[Lustre-devel] replacing Lustre pings with LNet Peer Health

[Nic Henke <nic@cray.com>]

Just floating an idea... I'd much appreciate any feedback

Given bug 12471 where the ptlrpc pinger traffic on a large system can 
approach the ridiculous (2.6M pings every 75s for 160 OSTs and 16K 
clients), I'd like to consider getting rid of the pings entirely.

The idea would be to extend the idea in the attached patch where we add 
an upper layer callback for lnet_notify() signaling a peer going down or 
up. The ptlrpc pinger code would be then changed to record the 'down' 
event for an import/export which would then start an eviction timer that 
started when the LNet peer was last_alive. If the nodes comes 'up' 
before the timer expires, no eviction. The eviction code would then only 
operate on nodes with 'down' events and trusting that the rest are all 
ok and functional.

Eric - I know this doesn't get us that far down the road toward your new 
health network, but does solve a near term issue with pinger rates on 
large systems.

Issues...

- lacks "proof" that peer nodes ptlrpc queues are moving forward, but 
not really sure that is all that important in terms of pinger evictions.

- LNet peer health is a bit "weird" in that it requires an upper layer 
sending a packet to trigger a node moving back to 'up'. We would need to 
address this for proper LNet peer health as it is.

- Might need some beefing up of the standard LNDs to ensure we have good 
peer health data.

Thoughts ?

Nic
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