Provided by: qemu-system-common_7.0+dfsg-7ubuntu2_amd64 bug

NAME

       qemu-block-drivers - QEMU block drivers reference

SYNOPSIS

       QEMU block driver reference manual

DESCRIPTION

   Disk image file formats
       QEMU supports many image file formats that can be used with VMs as well as with any of the
       tools (like qemu-img). This includes the preferred  formats  raw  and  qcow2  as  well  as
       formats   that  are  supported  for  compatibility  with  older  QEMU  versions  or  other
       hypervisors.

       Depending on the image format, different options can be  passed  to  qemu-img  create  and
       qemu-img  convert using the -o option.  This section describes each format and the options
       that are supported for it.

       raw    Raw disk image format. This format has the advantage of  being  simple  and  easily
              exportable  to all other emulators. If your file system supports holes (for example
              in ext2 or ext3 on Linux or NTFS on Windows), then only the  written  sectors  will
              reserve  space. Use qemu-img info to know the real size used by the image or ls -ls
              on Unix/Linux.

              Supported options:

              preallocation
                     Preallocation  mode  (allowed  values:  off,  falloc,  full).  falloc   mode
                     preallocates  space  for  image  by  calling  posix_fallocate().  full  mode
                     preallocates space for image by writing data  to  underlying  storage.  This
                     data may or may not be zero, depending on the storage location.

       qcow2  QEMU image format, the most versatile format. Use it to have smaller images (useful
              if your filesystem does not supports holes, for example  on  Windows),  zlib  based
              compression and support of multiple VM snapshots.

              Supported options:

              compat Determines  the qcow2 version to use. compat=0.10 uses the traditional image
                     format that can be read by any QEMU since 0.10.   compat=1.1  enables  image
                     format  extensions  that  only  QEMU  1.1  and newer understand (this is the
                     default). Amongst others, this includes zero clusters, which allow efficient
                     copy-on-read for sparse images.

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

              backing_fmt
                     Image format of the base image

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

              encrypt.format
                     If  this  is  set  to  luks,  it  requests that the qcow2 payload (not qcow2
                     header) be encrypted using the LUKS format. The passphrase to use to  unlock
                     the  LUKS  key  slot  is  given  by  the  encrypt.key-secret parameter. LUKS
                     encryption parameters can be tuned with the other encrypt.* parameters.

                     If this is set to aes, the image is encrypted  with  128-bit  AES-CBC.   The
                     encryption   key   is  given  by  the  encrypt.key-secret  parameter.   This
                     encryption  format  is  considered  to  be  flawed  by  modern  cryptography
                     standards, suffering from a number of design problems:

                     • The  AES-CBC  cipher is used with predictable initialization vectors based
                       on the sector number. This makes it vulnerable to chosen plaintext attacks
                       which can reveal the existence of encrypted data.

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

                     • In the event of the passphrase being compromised there is no way to change
                       the  passphrase  to  protect  data  in  any qcow images. The files must be
                       cloned, using a different encryption  passphrase  in  the  new  file.  The
                       original  file  must  then  be securely erased using a program like shred,
                       though even this is ineffective with many modern storage technologies.

                     The use of this is no longer supported in  system  emulators.  Support  only
                     remains  in  the command line utilities, for the purposes of data liberation
                     and interoperability with old versions of QEMU. The luks  format  should  be
                     used instead.

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

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

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

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

              encrypt.ivgen-hash-alg
                     Name  of  the hash algorithm to use with the initialization vector generator
                     (if required). Defaults to sha256. Only used when encrypt.format=luks.

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

              encrypt.iter-time
                     Amount  of  time,  in milliseconds, to use for PBKDF algorithm per key slot.
                     Defaults to 2000. Only used when encrypt.format=luks.

              cluster_size
                     Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster
                     sizes can improve the image file size whereas larger cluster sizes generally
                     provide better performance.

              preallocation
                     Preallocation mode (allowed values: off, metadata, falloc, full).  An  image
                     with  preallocated  metadata is initially larger but can improve performance
                     when the image needs to grow. falloc and full preallocations  are  like  the
                     same options of raw format, but sets up metadata also.

              lazy_refcounts
                     If  this option is set to on, reference count updates are postponed with the
                     goal  of  avoiding  metadata  I/O  and  improving   performance.   This   is
                     particularly   interesting   with  cache=writethrough  which  doesn't  batch
                     metadata updates. The tradeoff is that after a  host  crash,  the  reference
                     count  tables must be rebuilt, i.e. on the next open an (automatic) qemu-img
                     check -r all is required, which may take some time.

                     This option can only be enabled if compat=1.1 is specified.

              nocow  If this option is set to on, it will turn off COW of  the  file.  It's  only
                     valid on btrfs, no effect on other file systems.

                     Btrfs  has  low performance when hosting a VM image file, even more when the
                     guest on the VM also using btrfs as file system. Turning off COW is a way to
                     mitigate  this bad performance. Generally there are two ways to turn off COW
                     on btrfs:

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

                     • For  an  empty file, add the NOCOW file attribute. That's what this option
                       does.

                     Note: this option is only valid to new  or  empty  files.  If  there  is  an
                     existing  file  which  is  COW  and  has data blocks already, it couldn't be
                     changed to NOCOW by setting nocow=on. One can issue lsattr filename to check
                     if the NOCOW flag is set or not (Capital 'C' is NOCOW flag).

       qed    Old  QEMU image format with support for backing files and compact image files (when
              your filesystem or transport medium does not support holes).

              When converting QED  images  to  qcow2,  you  might  want  to  consider  using  the
              lazy_refcounts=on option to get a more QED-like behaviour.

              Supported options:

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

              backing_fmt
                     Image  file  format of backing file (optional).  Useful if the format cannot
                     be autodetected because it has no header, like some vhd/vpc files.

              cluster_size
                     Changes the cluster size (must be power-of-2 between 4K  and  64K).  Smaller
                     cluster  sizes  can improve the image file size whereas larger cluster sizes
                     generally provide better performance.

              table_size
                     Changes the number of clusters per L1/L2 table (must be power-of-2 between 1
                     and 16).  There is normally no need to change this value but this option can
                     between used for performance benchmarking.

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

              Supported options:

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

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

                 encrypt.format
                        If  this is set to aes, the image is encrypted with 128-bit AES-CBC.  The
                        encryption key  is  given  by  the  encrypt.key-secret  parameter.   This
                        encryption  format  is  considered  to  be  flawed by modern cryptography
                        standards,  suffering  from  a  number  of  design  problems   enumerated
                        previously against the qcow2 image format.

                        The  use of this is no longer supported in system emulators. Support only
                        remains  in  the  command  line  utilities,  for  the  purposes  of  data
                        liberation and interoperability with old versions of QEMU.

                        Users  requiring  native  encryption  should use the qcow2 format instead
                        with encrypt.format=luks.

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

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

              Supported options:

              key-secret
                     Provides the ID of a secret object that contains the passphrase.

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

              cipher-mode
                     Name of the encryption mode to use. Currently defaults to xts.

              ivgen-alg
                     Name of the initialization vector generator algorithm. Currently defaults to
                     plain64.

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

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

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

       vdi    VirtualBox 1.1 compatible image format.

              Supported options:

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

       vmdk   VMware 3 and 4 compatible image format.

              Supported options:

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

              compat6
                     Create a VMDK version 6 image (instead of version 4)

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

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

       vpc    VirtualPC compatible image format (VHD).

              Supported options:

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

       VHDX   Hyper-V compatible image format (VHDX).

              Supported options:

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

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

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

                     log_size
                            Log size; min 1 MB.

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

       bochs  Bochs images of growing type.

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

       dmg    Apple disk image.

       parallels
              Parallels disk image format.

   Using host drives
       In addition to disk image files, QEMU can directly access host devices. We  describe  here
       the usage for QEMU version >= 0.8.3.

   Linux
       On  Linux,  you can directly use the host device filename instead of a disk image filename
       provided you have enough privileges to access it. For example, use /dev/cdrom to access to
       the CDROM.

       CD     You  can  specify a CDROM device even if no CDROM is loaded. QEMU has specific code
              to detect CDROM insertion or removal. CDROM ejection by the guest OS is  supported.
              Currently only data CDs are supported.

       Floppy You  can  specify  a  floppy  device even if no floppy is loaded. Floppy removal is
              currently not detected accurately (if you change floppy without doing floppy access
              while  the  floppy  is  not loaded, the guest OS will think that the same floppy is
              loaded).  Use of the host's floppy device is deprecated, and support for it will be
              removed in a future release.

       Hard disks
              Hard  disks can be used. Normally you must specify the whole disk (/dev/hdb instead
              of /dev/hdb1) so that the guest OS can see  it  as  a  partitioned  disk.  WARNING:
              unless  you  know  what you do, it is better to only make READ-ONLY accesses to the
              hard disk otherwise you may corrupt your host data (use the -snapshot command  line
              option or modify the device permissions accordingly).

   Windows
       CD     The  preferred syntax is the drive letter (e.g. d:). The alternate syntax \\.\d: is
              supported. /dev/cdrom is supported as an alias to the first CDROM drive.

              Currently there is no specific code to handle removable media, so it is  better  to
              use the change or eject monitor commands to change or eject media.

       Hard disks
              Hard  disks  can  be  used with the syntax: \\.\PhysicalDriveN where N is the drive
              number (0 is the first hard disk).

              WARNING: unless you know what you do, it is better to only make READ-ONLY  accesses
              to  the  hard  disk  otherwise  you  may  corrupt your host data (use the -snapshot
              command line so that the modifications are written in a temporary file).

   Mac OS X
       /dev/cdrom is an alias to the first CDROM.

       Currently there is no specific code to handle removable media, so it is better to use  the
       change or eject monitor commands to change or eject media.

   Virtual FAT disk images
       QEMU  can automatically create a virtual FAT disk image from a directory tree. In order to
       use it, just type:

          qemu-system-x86_64 linux.img -hdb fat:/my_directory

       Then you access access to all the files in the /my_directory directory without  having  to
       copy  them  in  a  disk  image  or  to export them via SAMBA or NFS. The default access is
       read-only.

       Floppies can be emulated with the :floppy: option:

          qemu-system-x86_64 linux.img -fda fat:floppy:/my_directory

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

          qemu-system-x86_64 linux.img -fda fat:floppy:rw:/my_directory

       What you should never do:

       • use non-ASCII filenames

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

       • expect it to work when loadvm'ing

       • write to the FAT directory on the host system while accessing it with the guest system

   NBD access
       QEMU can access directly to block device exported using the Network Block Device protocol.

          qemu-system-x86_64 linux.img -hdb nbd://my_nbd_server.mydomain.org:1024/

       If the NBD server is located on the same host, you can use an unix socket  instead  of  an
       inet socket:

          qemu-system-x86_64 linux.img -hdb nbd+unix://?socket=/tmp/my_socket

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

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

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

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

       and then you can use it with two guests:

          qemu-system-x86_64 linux1.img -hdb nbd+unix://?socket=/tmp/my_socket
          qemu-system-x86_64 linux2.img -hdb nbd+unix://?socket=/tmp/my_socket

       If  the  nbd-server  uses  named  exports  (supported since NBD 2.9.18, or with QEMU's own
       embedded NBD server), you must specify an export name in the URI:

          qemu-system-x86_64 -cdrom nbd://localhost/debian-500-ppc-netinst
          qemu-system-x86_64 -cdrom nbd://localhost/openSUSE-11.1-ppc-netinst

       The URI syntax for NBD is supported  since  QEMU  1.3.   An  alternative  syntax  is  also
       available.  Here are some example of the older syntax:

          qemu-system-x86_64 linux.img -hdb nbd:my_nbd_server.mydomain.org:1024
          qemu-system-x86_64 linux2.img -hdb nbd:unix:/tmp/my_socket
          qemu-system-x86_64 -cdrom nbd:localhost:10809:exportname=debian-500-ppc-netinst

   iSCSI LUNs
       iSCSI is a popular protocol used to access SCSI devices across a computer network.

       There are two different ways iSCSI devices can be used by QEMU.

       The  first  method  is to mount the iSCSI LUN on the host, and make it appear as any other
       ordinary SCSI device on the host and then to access this device as a /dev/sd  device  from
       QEMU. How to do this differs between host OSes.

       The  second  method  involves  using  the  iSCSI  initiator  that is built into QEMU. This
       provides a mechanism that works the same way regardless of which host OS you  are  running
       QEMU on. This section will describe this second method of using iSCSI together with QEMU.

       In QEMU, iSCSI devices are described using special iSCSI URLs. URL syntax:

          iscsi://[<username>[%<password>]@]<host>[:<port>]/<target-iqn-name>/<lun>

       Username  and  password  are  optional  and  only used if your target is set up using CHAP
       authentication for access control.  Alternatively the username and password  can  also  be
       set via environment variables to have these not show up in the process list:

          export LIBISCSI_CHAP_USERNAME=<username>
          export LIBISCSI_CHAP_PASSWORD=<password>
          iscsi://<host>/<target-iqn-name>/<lun>

       Various  session  related  parameters  can  be  set  via  special  options,  either  in  a
       configuration file provided via '-readconfig' or directly on the command line.

       If  the  initiator-name  is  not   specified   qemu   will   use   a   default   name   of
       'iqn.2008-11.org.linux-kvm[:<uuid>']  where  <uuid> is the UUID of the virtual machine. If
       the UUID is not specified qemu will use 'iqn.2008-11.org.linux-kvm[:<name>'] where  <name>
       is the name of the virtual machine.

       Setting a specific initiator name to use when logging in to the target:

          -iscsi initiator-name=iqn.qemu.test:my-initiator

       Controlling which type of header digest to negotiate with the target:

          -iscsi header-digest=CRC32C|CRC32C-NONE|NONE-CRC32C|NONE

       These can also be set via a configuration file:

          [iscsi]
            user = "CHAP username"
            password = "CHAP password"
            initiator-name = "iqn.qemu.test:my-initiator"
            # header digest is one of CRC32C|CRC32C-NONE|NONE-CRC32C|NONE
            header-digest = "CRC32C"

       Setting the target name allows different options for different targets:

          [iscsi "iqn.target.name"]
            user = "CHAP username"
            password = "CHAP password"
            initiator-name = "iqn.qemu.test:my-initiator"
            # header digest is one of CRC32C|CRC32C-NONE|NONE-CRC32C|NONE
            header-digest = "CRC32C"

       How to use a configuration file to set iSCSI configuration options:

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

          qemu-system-x86_64 -drive file=iscsi://127.0.0.1/iqn.qemu.test/1 \
            -readconfig iscsi.conf

       How to set up a simple iSCSI target on loopback and access it via QEMU: this example shows
       how to set up an iSCSI target with one CDROM and one DISK using the  Linux  STGT  software
       target.   This   target   is   available   on   Red  Hat  based  systems  as  the  package
       'scsi-target-utils'.

          tgtd --iscsi portal=127.0.0.1:3260
          tgtadm --lld iscsi --op new --mode target --tid 1 -T iqn.qemu.test
          tgtadm --lld iscsi --mode logicalunit --op new --tid 1 --lun 1 \
              -b /IMAGES/disk.img --device-type=disk
          tgtadm --lld iscsi --mode logicalunit --op new --tid 1 --lun 2 \
              -b /IMAGES/cd.iso --device-type=cd
          tgtadm --lld iscsi --op bind --mode target --tid 1 -I ALL

          qemu-system-x86_64 -iscsi initiator-name=iqn.qemu.test:my-initiator \
            -boot d -drive file=iscsi://127.0.0.1/iqn.qemu.test/1 \
            -cdrom iscsi://127.0.0.1/iqn.qemu.test/2

   GlusterFS disk images
       GlusterFS is a user space distributed file system.

       You can boot from the GlusterFS disk image with the command:

       URI:

          qemu-system-x86_64 -drive file=gluster[+TYPE]://[HOST}[:PORT]]/VOLUME/PATH
                                       [?socket=...][,file.debug=9][,file.logfile=...]

       JSON:

          qemu-system-x86_64 'json:{"driver":"qcow2",
                                   "file":{"driver":"gluster",
                                            "volume":"testvol","path":"a.img","debug":9,"logfile":"...",
                                            "server":[{"type":"tcp","host":"...","port":"..."},
                                                      {"type":"unix","socket":"..."}]}}'

       gluster is the protocol.

       TYPE specifies the transport type used to connect to gluster management daemon (glusterd).
       Valid  transport  types  are  tcp  and  unix.  In  the URI form, if a transport type isn't
       specified, then tcp type is assumed.

       HOST specifies the server where  the  volume  file  specification  for  the  given  volume
       resides.  This  can  be  either a hostname or an ipv4 address.  If transport type is unix,
       then HOST field should not be specified.  Instead socket field needs to be populated  with
       the path to unix domain socket.

       PORT  is  the  port  number  on  which  glusterd is listening. This is optional and if not
       specified, it defaults to port 24007. If the transport type is unix, then PORT should  not
       be specified.

       VOLUME is the name of the gluster volume which contains the disk image.

       PATH is the path to the actual disk image that resides on gluster volume.

       debug  is  the  logging level of the gluster protocol driver. Debug levels are 0-9, with 9
       being the most verbose, and 0 representing no debugging output.  The default level  is  4.
       The  current logging levels defined in the gluster source are 0 - None, 1 - Emergency, 2 -
       Alert, 3 - Critical, 4 - Error, 5 - Warning, 6 - Notice, 7 - Info, 8 - Debug, 9 - Trace

       logfile is a commandline option to mention log file path which helps  in  logging  to  the
       specified file and also help in persisting the gfapi logs. The default is stderr.

       You can create a GlusterFS disk image with the command:

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

       Examples

          qemu-system-x86_64 -drive file=gluster://1.2.3.4/testvol/a.img
          qemu-system-x86_64 -drive file=gluster+tcp://1.2.3.4/testvol/a.img
          qemu-system-x86_64 -drive file=gluster+tcp://1.2.3.4:24007/testvol/dir/a.img
          qemu-system-x86_64 -drive file=gluster+tcp://[1:2:3:4:5:6:7:8]/testvol/dir/a.img
          qemu-system-x86_64 -drive file=gluster+tcp://[1:2:3:4:5:6:7:8]:24007/testvol/dir/a.img
          qemu-system-x86_64 -drive file=gluster+tcp://server.domain.com:24007/testvol/dir/a.img
          qemu-system-x86_64 -drive file=gluster+unix:///testvol/dir/a.img?socket=/tmp/glusterd.socket
          qemu-system-x86_64 -drive file=gluster+rdma://1.2.3.4:24007/testvol/a.img
          qemu-system-x86_64 -drive file=gluster://1.2.3.4/testvol/a.img,file.debug=9,file.logfile=/var/log/qemu-gluster.log
          qemu-system-x86_64 'json:{"driver":"qcow2",
                                   "file":{"driver":"gluster",
                                            "volume":"testvol","path":"a.img",
                                            "debug":9,"logfile":"/var/log/qemu-gluster.log",
                                            "server":[{"type":"tcp","host":"1.2.3.4","port":24007},
                                                      {"type":"unix","socket":"/var/run/glusterd.socket"}]}}'
          qemu-system-x86_64 -drive driver=qcow2,file.driver=gluster,file.volume=testvol,file.path=/path/a.img,
                                               file.debug=9,file.logfile=/var/log/qemu-gluster.log,
                                               file.server.0.type=tcp,file.server.0.host=1.2.3.4,file.server.0.port=24007,
                                               file.server.1.type=unix,file.server.1.socket=/var/run/glusterd.socket

   Secure Shell (ssh) disk images
       You can access disk images located on a remote ssh server by using the ssh protocol:

          qemu-system-x86_64 -drive file=ssh://[USER@]SERVER[:PORT]/PATH[?host_key_check=HOST_KEY_CHECK]

       Alternative syntax using properties:

          qemu-system-x86_64 -drive file.driver=ssh[,file.user=USER],file.host=SERVER[,file.port=PORT],file.path=PATH[,file.host_key_check=HOST_KEY_CHECK]

       ssh is the protocol.

       USER is the remote user.  If not specified, then the local username is tried.

       SERVER specifies the remote ssh server.  Any ssh server can be used, but it must implement
       the sftp-server protocol.  Most Unix/Linux systems should work without requiring any extra
       configuration.

       PORT is the port number on which sshd is listening.  By default the standard ssh port (22)
       is used.

       PATH is the path to the disk image.

       The optional HOST_KEY_CHECK parameter controls how the remote host's key is checked.   The
       default  is  yes  which  means to use the local .ssh/known_hosts file.  Setting this to no
       turns off known-hosts checking.  Or you can check that the host  key  matches  a  specific
       fingerprint.  The  fingerprint can be provided in md5, sha1, or sha256 format, however, it
       is strongly recommended to only  use  sha256,  since  the  other  options  are  considered
       insecure by modern standards. The fingerprint value must be given as a hex encoded string:

          host_key_check=sha256:04ce2ae89ff4295a6b9c4111640bdcb3297858ee55cb434d9dd88796e93aa795

       The key string may optionally contain ":" separators between each pair of hex digits.

       The  $HOME/.ssh/known_hosts  file  contains  the  base64  encoded  host keys. These can be
       converted into the format needed for QEMU using a command such as:

          $ for key in `grep 10.33.8.112 known_hosts | awk '{print $3}'`
            do
              echo $key | base64 -d | sha256sum
            done
            6c3aa525beda9dc83eadfbd7e5ba7d976ecb59575d1633c87cd06ed2ed6e366f  -
            12214fd9ea5b408086f98ecccd9958609bd9ac7c0ea316734006bc7818b45dc8  -
            d36420137bcbd101209ef70c3b15dc07362fbe0fa53c5b135eba6e6afa82f0ce  -

       Note that there can be multiple keys present per host, each with  different  key  ciphers.
       Care  is  needed  to  pick the key fingerprint that matches the cipher QEMU will negotiate
       with the remote server.

       Currently authentication must be done using ssh-agent.  Other authentication  methods  may
       be supported in future.

       Note:  Many  ssh  servers  do not support an fsync-style operation.  The ssh driver cannot
       guarantee that disk flush requests are obeyed, and this causes a risk of  disk  corruption
       if  the remote server or network goes down during writes.  The driver will print a warning
       when fsync is not supported:

          warning: ssh server ssh.example.com:22 does not support fsync

       With sufficiently new versions of libssh and OpenSSH, fsync is supported.

   NVMe disk images
       NVM Express (NVMe) storage controllers can be accessed directly by a userspace  driver  in
       QEMU.   This  bypasses  the  host kernel file system and block layers while retaining QEMU
       block layer functionalities, such as block jobs, I/O throttling, image formats, etc.  Disk
       I/O  performance  is  typically  higher than with -drive file=/dev/sda using either thread
       pool or linux-aio.

       The controller will be exclusively used by the QEMU process once started. To  be  able  to
       share storage between multiple VMs and other applications on the host, please use the file
       based protocols.

       Before starting QEMU, bind the host NVMe controller to  the  host  vfio-pci  driver.   For
       example:

          # modprobe vfio-pci
          # lspci -n -s 0000:06:0d.0
          06:0d.0 0401: 1102:0002 (rev 08)
          # echo 0000:06:0d.0 > /sys/bus/pci/devices/0000:06:0d.0/driver/unbind
          # echo 1102 0002 > /sys/bus/pci/drivers/vfio-pci/new_id

          # qemu-system-x86_64 -drive file=nvme://HOST:BUS:SLOT.FUNC/NAMESPACE

       Alternative syntax using properties:

          qemu-system-x86_64 -drive file.driver=nvme,file.device=HOST:BUS:SLOT.FUNC,file.namespace=NAMESPACE

       HOST:BUS:SLOT.FUNC is the NVMe controller's PCI device address on the host.

       NAMESPACE is the NVMe namespace number, starting from 1.

   Disk image file locking
       By  default,  QEMU tries to protect image files from unexpected concurrent access, as long
       as it's supported by the block protocol driver and host operating system. If multiple QEMU
       processes  (including  QEMU  emulators  and  utilities)  try  to  open the same image with
       conflicting accessing modes, all but the first one will get an error.

       This feature is currently supported by the file protocol  on  Linux  with  the  Open  File
       Descriptor  (OFD)  locking API, and can be configured to fall back to POSIX locking if the
       POSIX host doesn't support Linux OFD locking.

       To explicitly enable image locking, specify  "locking=on"  in  the  file  protocol  driver
       options.  If  OFD locking is not possible, a warning will be printed and the POSIX locking
       API will be used. In this case there is a risk that the lock will get silently  lost  when
       doing hot plugging and block jobs, due to the shortcomings of the POSIX locking API.

       QEMU  transparently  handles  lock  handover  during shared storage migration.  For shared
       virtual disk images between multiple VMs, the "share-rw" device option should be used.

       By default, the guest has exclusive write access to its  disk  image.  If  the  guest  can
       safely  share  the disk image with other writers the -device ...,share-rw=on parameter can
       be used.  This is only safe if the guest is running  software,  such  as  a  cluster  file
       system, that coordinates disk accesses to avoid corruption.

       Note  that  share-rw=on  only  declares  the guest's ability to share the disk.  Some QEMU
       features, such as image file formats, require exclusive write access to the disk image and
       this is unaffected by the share-rw=on option.

       Alternatively,  locking can be fully disabled by "locking=off" block device option. In the
       command line, the option is usually in the form  of  "file.locking=off"  as  the  protocol
       driver is normally placed as a "file" child under a format driver. For example:

          -blockdev driver=qcow2,file.filename=/path/to/image,file.locking=off,file.driver=file

       To check if image locking is active, check the output of the "lslocks" command on host and
       see if there are locks held by the QEMU process on the image file.   More  than  one  byte
       could  be locked by the QEMU instance, each byte of which reflects a particular permission
       that is acquired or protected by the running block driver.

   Filter drivers
       QEMU supports several filter drivers,  which  don't  store  any  data,  but  perform  some
       additional tasks, hooking io requests.

       preallocate
              The  preallocate  filter  driver  is  intended  to  be  inserted between format and
              protocol nodes and preallocates some additional space (expanding the protocol file)
              when  writing  past  the  file’s end. This can be useful for file-systems with slow
              allocation.

              Supported options:

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

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

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

       2022, The QEMU Project Developers