dif- ferent encryption passphrase in the new file. The orig- inal 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 emula- tors. 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 (en- crypt.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 en- crypt.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 De- faults to sha256. Only used when encrypt.format=luks. encrypt.iter-time Amount of time, in milliseconds, to use for PBKDF algo- rithm per key slot. Defaults to 2000. Only used when en- crypt.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 bet- ter performance. preallocation Preallocation mode (allowed values: off, metadata, fal- loc, full). An image with preallocated metadata is ini- tially 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 im- proving performance. This is particularly interesting with cache=writethrough which doesn't batch metadata up- dates. The tradeoff is that after a host crash, the ref- erence 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 speci- fied. 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 set- ting 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 behav- iour. 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 en- crypt.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 emu- lators. Support only remains in the command line util- ities, for the purposes of data liberation and inter- operability 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/crypt- setup 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 De- faults to sha256. iter-time Amount of time, in milliseconds, to use for PBKDF algo- rithm 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 can- not be enabled if hwversion is specified. subformat Specifies which VMDK subformat to use. Valid options are monolithicSparse (default), monolithicFlat, twoGbMaxEx- tentSparse, 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 PAY- LOAD_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 com- pressed 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 oth- erwise you may corrupt your host data (use the -snapshot command line option or modify the device permissions accordingly). Zoned block devices Zoned block devices can be passed through to the guest if the emulated storage controller supports zoned storage. Use --block- dev host_device, node-name=drive0,filename=/dev/nullb0,cache.di- rect=on to pass through /dev/nullb0 as drive0. Windows CD The preferred syntax is the drive letter (e.g. d:). The alter- nate 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 modi- fications 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 com- puter 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 ap- pear 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 be- tween 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 syn- tax: 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, ei- ther 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 op- tional and if not specified, it defaults to port 24007. If the trans- port 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 vol- ume. 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 - Criti- cal, 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 sys- tems 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 check- ing. Or you can check that the host key matches a specific finger- print. 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 dif- ferent 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 authenti- cation 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 sup- ported. 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 perfor- mance 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 ap- plications 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 concur- rent access, as long as it's supported by the block protocol driver and host operating system. If multiple QEMU processes (including QEMU emu- lators and utilities) try to open the same image with conflicting ac- cessing 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 migra- tion. 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 -de- vice ...,share-rw=on parameter can be used. This is only safe if the guest is running software, such as a cluster file system, that coordi- nates 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 exclu- sive 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 de- vice 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 in- stance, each byte of which reflects a particular permission that is ac- quired 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 files 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 COPYRIGHT 2023, The QEMU Project Developers 8.2.10 Apr 12, 2025 QEMU-BLOCK-DRIVERS(7)
NAME | SYNOPSIS | DESCRIPTION | SEE ALSO | AUTHOR | COPYRIGHT
Want to link to this manual page? Use this URL:
<https://man.freebsd.org/cgi/man.cgi?query=qemu-block-drivers&sektion=7&manpath=FreeBSD+Ports+14.3.quarterly>