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

Alexey Lyashkov alexey_lyashkov at xyratex.com
Sun May 15 00:44:28 PDT 2011

One problem.

LNet layer can report - node is live, but one or more ptlrpc services on that node is dead (due a LBUG hit by example).
But yes, generate a LNet event about node is dead is usefull to reduce time of detecting timeout of requests.

On May 12, 2011, at 21:37, Christopher J. Morrone wrote:

> I think Eric's approach is the only sane way I've heard to reduce pings.
> Here are some issues that I see with this:
> 1)  For your solution to work, you require that the lnet layer take on 
> pinging duties.  Usually the network, be it IB, TCP, whatever, will not 
> provide any active notification of a peer failure.  To notice that a 
> peer has died, the lnet LND must, you guessed it, ping.
> Usually the LNDs try to be smart.  They only generate their own pings if 
> no traffic has been sent to the peer in a certain period of time.  So 
> once you eliminate the higher-level pings, they will partly be replaced 
> by lower-level pings.
> 2)  Doesn't work in a routed environment.  Would need a health network 
> for clients behind routers to learn that a server has died, and vice versa.
> On 05/12/2011 07:57 AM, Nic Henke wrote:
>> 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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Alexey Lyashkov
alexey_lyashkov at xyratex.com

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