QEMU block drivers reference — QEMU documentation

# 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).

Zoned block devices

  Zoned block devices can be passed through to the guest if the emulated storage controller supports zoned storage. Use `--blockdev host_device, node-name=drive0,filename=/dev/nullb0,cache.direct=on` to pass through `/dev/nullb0` as `drive0`.

#### 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

### 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.