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diff --git a/docs/pr-manager.rst b/docs/pr-manager.rst new file mode 100644 index 0000000000..b6089fb57c --- /dev/null +++ b/docs/pr-manager.rst @@ -0,0 +1,51 @@ +====================================== +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 |