1QEMU-BLOCK-DRIVERS(7) QEMU QEMU-BLOCK-DRIVERS(7)
2
6 qemu-block-drivers - QEMU block drivers reference
7
9 QEMU block driver reference manual
10
12 Disk image file formats
13 QEMU supports many image file formats that can be used with VMs as well
14 as with any of the tools (like qemu-img). This includes the preferred
15 formats raw and qcow2 as well as formats that are supported for compat‐
16 ibility with older QEMU versions or other hypervisors.
17 Depending on the image format, different options can be passed to
19 qemu-img create and qemu-img convert using the -o option. This section
20 describes each format and the options that are supported for it.
21 raw Raw disk image format. This format has the advantage of being
23 simple and easily exportable to all other emulators. If your
24 file system supports holes (for example in ext2 or ext3 on Linux
25 or NTFS on Windows), then only the written sectors will reserve
26 space. Use qemu-img info to know the real size used by the image
27 or ls -ls on Unix/Linux.
28 Supported options:
30 preallocation
32 Preallocation mode (allowed values: off, falloc, full).
33 falloc mode preallocates space for image by calling
34 posix_fallocate(). full mode preallocates space for image
35 by writing data to underlying storage. This data may or
36 may not be zero, depending on the storage location.
37 qcow2 QEMU image format, the most versatile format. Use it to have
39 smaller images (useful if your filesystem does not supports
40 holes, for example on Windows), zlib based compression and sup‐
41 port of multiple VM snapshots.
42 Supported options:
44 compat Determines the qcow2 version to use. compat=0.10 uses the
46 traditional image format that can be read by any QEMU
47 since 0.10. compat=1.1 enables image format extensions
48 that only QEMU 1.1 and newer understand (this is the de‐
49 fault). Amongst others, this includes zero clusters,
50 which allow efficient copy-on-read for sparse images.
51 backing_file
53 File name of a base image (see create subcommand)
54 backing_fmt
56 Image format of the base image
57 encryption
59 This option is deprecated and equivalent to encrypt.for‐
60 mat=aes
61 encrypt.format
63 If this is set to luks, it requests that the qcow2 pay‐
64 load (not qcow2 header) be encrypted using the LUKS for‐
65 mat. The passphrase to use to unlock the LUKS key slot is
66 given by the encrypt.key-secret parameter. LUKS encryp‐
67 tion parameters can be tuned with the other encrypt.* pa‐
68 rameters.
69 If this is set to aes, the image is encrypted with
71 128-bit AES-CBC. The encryption key is given by the en‐
72 crypt.key-secret parameter. This encryption format is
73 considered to be flawed by modern cryptography standards,
74 suffering from a number of design problems:
75 • The AES-CBC cipher is used with predictable initializa‐
77 tion vectors based on the sector number. This makes it
78 vulnerable to chosen plaintext attacks which can reveal
79 the existence of encrypted data.
80 • The user passphrase is directly used as the encryption
82 key. A poorly chosen or short passphrase will compro‐
83 mise the security of the encryption.
84 • In the event of the passphrase being compromised there
86 is no way to change the passphrase to protect data in
87 any qcow images. The files must be cloned, using a dif‐
88 ferent encryption passphrase in the new file. The orig‐
89 inal file must then be securely erased using a program
90 like shred, though even this is ineffective with many
91 modern storage technologies.
92 The use of this is no longer supported in system emula‐
94 tors. Support only remains in the command line utilities,
95 for the purposes of data liberation and interoperability
96 with old versions of QEMU. The luks format should be used
97 instead.
98 encrypt.key-secret
100 Provides the ID of a secret object that contains the
101 passphrase (encrypt.format=luks) or encryption key (en‐
102 crypt.format=aes).
103 encrypt.cipher-alg
105 Name of the cipher algorithm and key length. Currently
106 defaults to aes-256. Only used when encrypt.format=luks.
107 encrypt.cipher-mode
109 Name of the encryption mode to use. Currently defaults to
110 xts. Only used when encrypt.format=luks.
111 encrypt.ivgen-alg
113 Name of the initialization vector generator algorithm.
114 Currently defaults to plain64. Only used when en‐
115 crypt.format=luks.
116 encrypt.ivgen-hash-alg
118 Name of the hash algorithm to use with the initialization
119 vector generator (if required). Defaults to sha256. Only
120 used when encrypt.format=luks.
121 encrypt.hash-alg
123 Name of the hash algorithm to use for PBKDF algorithm De‐
124 faults to sha256. Only used when encrypt.format=luks.
125 encrypt.iter-time
127 Amount of time, in milliseconds, to use for PBKDF algo‐
128 rithm per key slot. Defaults to 2000. Only used when en‐
129 crypt.format=luks.
130 cluster_size
132 Changes the qcow2 cluster size (must be between 512 and
133 2M). Smaller cluster sizes can improve the image file
134 size whereas larger cluster sizes generally provide bet‐
135 ter performance.
136 preallocation
138 Preallocation mode (allowed values: off, metadata, fal‐
139 loc, full). An image with preallocated metadata is ini‐
140 tially larger but can improve performance when the image
141 needs to grow. falloc and full preallocations are like
142 the same options of raw format, but sets up metadata
143 also.
144 lazy_refcounts
146 If this option is set to on, reference count updates are
147 postponed with the goal of avoiding metadata I/O and im‐
148 proving performance. This is particularly interesting
149 with cache=writethrough which doesn't batch metadata up‐
150 dates. The tradeoff is that after a host crash, the ref‐
151 erence count tables must be rebuilt, i.e. on the next
152 open an (automatic) qemu-img check -r all is required,
153 which may take some time.
154 This option can only be enabled if compat=1.1 is speci‐
156 fied.
157 nocow If this option is set to on, it will turn off COW of the
159 file. It's only valid on btrfs, no effect on other file
160 systems.
161 Btrfs has low performance when hosting a VM image file,
163 even more when the guest on the VM also using btrfs as
164 file system. Turning off COW is a way to mitigate this
165 bad performance. Generally there are two ways to turn off
166 COW on btrfs:
167 • Disable it by mounting with nodatacow, then all newly
169 created files will be NOCOW.
170 • For an empty file, add the NOCOW file attribute. That's
172 what this option does.
173 Note: this option is only valid to new or empty files. If
175 there is an existing file which is COW and has data
176 blocks already, it couldn't be changed to NOCOW by set‐
177 ting nocow=on. One can issue lsattr filename to check if
178 the NOCOW flag is set or not (Capital 'C' is NOCOW flag).
179 qed Old QEMU image format with support for backing files and compact
181 image files (when your filesystem or transport medium does not
182 support holes).
183 When converting QED images to qcow2, you might want to consider
185 using the lazy_refcounts=on option to get a more QED-like behav‐
186 iour.
187 Supported options:
189 backing_file
191 File name of a base image (see create subcommand).
192 backing_fmt
194 Image file format of backing file (optional). Useful if
195 the format cannot be autodetected because it has no
196 header, like some vhd/vpc files.
197 cluster_size
199 Changes the cluster size (must be power-of-2 between 4K
200 and 64K). Smaller cluster sizes can improve the image
201 file size whereas larger cluster sizes generally provide
202 better performance.
203 table_size
205 Changes the number of clusters per L1/L2 table (must be
206 power-of-2 between 1 and 16). There is normally no need
207 to change this value but this option can between used for
208 performance benchmarking.
209 qcow Old QEMU image format with support for backing files, compact
211 image files, encryption and compression.
212 Supported options:
214 backing_file
216 File name of a base image (see create subcommand)
217 encryption
219 This option is deprecated and equivalent to en‐
220 crypt.format=aes
221 encrypt.format
223 If this is set to aes, the image is encrypted with
224 128-bit AES-CBC. The encryption key is given by the
225 encrypt.key-secret parameter. This encryption format
226 is considered to be flawed by modern cryptography
227 standards, suffering from a number of design problems
228 enumerated previously against the qcow2 image format.
229 The use of this is no longer supported in system emu‐
231 lators. Support only remains in the command line util‐
232 ities, for the purposes of data liberation and inter‐
233 operability with old versions of QEMU.
234 Users requiring native encryption should use the qcow2
236 format instead with encrypt.format=luks.
237 encrypt.key-secret
239 Provides the ID of a secret object that contains the
240 encryption key (encrypt.format=aes).
241 luks LUKS v1 encryption format, compatible with Linux dm-crypt/crypt‐
243 setup
244 Supported options:
246 key-secret
248 Provides the ID of a secret object that contains the
249 passphrase.
250 cipher-alg
252 Name of the cipher algorithm and key length. Currently
253 defaults to aes-256.
254 cipher-mode
256 Name of the encryption mode to use. Currently defaults to
257 xts.
258 ivgen-alg
260 Name of the initialization vector generator algorithm.
261 Currently defaults to plain64.
262 ivgen-hash-alg
264 Name of the hash algorithm to use with the initialization
265 vector generator (if required). Defaults to sha256.
266 hash-alg
268 Name of the hash algorithm to use for PBKDF algorithm De‐
269 faults to sha256.
270 iter-time
272 Amount of time, in milliseconds, to use for PBKDF algo‐
273 rithm per key slot. Defaults to 2000.
274 vdi VirtualBox 1.1 compatible image format.
276 Supported options:
278 static If this option is set to on, the image is created with
280 metadata preallocation.
281 vmdk VMware 3 and 4 compatible image format.
283 Supported options:
285 backing_file
287 File name of a base image (see create subcommand).
288 compat6
290 Create a VMDK version 6 image (instead of version 4)
291 hwversion
293 Specify vmdk virtual hardware version. Compat6 flag can‐
294 not be enabled if hwversion is specified.
295 subformat
297 Specifies which VMDK subformat to use. Valid options are
298 monolithicSparse (default), monolithicFlat, twoGbMaxEx‐
299 tentSparse, twoGbMaxExtentFlat and streamOptimized.
300 vpc VirtualPC compatible image format (VHD).
302 Supported options:
304 subformat
306 Specifies which VHD subformat to use. Valid options are
307 dynamic (default) and fixed.
308 VHDX Hyper-V compatible image format (VHDX).
310 Supported options:
312 subformat
314 Specifies which VHDX subformat to use. Valid options are
315 dynamic (default) and fixed.
316 block_state_zero
318 Force use of payload blocks of type 'ZERO'. Can
319 be set to on (default) or off. When set to off,
320 new blocks will be created as PAY‐
321 LOAD_BLOCK_NOT_PRESENT, which means parsers are
322 free to return arbitrary data for those blocks.
323 Do not set to off when using qemu-img convert with
324 subformat=dynamic.
325 block_size
327 Block size; min 1 MB, max 256 MB. 0 means
328 auto-calculate based on image size.
329 log_size
331 Log size; min 1 MB.
332 Read-only formats
334 More disk image file formats are supported in a read-only mode.
335 bochs Bochs images of growing type.
337 cloop Linux Compressed Loop image, useful only to reuse directly com‐
339 pressed CD-ROM images present for example in the Knoppix
340 CD-ROMs.
341 dmg Apple disk image.
343 parallels
345 Parallels disk image format.
346 Using host drives
348 In addition to disk image files, QEMU can directly access host devices.
349 We describe here the usage for QEMU version >= 0.8.3.
350 Linux
352 On Linux, you can directly use the host device filename instead of a
353 disk image filename provided you have enough privileges to access it.
354 For example, use /dev/cdrom to access to the CDROM.
355 CD You can specify a CDROM device even if no CDROM is loaded. QEMU
357 has specific code to detect CDROM insertion or removal. CDROM
358 ejection by the guest OS is supported. Currently only data CDs
359 are supported.
360 Floppy You can specify a floppy device even if no floppy is loaded.
362 Floppy removal is currently not detected accurately (if you
363 change floppy without doing floppy access while the floppy is
364 not loaded, the guest OS will think that the same floppy is
365 loaded). Use of the host's floppy device is deprecated, and
366 support for it will be removed in a future release.
367 Hard disks
369 Hard disks can be used. Normally you must specify the whole disk
370 (/dev/hdb instead of /dev/hdb1) so that the guest OS can see it
371 as a partitioned disk. WARNING: unless you know what you do, it
372 is better to only make READ-ONLY accesses to the hard disk oth‐
373 erwise you may corrupt your host data (use the -snapshot command
374 line option or modify the device permissions accordingly).
375 Windows
377 CD The preferred syntax is the drive letter (e.g. d:). The alter‐
378 nate syntax \\.\d: is supported. /dev/cdrom is supported as an
379 alias to the first CDROM drive.
380 Currently there is no specific code to handle removable media,
382 so it is better to use the change or eject monitor commands to
383 change or eject media.
384 Hard disks
386 Hard disks can be used with the syntax: \\.\PhysicalDriveN where
387 N is the drive number (0 is the first hard disk).
388 WARNING: unless you know what you do, it is better to only make
390 READ-ONLY accesses to the hard disk otherwise you may corrupt
391 your host data (use the -snapshot command line so that the modi‐
392 fications are written in a temporary file).
393 Mac OS X
395 /dev/cdrom is an alias to the first CDROM.
396 Currently there is no specific code to handle removable media, so it is
398 better to use the change or eject monitor commands to change or eject
399 media.
400 Virtual FAT disk images
402 QEMU can automatically create a virtual FAT disk image from a directory
403 tree. In order to use it, just type:
404 qemu-system-x86_64 linux.img -hdb fat:/my_directory
406 Then you access access to all the files in the /my_directory directory
408 without having to copy them in a disk image or to export them via SAMBA
409 or NFS. The default access is read-only.
410 Floppies can be emulated with the :floppy: option:
412 qemu-system-x86_64 linux.img -fda fat:floppy:/my_directory
414 A read/write support is available for testing (beta stage) with the
416 :rw: option:
417 qemu-system-x86_64 linux.img -fda fat:floppy:rw:/my_directory
419 What you should never do:
421 • use non-ASCII filenames
423 • use "-snapshot" together with ":rw:"
425 • expect it to work when loadvm'ing
427 • write to the FAT directory on the host system while accessing it with
429 the guest system
430 NBD access
432 QEMU can access directly to block device exported using the Network
433 Block Device protocol.
434 qemu-system-x86_64 linux.img -hdb nbd://my_nbd_server.mydomain.org:1024/
436 If the NBD server is located on the same host, you can use an unix
438 socket instead of an inet socket:
439 qemu-system-x86_64 linux.img -hdb nbd+unix://?socket=/tmp/my_socket
441 In this case, the block device must be exported using qemu-nbd:
443 qemu-nbd --socket=/tmp/my_socket my_disk.qcow2
445 The use of qemu-nbd allows sharing of a disk between several guests:
447 qemu-nbd --socket=/tmp/my_socket --share=2 my_disk.qcow2
449 and then you can use it with two guests:
451 qemu-system-x86_64 linux1.img -hdb nbd+unix://?socket=/tmp/my_socket
453 qemu-system-x86_64 linux2.img -hdb nbd+unix://?socket=/tmp/my_socket
454 If the nbd-server uses named exports (supported since NBD 2.9.18, or
456 with QEMU's own embedded NBD server), you must specify an export name
457 in the URI:
458 qemu-system-x86_64 -cdrom nbd://localhost/debian-500-ppc-netinst
460 qemu-system-x86_64 -cdrom nbd://localhost/openSUSE-11.1-ppc-netinst
461 The URI syntax for NBD is supported since QEMU 1.3. An alternative
463 syntax is also available. Here are some example of the older syntax:
464 qemu-system-x86_64 linux.img -hdb nbd:my_nbd_server.mydomain.org:1024
466 qemu-system-x86_64 linux2.img -hdb nbd:unix:/tmp/my_socket
467 qemu-system-x86_64 -cdrom nbd:localhost:10809:exportname=debian-500-ppc-netinst
468 iSCSI LUNs
470 iSCSI is a popular protocol used to access SCSI devices across a com‐
471 puter network.
472 There are two different ways iSCSI devices can be used by QEMU.
474 The first method is to mount the iSCSI LUN on the host, and make it ap‐
476 pear as any other ordinary SCSI device on the host and then to access
477 this device as a /dev/sd device from QEMU. How to do this differs be‐
478 tween host OSes.
479 The second method involves using the iSCSI initiator that is built into
481 QEMU. This provides a mechanism that works the same way regardless of
482 which host OS you are running QEMU on. This section will describe this
483 second method of using iSCSI together with QEMU.
484 In QEMU, iSCSI devices are described using special iSCSI URLs. URL syn‐
486 tax:
487 iscsi://[<username>[%<password>]@]<host>[:<port>]/<target-iqn-name>/<lun>
489 Username and password are optional and only used if your target is set
491 up using CHAP authentication for access control. Alternatively the
492 username and password can also be set via environment variables to have
493 these not show up in the process list:
494 export LIBISCSI_CHAP_USERNAME=<username>
496 export LIBISCSI_CHAP_PASSWORD=<password>
497 iscsi://<host>/<target-iqn-name>/<lun>
498 Various session related parameters can be set via special options, ei‐
500 ther in a configuration file provided via '-readconfig' or directly on
501 the command line.
502 If the initiator-name is not specified qemu will use a default name of
504 'iqn.2008-11.org.linux-kvm[:<uuid>'] where <uuid> is the UUID of the
505 virtual machine. If the UUID is not specified qemu will use
506 'iqn.2008-11.org.linux-kvm[:<name>'] where <name> is the name of the
507 virtual machine.
508 Setting a specific initiator name to use when logging in to the target:
510 -iscsi initiator-name=iqn.qemu.test:my-initiator
512 Controlling which type of header digest to negotiate with the target:
514 -iscsi header-digest=CRC32C|CRC32C-NONE|NONE-CRC32C|NONE
516 These can also be set via a configuration file:
518 [iscsi]
520 user = "CHAP username"
521 password = "CHAP password"
522 initiator-name = "iqn.qemu.test:my-initiator"
523 # header digest is one of CRC32C|CRC32C-NONE|NONE-CRC32C|NONE
524 header-digest = "CRC32C"
525 Setting the target name allows different options for different targets:
527 [iscsi "iqn.target.name"]
529 user = "CHAP username"
530 password = "CHAP password"
531 initiator-name = "iqn.qemu.test:my-initiator"
532 # header digest is one of CRC32C|CRC32C-NONE|NONE-CRC32C|NONE
533 header-digest = "CRC32C"
534 How to use a configuration file to set iSCSI configuration options:
536 cat >iscsi.conf <<EOF
538 [iscsi]
539 user = "me"
540 password = "my password"
541 initiator-name = "iqn.qemu.test:my-initiator"
542 header-digest = "CRC32C"
543 EOF
544 qemu-system-x86_64 -drive file=iscsi://127.0.0.1/iqn.qemu.test/1 \
546 -readconfig iscsi.conf
547 How to set up a simple iSCSI target on loopback and access it via QEMU:
549 this example shows how to set up an iSCSI target with one CDROM and one
550 DISK using the Linux STGT software target. This target is available on
551 Red Hat based systems as the package 'scsi-target-utils'.
552 tgtd --iscsi portal=127.0.0.1:3260
554 tgtadm --lld iscsi --op new --mode target --tid 1 -T iqn.qemu.test
555 tgtadm --lld iscsi --mode logicalunit --op new --tid 1 --lun 1 \
556 -b /IMAGES/disk.img --device-type=disk
557 tgtadm --lld iscsi --mode logicalunit --op new --tid 1 --lun 2 \
558 -b /IMAGES/cd.iso --device-type=cd
559 tgtadm --lld iscsi --op bind --mode target --tid 1 -I ALL
560 qemu-system-x86_64 -iscsi initiator-name=iqn.qemu.test:my-initiator \
562 -boot d -drive file=iscsi://127.0.0.1/iqn.qemu.test/1 \
563 -cdrom iscsi://127.0.0.1/iqn.qemu.test/2
564 GlusterFS disk images
566 GlusterFS is a user space distributed file system.
567 You can boot from the GlusterFS disk image with the command:
569 URI:
571 qemu-system-x86_64 -drive file=gluster[+TYPE]://[HOST}[:PORT]]/VOLUME/PATH
573 [?socket=...][,file.debug=9][,file.logfile=...]
574 JSON:
576 qemu-system-x86_64 'json:{"driver":"qcow2",
578 "file":{"driver":"gluster",
579 "volume":"testvol","path":"a.img","debug":9,"logfile":"...",
580 "server":[{"type":"tcp","host":"...","port":"..."},
581 {"type":"unix","socket":"..."}]}}'
582 gluster is the protocol.
584 TYPE specifies the transport type used to connect to gluster management
586 daemon (glusterd). Valid transport types are tcp and unix. In the URI
587 form, if a transport type isn't specified, then tcp type is assumed.
588 HOST specifies the server where the volume file specification for the
590 given volume resides. This can be either a hostname or an ipv4 address.
591 If transport type is unix, then HOST field should not be specified.
592 Instead socket field needs to be populated with the path to unix domain
593 socket.
594 PORT is the port number on which glusterd is listening. This is op‐
596 tional and if not specified, it defaults to port 24007. If the trans‐
597 port type is unix, then PORT should not be specified.
598 VOLUME is the name of the gluster volume which contains the disk image.
600 PATH is the path to the actual disk image that resides on gluster vol‐
602 ume.
603 debug is the logging level of the gluster protocol driver. Debug levels
605 are 0-9, with 9 being the most verbose, and 0 representing no debugging
606 output. The default level is 4. The current logging levels defined in
607 the gluster source are 0 - None, 1 - Emergency, 2 - Alert, 3 - Criti‐
608 cal, 4 - Error, 5 - Warning, 6 - Notice, 7 - Info, 8 - Debug, 9 - Trace
609 logfile is a commandline option to mention log file path which helps in
611 logging to the specified file and also help in persisting the gfapi
612 logs. The default is stderr.
613 You can create a GlusterFS disk image with the command:
615 qemu-img create gluster://HOST/VOLUME/PATH SIZE
617 Examples
619 qemu-system-x86_64 -drive file=gluster://1.2.3.4/testvol/a.img
621 qemu-system-x86_64 -drive file=gluster+tcp://1.2.3.4/testvol/a.img
622 qemu-system-x86_64 -drive file=gluster+tcp://1.2.3.4:24007/testvol/dir/a.img
623 qemu-system-x86_64 -drive file=gluster+tcp://[1:2:3:4:5:6:7:8]/testvol/dir/a.img
624 qemu-system-x86_64 -drive file=gluster+tcp://[1:2:3:4:5:6:7:8]:24007/testvol/dir/a.img
625 qemu-system-x86_64 -drive file=gluster+tcp://server.domain.com:24007/testvol/dir/a.img
626 qemu-system-x86_64 -drive file=gluster+unix:///testvol/dir/a.img?socket=/tmp/glusterd.socket
627 qemu-system-x86_64 -drive file=gluster+rdma://1.2.3.4:24007/testvol/a.img
628 qemu-system-x86_64 -drive file=gluster://1.2.3.4/testvol/a.img,file.debug=9,file.logfile=/var/log/qemu-gluster.log
629 qemu-system-x86_64 'json:{"driver":"qcow2",
630 "file":{"driver":"gluster",
631 "volume":"testvol","path":"a.img",
632 "debug":9,"logfile":"/var/log/qemu-gluster.log",
633 "server":[{"type":"tcp","host":"1.2.3.4","port":24007},
634 {"type":"unix","socket":"/var/run/glusterd.socket"}]}}'
635 qemu-system-x86_64 -drive driver=qcow2,file.driver=gluster,file.volume=testvol,file.path=/path/a.img,
636 file.debug=9,file.logfile=/var/log/qemu-gluster.log,
637 file.server.0.type=tcp,file.server.0.host=1.2.3.4,file.server.0.port=24007,
638 file.server.1.type=unix,file.server.1.socket=/var/run/glusterd.socket
639 Secure Shell (ssh) disk images
641 You can access disk images located on a remote ssh server by using the
642 ssh protocol:
643 qemu-system-x86_64 -drive file=ssh://[USER@]SERVER[:PORT]/PATH[?host_key_check=HOST_KEY_CHECK]
645 Alternative syntax using properties:
647 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]
649 ssh is the protocol.
651 USER is the remote user. If not specified, then the local username is
653 tried.
654 SERVER specifies the remote ssh server. Any ssh server can be used,
656 but it must implement the sftp-server protocol. Most Unix/Linux sys‐
657 tems should work without requiring any extra configuration.
658 PORT is the port number on which sshd is listening. By default the
660 standard ssh port (22) is used.
661 PATH is the path to the disk image.
663 The optional HOST_KEY_CHECK parameter controls how the remote host's
665 key is checked. The default is yes which means to use the local
666 .ssh/known_hosts file. Setting this to no turns off known-hosts check‐
667 ing. Or you can check that the host key matches a specific finger‐
668 print:
669 host_key_check=md5:78:45:8e:14:57:4f:d5:45:83:0a:0e:f3:49:82:c9:c8
670 (sha1: can also be used as a prefix, but note that OpenSSH tools only
671 use MD5 to print fingerprints).
672 Currently authentication must be done using ssh-agent. Other authenti‐
674 cation methods may be supported in future.
675 Note: Many ssh servers do not support an fsync-style operation. The
677 ssh driver cannot guarantee that disk flush requests are obeyed, and
678 this causes a risk of disk corruption if the remote server or network
679 goes down during writes. The driver will print a warning when fsync is
680 not supported:
681 warning: ssh server ssh.example.com:22 does not support fsync
683 With sufficiently new versions of libssh and OpenSSH, fsync is sup‐
685 ported.
686 NVMe disk images
688 NVM Express (NVMe) storage controllers can be accessed directly by a
689 userspace driver in QEMU. This bypasses the host kernel file system
690 and block layers while retaining QEMU block layer functionalities, such
691 as block jobs, I/O throttling, image formats, etc. Disk I/O perfor‐
692 mance is typically higher than with -drive file=/dev/sda using either
693 thread pool or linux-aio.
694 The controller will be exclusively used by the QEMU process once
696 started. To be able to share storage between multiple VMs and other ap‐
697 plications on the host, please use the file based protocols.
698 Before starting QEMU, bind the host NVMe controller to the host
700 vfio-pci driver. For example:
701 # modprobe vfio-pci
703 # lspci -n -s 0000:06:0d.0
704 06:0d.0 0401: 1102:0002 (rev 08)
705 # echo 0000:06:0d.0 > /sys/bus/pci/devices/0000:06:0d.0/driver/unbind
706 # echo 1102 0002 > /sys/bus/pci/drivers/vfio-pci/new_id
707 # qemu-system-x86_64 -drive file=nvme://HOST:BUS:SLOT.FUNC/NAMESPACE
709 Alternative syntax using properties:
711 qemu-system-x86_64 -drive file.driver=nvme,file.device=HOST:BUS:SLOT.FUNC,file.namespace=NAMESPACE
713 HOST:BUS:SLOT.FUNC is the NVMe controller's PCI device address on the
715 host.
716 NAMESPACE is the NVMe namespace number, starting from 1.
718 Disk image file locking
720 By default, QEMU tries to protect image files from unexpected concur‐
721 rent access, as long as it's supported by the block protocol driver and
722 host operating system. If multiple QEMU processes (including QEMU emu‐
723 lators and utilities) try to open the same image with conflicting ac‐
724 cessing modes, all but the first one will get an error.
725 This feature is currently supported by the file protocol on Linux with
727 the Open File Descriptor (OFD) locking API, and can be configured to
728 fall back to POSIX locking if the POSIX host doesn't support Linux OFD
729 locking.
730 To explicitly enable image locking, specify "locking=on" in the file
732 protocol driver options. If OFD locking is not possible, a warning will
733 be printed and the POSIX locking API will be used. In this case there
734 is a risk that the lock will get silently lost when doing hot plugging
735 and block jobs, due to the shortcomings of the POSIX locking API.
736 QEMU transparently handles lock handover during shared storage migra‐
738 tion. For shared virtual disk images between multiple VMs, the
739 "share-rw" device option should be used.
740 By default, the guest has exclusive write access to its disk image. If
742 the guest can safely share the disk image with other writers the -de‐
743 vice ...,share-rw=on parameter can be used. This is only safe if the
744 guest is running software, such as a cluster file system, that coordi‐
745 nates disk accesses to avoid corruption.
746 Note that share-rw=on only declares the guest's ability to share the
748 disk. Some QEMU features, such as image file formats, require exclu‐
749 sive write access to the disk image and this is unaffected by the
750 share-rw=on option.
751 Alternatively, locking can be fully disabled by "locking=off" block de‐
753 vice option. In the command line, the option is usually in the form of
754 "file.locking=off" as the protocol driver is normally placed as a
755 "file" child under a format driver. For example:
756 -blockdev driver=qcow2,file.filename=/path/to/image,file.locking=off,file.driver=file
758 To check if image locking is active, check the output of the "lslocks"
760 command on host and see if there are locks held by the QEMU process on
761 the image file. More than one byte could be locked by the QEMU in‐
762 stance, each byte of which reflects a particular permission that is ac‐
763 quired or protected by the running block driver.
764 Filter drivers
766 QEMU supports several filter drivers, which don't store any data, but
767 perform some additional tasks, hooking io requests.
768 preallocate
770 The preallocate filter driver is intended to be inserted between
771 format and protocol nodes and preallocates some additional space
772 (expanding the protocol file) when writing past the file’s end.
773 This can be useful for file-systems with slow allocation.
774 Supported options:
776 prealloc-align
778 On preallocation, align the file length to this value (in
779 bytes), default 1M.
780 prealloc-size
782 How much to preallocate (in bytes), default 128M.
783
785 The HTML documentation of QEMU for more precise information and Linux
786 user mode emulator invocation.
787
789 Fabrice Bellard and the QEMU Project developers
790
792 2022, The QEMU Project Developers
7936.2.0 Jun 11, 2022 QEMU-BLOCK-DRIVERS(7)