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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 comes up. The device mapper utilities provide a library to do so; this patch makes qemu-pr-helper.c detect multipath devices and, when one is found, delegate the operation to libmpathpersist. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
112 lines
4.7 KiB
ReStructuredText
112 lines
4.7 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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----------------------------------
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Invoking :program:`qemu-pr-helper`
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----------------------------------
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QEMU provides an implementation of the persistent reservation helper,
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called :program:`qemu-pr-helper`. The helper should be started as a
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system service and supports the following option:
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-d, --daemon run in the background
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-q, --quiet decrease verbosity
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-v, --verbose increase verbosity
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-f, --pidfile=path PID file when running as a daemon
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-k, --socket=path path to the socket
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-T, --trace=trace-opts tracing options
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By default, the socket and PID file are placed in the runtime state
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directory, for example :file:`/var/run/qemu-pr-helper.sock` and
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:file:`/var/run/qemu-pr-helper.pid`. The PID file is not created
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unless :option:`-d` is passed too.
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:program:`qemu-pr-helper` can also use the systemd socket activation
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protocol. In this case, the systemd socket unit should specify a
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Unix stream socket, like this::
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[Socket]
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ListenStream=/var/run/qemu-pr-helper.sock
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After connecting to the socket, :program:`qemu-pr-helper`` can optionally drop
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root privileges, except for those capabilities that are needed for
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its operation. To do this, add the following options:
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-u, --user=user user to drop privileges to
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-g, --group=group group to drop privileges to
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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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