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Fix a couple of nits in pr-manager.rst: * the title marker for the top level heading is overlength * stray capital 'R' in the middle of a sentence Signed-off-by: Peter Maydell <peter.maydell@linaro.org> Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
84 lines
3.6 KiB
ReStructuredText
84 lines
3.6 KiB
ReStructuredText
===============================
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Persistent reservation managers
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===============================
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SCSI persistent reservations allow restricting access to block devices
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to specific initiators in a shared storage setup. When implementing
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clustering of virtual machines, it is a common requirement for virtual
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machines to send persistent reservation SCSI commands. However,
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the operating system restricts sending these commands to unprivileged
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programs because incorrect usage can disrupt regular operation of the
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storage fabric.
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For this reason, QEMU's SCSI passthrough devices, ``scsi-block``
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and ``scsi-generic`` (both are only available on Linux) can delegate
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implementation of persistent reservations to a separate object,
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the "persistent reservation manager". Only PERSISTENT RESERVE OUT and
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PERSISTENT RESERVE IN commands are passed to the persistent reservation
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manager object; other commands are processed by QEMU as usual.
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-----------------------------------------
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Defining a persistent reservation manager
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-----------------------------------------
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A persistent reservation manager is an instance of a subclass of the
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"pr-manager" QOM class.
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Right now only one subclass is defined, ``pr-manager-helper``, which
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forwards the commands to an external privileged helper program
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over Unix sockets. The helper program only allows sending persistent
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reservation commands to devices for which QEMU has a file descriptor,
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so that QEMU will not be able to effect persistent reservations
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unless it has access to both the socket and the device.
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``pr-manager-helper`` has a single string property, ``path``, which
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accepts the path to the helper program's Unix socket. For example,
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the following command line defines a ``pr-manager-helper`` object and
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attaches it to a SCSI passthrough device::
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$ qemu-system-x86_64
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-device virtio-scsi \
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-object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
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-drive if=none,id=hd,driver=raw,file.filename=/dev/sdb,file.pr-manager=helper0
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-device scsi-block,drive=hd
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Alternatively, using ``-blockdev``::
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$ qemu-system-x86_64
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-device virtio-scsi \
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-object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock
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-blockdev node-name=hd,driver=raw,file.driver=host_device,file.filename=/dev/sdb,file.pr-manager=helper0
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-device scsi-block,drive=hd
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You will also need to ensure that the helper program
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:command:`qemu-pr-helper` is running, and that it has been
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set up to use the same socket filename as your QEMU commandline
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specifies. See the qemu-pr-helper documentation or manpage for
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further details.
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---------------------------------------------
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Multipath devices and persistent reservations
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---------------------------------------------
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Proper support of persistent reservation for multipath devices requires
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communication with the multipath daemon, so that the reservation is
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registered and applied when a path is newly discovered or becomes online
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again. :command:`qemu-pr-helper` can do this if the ``libmpathpersist``
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library was available on the system at build time.
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As of August 2017, a reservation key must be specified in ``multipath.conf``
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for ``multipathd`` to check for persistent reservation for newly
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discovered paths or reinstated paths. The attribute can be added
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to the ``defaults`` section or the ``multipaths`` section; for example::
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multipaths {
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multipath {
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wwid XXXXXXXXXXXXXXXX
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alias yellow
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reservation_key 0x123abc
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}
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}
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Linking :program:`qemu-pr-helper` to ``libmpathpersist`` does not impede
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its usage on regular SCSI devices.
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