Implementing support for fencing is likely needed also when using Raft and
adding support for fencing should also be a smaller task, doable in the 5.1
release.
If fencing support is implemented the identified split-brain cases will be
handled.
One use case for Raft can also be to reduce complextity in OpenSAF services,
e.g. AMFs state machines, counters etc. could be replaced with Raft instead
of the node director and director messaging, message based checkpointing and
IMM messages. Probably the same for the other services.
---
** [tickets:#1859] fm: Add support for STONITH fencing**
**Status:** assigned
**Milestone:** 5.1.FC
**Created:** Wed Jun 01, 2016 01:49 PM UTC by Hans Nordebäck
**Last Updated:** Thu Jun 02, 2016 07:03 AM UTC
**Owner:** Hans Nordebäck
Split brain can occur in OpenSAF if either both links between the two
controllers are "lost"
or one of the controller "live hangs".
OpenSAF handles and detects split-brain via FM and uses PLM to fence the other
system controller using reboot. PLM only supports target environments running
on particular hardware
Only a few split-brain cases has been seen and only when running in virtualized
environments:
1) Virtual switches problems that makes SCs isolated from each other.
2) Both TIPC links between the SCs are "down/lost", e.g. TIPC tolerance time
too low, non-redundant links, other latencies etc.
3) A system controller in a virtual machine is "live hanging" for several
seconds, e.g. due to
live migration/snapshotting.
To be able to do power fencing in a virtualized environment this ticket suggests
to use STONITH.
When FM detects its peer is not available, both active and standby, in an
virtualized environment the active FM system controller will use STONITH to
power fence the FM standby system controller, (not power reset). The FM standby
system controller will also power fence, but with a delay, the active FM
system controller.
This will solve the above identified split-brain cases.
It will also fit well with the roaming feature. E.g. after power fencing a
standby controller a new standby controller will automatically be selected by
the roaming feature.
---
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