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-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,
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,
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:
-device virtio-scsi \
-device virtio-scsi \
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
library was available on the system at build time.
As of August 2017, a reservation key must be specified in
multipathd to check for persistent reservation for newly
discovered paths or reinstated paths. The attribute can be added
defaults section or the
multipaths section; for example:
Linking qemu-pr-helper to
libmpathpersist does not impede
its usage on regular SCSI devices.