mirror of https://github.com/proxmox/mirror_qemu
52 lines
2.4 KiB
ReStructuredText
52 lines
2.4 KiB
ReStructuredText
![]() |
======================================
|
||
|
Persistent reservation managers
|
||
|
======================================
|
||
|
|
||
|
SCSI persistent Reservations allow restricting access to block devices
|
||
|
to specific initiators in a shared storage setup. When implementing
|
||
|
clustering of virtual machines, it is a common requirement for virtual
|
||
|
machines to send persistent reservation SCSI commands. However,
|
||
|
the operating system restricts sending these commands to unprivileged
|
||
|
programs because incorrect usage can disrupt regular operation of the
|
||
|
storage fabric.
|
||
|
|
||
|
For this reason, QEMU's SCSI passthrough devices, ``scsi-block``
|
||
|
and ``scsi-generic`` (both are only available on Linux) can delegate
|
||
|
implementation of persistent reservations to a separate object,
|
||
|
the "persistent reservation manager". Only PERSISTENT RESERVE OUT and
|
||
|
PERSISTENT RESERVE IN commands are passed to the persistent reservation
|
||
|
manager object; other commands are processed by QEMU as usual.
|
||
|
|
||
|
-----------------------------------------
|
||
|
Defining a persistent reservation manager
|
||
|
-----------------------------------------
|
||
|
|
||
|
A persistent reservation manager is an instance of a subclass of the
|
||
|
"pr-manager" QOM class.
|
||
|
|
||
|
Right now only one subclass is defined, ``pr-manager-helper``, which
|
||
|
forwards the commands to an external privileged helper program
|
||
|
over Unix sockets. The helper program only allows sending persistent
|
||
|
reservation commands to devices for which QEMU has a file descriptor,
|
||
|
so that QEMU will not be able to effect persistent reservations
|
||
|
unless it has access to both the socket and the device.
|
||
|
|
||
|
``pr-manager-helper`` has a single string property, ``path``, which
|
||
|
accepts the path to the helper program's Unix socket. For example,
|
||
|
the following command line defines a ``pr-manager-helper`` object and
|
||
|
attaches it to a SCSI passthrough device::
|
||
|
|
||
|
$ qemu-system-x86_64
|
||
|
-device virtio-scsi \
|
||
|
-object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
|
||
|
-drive if=none,id=hd,driver=raw,file.filename=/dev/sdb,file.pr-manager=helper0
|
||
|
-device scsi-block,drive=hd
|
||
|
|
||
|
Alternatively, using ``-blockdev``::
|
||
|
|
||
|
$ qemu-system-x86_64
|
||
|
-device virtio-scsi \
|
||
|
-object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
|
||
|
-blockdev node-name=hd,driver=raw,file.driver=host_device,file.filename=/dev/sdb,file.pr-manager=helper0
|
||
|
-device scsi-block,drive=hd
|