# 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

You will also need to ensure that the helper program qemu-pr-helper is running, and that it has been set up to use the same socket filename as your QEMU commandline specifies. See the qemu-pr-helper documentation or manpage for further details.

## Multipath devices and persistent reservations

Proper support of persistent reservation for multipath devices requires communication with the multipath daemon, so that the reservation is registered and applied when a path is newly discovered or becomes online again. qemu-pr-helper can do this if the `libmpathpersist` library was available on the system at build time.

As of August 2017, a reservation key must be specified in `multipath.conf` for `multipathd` to check for persistent reservation for newly discovered paths or reinstated paths. The attribute can be added to the `defaults` section or the `multipaths` section; for example:

    multipaths {
        multipath {
            wwid   XXXXXXXXXXXXXXXX
            alias      yellow
            reservation_key  0x123abc
        }
    }

Linking qemu-pr-helper to `libmpathpersist` does not impede its usage on regular SCSI devices.