qemu-storage-daemon - QEMU storage daemon

Copyright

       2024, The QEMU Project Developers

9.2.1                                             Apr 09, 2025                            QEMU-STORAGE-DAEMON(1)

Description

qemu-storage-daemon provides disk image functionality from QEMU, qemu-img, and qemu-nbd in a long-running
       process  controlled  via  QMP commands without running a virtual machine.  It can export disk images, run
       block job operations, and perform other disk-related operations. The  daemon  is  controlled  via  a  QMP
       monitor and initial configuration from the command-line.

       The daemon offers the following subset of QEMU features:

       • Block nodes

       • Block jobs

       • Block exports

       • Throttle groups

       • Character devices

       • Crypto and secrets

       • QMP

       • IOThreads

       Commands    can    be    sent    over    a    QEMU   Monitor   Protocol   (QMP)   connection.   See   the
       qemu-storage-daemon-qmp-ref(7) manual page for a description of the commands.

       The daemon runs until it is stopped using the quit QMP command or SIGINT/SIGHUP/SIGTERM.

       Warning: Never modify images in use by a running virtual machine or any other process; this  may  destroy
       the  image. Also, be aware that querying an image that is being modified by another process may encounter
       inconsistent state.

Examples

       Launch the daemon with QMP monitor socket qmp.sock so clients can execute QMP commands:

          $ qemu-storage-daemon \
              --chardev socket,path=qmp.sock,server=on,wait=off,id=char1 \
              --monitor chardev=char1

       Launch the daemon from Python with a QMP monitor socket using file descriptor passing so there is no need
       to busy wait for the QMP monitor to become available:

          #!/usr/bin/env python3
          import subprocess
          import socket

          sock_path = '/var/run/qmp.sock'

          with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as listen_sock:
              listen_sock.bind(sock_path)
              listen_sock.listen()

              fd = listen_sock.fileno()

              subprocess.Popen(
                  ['qemu-storage-daemon',
                   '--chardev', f'socket,fd={fd},server=on,id=char1',
                   '--monitor', 'chardev=char1'],
                  pass_fds=[fd],
              )

          # listen_sock was automatically closed when leaving the 'with' statement
          # body. If the daemon process terminated early then the following connect()
          # will fail with "Connection refused" because no process has the listen
          # socket open anymore. Launch errors can be detected this way.

          qmp_sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
          qmp_sock.connect(sock_path)
          ...QMP interaction...

       The same socket spawning  approach  also  works  with  the  --nbd-serveraddr.type=fd,addr.str=<fd>  and
       --exporttype=vhost-user-blk,addr.type=fd,addr.str=<fd> options.

       Export raw image file disk.img over NBD UNIX domain socket nbd.sock:

          $ qemu-storage-daemon \
              --blockdev driver=file,node-name=disk,filename=disk.img \
              --nbd-server addr.type=unix,addr.path=nbd.sock \
              --export type=nbd,id=export,node-name=disk,writable=on

       Export   a   qcow2   image   file   disk.qcow2  as  a  vhost-user-blk  device  over  UNIX  domain  socket
       vhost-user-blk.sock:

          $ qemu-storage-daemon \
              --blockdev driver=file,node-name=file,filename=disk.qcow2 \
              --blockdev driver=qcow2,node-name=qcow2,file=file \
              --export type=vhost-user-blk,id=export,addr.type=unix,addr.path=vhost-user-blk.sock,node-name=qcow2

       Export a qcow2 image file disk.qcow2 via FUSE on itself, so the disk image file will then appear as a raw
       image:

          $ qemu-storage-daemon \
              --blockdev driver=file,node-name=file,filename=disk.qcow2 \
              --blockdev driver=qcow2,node-name=qcow2,file=file \
              --export type=fuse,id=export,node-name=qcow2,mountpoint=disk.qcow2,writable=on

Name

       qemu-storage-daemon - QEMU storage daemon

Options

       Standard options:

       -h,--help
              Display help and exit

       -V,--version
              Display version information and exit

       -T,--trace[[enable=]PATTERN][,events=FILE][,file=FILE]
              Specify tracing options.

              [enable=]PATTERN
                 Immediately enable events matching PATTERN (either event name or  a  globbing  pattern).   This
                 option  is  only  available  if  QEMU  has been compiled with the simple, log or ftrace tracing
                 backend.  To specify multiple events or patterns, specify the -trace option multiple times.

                 Use -tracehelp to print a list of names of trace points.

              events=FILE
                 Immediately enable events listed in FILE.  The file must contain one event name (as  listed  in
                 the  trace-events-all  file) per line; globbing patterns are accepted too.  This option is only
                 available if QEMU has been compiled with the simple, log or ftrace tracing backend.

              file=FILE
                 Log output traces to FILE.  This option is only available if QEMU has been  compiled  with  the
                 simple tracing backend.

       --blockdevBLOCKDEVDEF
              is a block node definition. See the qemu-system(1) manual page (in qemu-system-common package) for
              a description of block node properties and the qemu-block-drivers(7) manual page for a description
              of driver-specific parameters.

       --chardevCHARDEVDEF
              is  a  character  device  definition.  See  the  qemu-system(1) manual page (in qemu-system-common
              package) for a description of character device properties. A common  character  device  definition
              configures a UNIX domain socket:

                 --chardev socket,id=char1,path=/var/run/qsd-qmp.sock,server=on,wait=off

       --export[type=]nbd,id=<id>,node-name=<node-name>[,name=<export-name>][,writable=on|off][,bitmap=<name>]--export[type=]vhost-user-blk,id=<id>,node-name=<node-name>,addr.type=unix,addr.path=<socket-path>[,writable=on|off][,logical-block-size=<block-size>][,num-queues=<num-queues>]--export[type=]vhost-user-blk,id=<id>,node-name=<node-name>,addr.type=fd,addr.str=<fd>[,writable=on|off][,logical-block-size=<block-size>][,num-queues=<num-queues>]--export[type=]fuse,id=<id>,node-name=<node-name>,mountpoint=<file>[,growable=on|off][,writable=on|off][,allow-other=on|off|auto]--export[type=]vduse-blk,id=<id>,node-name=<node-name>,name=<vduse-name>[,writable=on|off][,num-queues=<num-queues>][,queue-size=<queue-size>][,logical-block-size=<block-size>][,serial=<serial-number>]
              is  a  block  export  definition.  node-name  is  the block node that should be exported. writable
              determines whether or not the export allows write requests for  modifying  data  (the  default  is
              off).

              The  nbd  export  type  requires  --nbd-server  (see  below).  name is the NBD export name (if not
              specified, it defaults to the given node-name). bitmap is the name of  a  dirty  bitmap  reachable
              from  the block node, so the NBD client can use NBD_OPT_SET_META_CONTEXT with the metadata context
              name "qemu:dirty-bitmap:BITMAP" to inspect the bitmap.

              The vhost-user-blk export type takes a vhost-user socket  address  on  which  it  accept  incoming
              connections.   Both   addr.type=unix,addr.path=<socket-path>   for   UNIX   domain   sockets   and
              addr.type=fd,addr.str=<fd> for file descriptor passing are supported.  logical-block-size sets the
              logical block size in bytes (the default is 512). num-queues sets the number  of  virtqueues  (the
              default is 1).

              The  fuse  export  type  takes a mount point, which must be a regular file, on which to export the
              given block node. That file will not be changed, it will just appear  to  have  the  block  node's
              content  while  the export is active (very much like mounting a filesystem on a directory does not
              change what the directory contains, it only shows a different  content  while  the  filesystem  is
              mounted).  Consequently,  applications  that  have  opened the given file before the export became
              active will continue to see its original content. If growable is set, writes after the end of  the
              exported  file  will  grow  the  block node to fit.  The allow-other option controls whether users
              other than the user running the process will be allowed to access the export.  Note that  enabling
              this  option  as  a  non-root  user  requires  enabling  the user_allow_other option in the global
              fuse.conf configuration file.  Setting allow-other to auto (the default) will  try  enabling  this
              option, and on error fall back to disabling it.

              The  vduse-blk  export  type  takes  a  name  (must be unique across the host) to create the VDUSE
              device.  num-queues sets the number of  virtqueues  (the  default  is  1).   queue-size  sets  the
              virtqueue descriptor table size (the default is 256).

              The  instantiated  VDUSE  device must then be added to the vDPA bus using the vdpa(8) command from
              the iproute2 project:

                 # vdpa dev add name <id> mgmtdev vduse

              The device can be removed from the vDPA bus later as follows:

                 # vdpa dev del <id>

              For more information about attaching vDPA devices to the host  with  virtio_vdpa.ko  or  attaching
              them to guests with vhost_vdpa.ko, see https://vdpa-dev.gitlab.io/.

              For more information about VDUSE, see https://docs.kernel.org/userspace-api/vduse.html.

       --monitorMONITORDEF
              is  a  QMP  monitor definition. See the qemu-system(1) manual page (in qemu-system-common package)
              for a description of QMP monitor properties. A common QMP monitor definition configures a  monitor
              on character device char1:

                 --monitor chardev=char1

       --nbd-serveraddr.type=inet,addr.host=<host>,addr.port=<port>[,tls-creds=<id>][,tls-authz=<id>][,max-connections=<n>]--nbd-serveraddr.type=unix,addr.path=<path>[,tls-creds=<id>][,tls-authz=<id>][,max-connections=<n>]--nbd-serveraddr.type=fd,addr.str=<fd>[,tls-creds=<id>][,tls-authz=<id>][,max-connections=<n>]
              is  a server for NBD exports. Both TCP and UNIX domain sockets are supported.  A listen socket can
              be provided via file descriptor passing (see Examples below). TLS  encryption  can  be  configured
              using --object tls-creds-* and authz-* secrets (see below).

              To configure an NBD server on UNIX domain socket path /var/run/qsd-nbd.sock:

                 --nbd-server addr.type=unix,addr.path=/var/run/qsd-nbd.sock

       --objecthelp--object<type>,help--object<type>[,<property>=<value>...]
              is  a  QEMU user creatable object definition. List object types with help.  List object properties
              with <type>,help. See the qemu(1) manual page for a description of the object properties.

       --pidfilePATH
              is the path to a file where the daemon writes its pid. This allows scripts to stop the  daemon  by
              sending a signal:

                 $ kill -SIGTERM $(<path/to/qsd.pid)

              A  file  lock  is  applied to the file so only one instance of the daemon can run with a given pid
              file path. The daemon unlinks its pid file when terminating.

              The pid file is written after chardevs, exports, and NBD servers  have  been  created  but  before
              accepting  connections.  The  daemon  has  started  successfully  when the pid file is written and
              clients may begin connecting.

       --daemonize
              Daemonize the process. The parent process will exit once startup is complete (i.e., after the  pid
              file  has  been  or would have been written) or failure occurs. Its exit code reflects whether the
              child has started up successfully or failed to do so.