pr-manager.rst (3681B)
1 =============================== 2 Persistent reservation managers 3 =============================== 4 5 SCSI persistent reservations allow restricting access to block devices 6 to specific initiators in a shared storage setup. When implementing 7 clustering of virtual machines, it is a common requirement for virtual 8 machines to send persistent reservation SCSI commands. However, 9 the operating system restricts sending these commands to unprivileged 10 programs because incorrect usage can disrupt regular operation of the 11 storage fabric. 12 13 For this reason, QEMU's SCSI passthrough devices, ``scsi-block`` 14 and ``scsi-generic`` (both are only available on Linux) can delegate 15 implementation of persistent reservations to a separate object, 16 the "persistent reservation manager". Only PERSISTENT RESERVE OUT and 17 PERSISTENT RESERVE IN commands are passed to the persistent reservation 18 manager object; other commands are processed by QEMU as usual. 19 20 ----------------------------------------- 21 Defining a persistent reservation manager 22 ----------------------------------------- 23 24 A persistent reservation manager is an instance of a subclass of the 25 "pr-manager" QOM class. 26 27 Right now only one subclass is defined, ``pr-manager-helper``, which 28 forwards the commands to an external privileged helper program 29 over Unix sockets. The helper program only allows sending persistent 30 reservation commands to devices for which QEMU has a file descriptor, 31 so that QEMU will not be able to effect persistent reservations 32 unless it has access to both the socket and the device. 33 34 ``pr-manager-helper`` has a single string property, ``path``, which 35 accepts the path to the helper program's Unix socket. For example, 36 the following command line defines a ``pr-manager-helper`` object and 37 attaches it to a SCSI passthrough device:: 38 39 $ qemu-system-x86_64 40 -device virtio-scsi \ 41 -object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock 42 -drive if=none,id=hd,driver=raw,file.filename=/dev/sdb,file.pr-manager=helper0 43 -device scsi-block,drive=hd 44 45 Alternatively, using ``-blockdev``:: 46 47 $ qemu-system-x86_64 48 -device virtio-scsi \ 49 -object pr-manager-helper,id=helper0,path=/var/run/qemu-pr-helper.sock 50 -blockdev node-name=hd,driver=raw,file.driver=host_device,file.filename=/dev/sdb,file.pr-manager=helper0 51 -device scsi-block,drive=hd 52 53 You will also need to ensure that the helper program 54 :command:`qemu-pr-helper` is running, and that it has been 55 set up to use the same socket filename as your QEMU commandline 56 specifies. See the qemu-pr-helper documentation or manpage for 57 further details. 58 59 --------------------------------------------- 60 Multipath devices and persistent reservations 61 --------------------------------------------- 62 63 Proper support of persistent reservation for multipath devices requires 64 communication with the multipath daemon, so that the reservation is 65 registered and applied when a path is newly discovered or becomes online 66 again. :command:`qemu-pr-helper` can do this if the ``libmpathpersist`` 67 library was available on the system at build time. 68 69 As of August 2017, a reservation key must be specified in ``multipath.conf`` 70 for ``multipathd`` to check for persistent reservation for newly 71 discovered paths or reinstated paths. The attribute can be added 72 to the ``defaults`` section or the ``multipaths`` section; for example:: 73 74 multipaths { 75 multipath { 76 wwid XXXXXXXXXXXXXXXX 77 alias yellow 78 reservation_key 0x123abc 79 } 80 } 81 82 Linking :program:`qemu-pr-helper` to ``libmpathpersist`` does not impede 83 its usage on regular SCSI devices.