path: root/docs/system/qemu-block-drivers.rst.inc

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.

.. program:: image-formats
.. option:: 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:

  .. program:: raw
  .. option:: 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.

.. program:: image-formats
.. option:: 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:

  .. program:: qcow2
  .. option:: 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.

  .. option:: backing_file

    File name of a base image (see ``create`` subcommand)

  .. option:: backing_fmt

    Image format of the base image

  .. option:: encryption

    This option is deprecated and equivalent to ``encrypt.format=aes``

  .. option:: 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.

  .. option:: encrypt.key-secret

    Provides the ID of a ``secret`` object that contains the passphrase
    (``encrypt.format=luks``) or encryption key (``encrypt.format=aes``).

  .. option:: encrypt.cipher-alg

    Name of the cipher algorithm and key length. Currently defaults
    to ``aes-256``. Only used when ``encrypt.format=luks``.

  .. option:: encrypt.cipher-mode

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

  .. option:: encrypt.ivgen-alg

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

  .. option:: 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``.

  .. option:: encrypt.hash-alg

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

  .. option:: 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``.

  .. option:: 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.

  .. option:: 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.

  .. option:: 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 :option:`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.

  .. option:: 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).

.. program:: image-formats
.. option:: 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:

   .. program:: qed
   .. option:: backing_file

      File name of a base image (see ``create`` subcommand).

   .. option:: 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.

   .. option:: 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.

   .. option:: 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.

.. program:: image-formats
.. option:: qcow

  Old QEMU image format with support for backing files, compact image files,
  encryption and compression.

  Supported options:

   .. program:: qcow
   .. option:: backing_file

     File name of a base image (see ``create`` subcommand)

   .. option:: encryption

     This option is deprecated and equivalent to ``encrypt.format=aes``

   .. option:: 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``.

   .. option:: encrypt.key-secret

     Provides the ID of a ``secret`` object that contains the encryption
     key (``encrypt.format=aes``).

.. program:: image-formats
.. option:: luks

  LUKS v1 encryption format, compatible with Linux dm-crypt/cryptsetup

  Supported options:

  .. program:: luks
  .. option:: key-secret

    Provides the ID of a ``secret`` object that contains the passphrase.

  .. option:: cipher-alg

    Name of the cipher algorithm and key length. Currently defaults
    to ``aes-256``.

  .. option:: cipher-mode

    Name of the encryption mode to use. Currently defaults to ``xts``.

  .. option:: ivgen-alg

    Name of the initialization vector generator algorithm. Currently defaults
    to ``plain64``.

  .. option:: ivgen-hash-alg

    Name of the hash algorithm to use with the initialization vector generator
    (if required). Defaults to ``sha256``.

  .. option:: hash-alg

    Name of the hash algorithm to use for PBKDF algorithm
    Defaults to ``sha256``.

  .. option:: iter-time

    Amount of time, in milliseconds, to use for PBKDF algorithm per key slot.
    Defaults to ``2000``.

.. program:: image-formats
.. option:: vdi

  VirtualBox 1.1 compatible image format.

  Supported options:

  .. program:: vdi
  .. option:: static

    If this option is set to ``on``, the image is created with metadata
    preallocation.

.. program:: image-formats
.. option:: vmdk

  VMware 3 and 4 compatible image format.

  Supported options:

  .. program: vmdk
  .. option:: backing_file

    File name of a base image (see ``create`` subcommand).

  .. option:: compat6

    Create a VMDK version 6 image (instead of version 4)

  .. option:: hwversion

    Specify vmdk virtual hardware version. Compat6 flag cannot be enabled
    if hwversion is specified.

  .. option:: subformat

    Specifies which VMDK subformat to use. Valid options are
    ``monolithicSparse`` (default),
    ``monolithicFlat``,
    ``twoGbMaxExtentSparse``,
    ``twoGbMaxExtentFlat`` and
    ``streamOptimized``.

.. program:: image-formats
.. option:: vpc

  VirtualPC compatible image format (VHD).

  Supported options:

  .. program:: vpc
  .. option:: subformat

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

.. program:: image-formats
.. option:: VHDX

  Hyper-V compatible image format (VHDX).

  Supported options:

  .. program:: VHDX
  .. option:: subformat

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

    .. option:: 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``.

    .. option:: block_size

      Block size; min 1 MB, max 256 MB.  0 means auto-calculate based on
      image size.

    .. option:: log_size

      Log size; min 1 MB.

Read-only formats
~~~~~~~~~~~~~~~~~

More disk image file formats are supported in a read-only mode.

.. program:: image-formats
.. option:: bochs

  Bochs images of ``growing`` type.

.. program:: image-formats
.. option:: cloop

  Linux Compressed Loop image, useful only to reuse directly compressed
  CD-ROM images present for example in the Knoppix CD-ROMs.

.. program:: image-formats
.. option:: dmg

  Apple disk image.

.. program:: image-formats
.. option:: 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:

.. parsed-literal::

  |qemu_system| 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:

.. parsed-literal::

  |qemu_system| linux.img -fda fat:floppy:/my_directory

A read/write support is available for testing (beta stage) with the
``:rw:`` option:

.. parsed-literal::

  |qemu_system| 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.

.. parsed-literal::

  |qemu_system| 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:

.. parsed-literal::

  |qemu_system| linux.img -hdb nbd+unix://?socket=/tmp/my_socket

In this case, the block device must be exported using ``qemu-nbd``:

.. parsed-literal::

  qemu-nbd --socket=/tmp/my_socket my_disk.qcow2

The use of ``qemu-nbd`` allows sharing of a disk between several guests:

.. parsed-literal::

  qemu-nbd --socket=/tmp/my_socket --share=2 my_disk.qcow2

and then you can use it with two guests:

.. parsed-literal::

  |qemu_system| linux1.img -hdb nbd+unix://?socket=/tmp/my_socket
  |qemu_system| 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:

.. parsed-literal::

  |qemu_system| -cdrom nbd://localhost/debian-500-ppc-netinst
  |qemu_system| -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:

.. parsed-literal::

  |qemu_system| linux.img -hdb nbd:my_nbd_server.mydomain.org:1024
  |qemu_system| linux2.img -hdb nbd:unix:/tmp/my_socket
  |qemu_system| -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:

.. parsed-literal::

  cat >iscsi.conf <<EOF
  [iscsi]
    user = "me"
    password = "my password"
    initiator-name = "iqn.qemu.test:my-initiator"
    header-digest = "CRC32C"
  EOF

  |qemu_system| -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'.

.. parsed-literal::

  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| -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:

.. parsed-literal::

  |qemu_system| -drive file=gluster[+TYPE]://[HOST}[:PORT]]/VOLUME/PATH
                               [?socket=...][,file.debug=9][,file.logfile=...]

JSON:

.. parsed-literal::

  |qemu_system| '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:

.. parsed-literal::

  qemu-img create gluster://HOST/VOLUME/PATH SIZE

Examples

.. parsed-literal::

  |qemu_system| -drive file=gluster://1.2.3.4/testvol/a.img
  |qemu_system| -drive file=gluster+tcp://1.2.3.4/testvol/a.img
  |qemu_system| -drive file=gluster+tcp://1.2.3.4:24007/testvol/dir/a.img
  |qemu_system| -drive file=gluster+tcp://[1:2:3:4:5:6:7:8]/testvol/dir/a.img
  |qemu_system| -drive file=gluster+tcp://[1:2:3:4:5:6:7:8]:24007/testvol/dir/a.img
  |qemu_system| -drive file=gluster+tcp://server.domain.com:24007/testvol/dir/a.img
  |qemu_system| -drive file=gluster+unix:///testvol/dir/a.img?socket=/tmp/glusterd.socket
  |qemu_system| -drive file=gluster+rdma://1.2.3.4:24007/testvol/a.img
  |qemu_system| -drive file=gluster://1.2.3.4/testvol/a.img,file.debug=9,file.logfile=/var/log/qemu-gluster.log
  |qemu_system| '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| -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:

.. parsed-literal::

  |qemu_system| -drive file=ssh://[USER@]SERVER[:PORT]/PATH[?host_key_check=HOST_KEY_CHECK]

Alternative syntax using properties:

.. parsed-literal::

  |qemu_system| -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:

.. parsed-literal::

  # 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| -drive file=nvme://HOST:BUS:SLOT.FUNC/NAMESPACE

Alternative syntax using properties:

.. parsed-literal::

  |qemu_system| -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.

.. program:: filter-drivers
.. option:: 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:

  .. program:: preallocate
  .. option:: prealloc-align

    On preallocation, align the file length to this value (in bytes), default 1M.

  .. program:: preallocate
  .. option:: prealloc-size

    How much to preallocate (in bytes), default 128M.