Re: [Qemu-devel] [PATCH 06/10] scsi, file-posix: add support for persistent reservation management

[Fam Zheng <famz@redhat.com>]

On Tue, 08/22 15:18, Paolo Bonzini wrote:
> diff --git a/docs/pr-manager.rst b/docs/pr-manager.rst

Is docs/interop/persistent-reservation-manager.rst better? (Move to interop/ and
de-abbreviate) ...

> 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.

Or is this abstraction class the reason it is not under interop? Why not just
define the protocol?

> +
> +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

Fam