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From Patrick Hunt <ph...@apache.org>
Subject Re: Zookeeper outage recap & questions
Date Thu, 01 Jul 2010 07:37:04 GMT
Hi Travis, as Flavio suggested would be great to get the logs. A few 
questions:

1) how did you eventually recover, restart the zk servers?

2) was the cluster losing quorum during this time? leader re-election?

3) Any chance this could have been initially triggered by a long GC 
pause on one of the servers? (is gc logging turned on, any sort of heap 
monitoring?) Has the GC been tuned on the servers, for example CMS and 
incremental?

4) what are the clients using for timeout on the sessions?

3.4 probably not for a few months yet, but we are planning for a 3.3.2 
in a few weeks to fix a couple critical issues (which don't seem related 
to what you saw). If we can identify the problem here we should be able 
to include it in any fix release we do.

fixing something like 517 might help, but it's not clear how we got to 
this state in the first place. fixing 517 might not have any effect if 
the root cause is not addressed. 662 has only ever been reported once 
afaik, and we weren't able to identify the root cause for that one.

One thing we might also consider is modifying the zk client lib to 
backoff connection attempts if they keep failing (timing out say). Today 
the clients are pretty aggressive on reconnection attempts. Having some 
sort of backoff (exponential?) would provide more breathing room to the 
server in this situation.

Patrick

On 06/30/2010 11:13 PM, Travis Crawford wrote:
> Hey zookeepers -
>
> We just experienced a total zookeeper outage, and here's a quick
> post-mortem of the issue, and some questions about preventing it going
> forward. Quick overview of the setup:
>
> - RHEL5 2.6.18 kernel
> - Zookeeper 3.3.0
> - ulimit raised to 65k files
> - 3 cluster members
> - 4-5k connections in steady-state
> - Primarily C and python clients, plus some java
>
> In chronological order, the issue manifested itself as alert about RW
> tests failing. Logs were full of too many files errors, and the output
> of netstat showed lots of CLOSE_WAIT and SYN_RECV sockets. CPU was
> 100%. Application logs showed lots of connection timeouts. This
> suggests an event happened that caused applications to dogpile on
> Zookeeper, and eventually the CLOSE_WAIT timeout caused file handles
> to run out and basically game over.
>
> I looked through lots of logs (clients+servers) and did not see a
> clear indication of what happened. Graphs show a sudden decrease in
> network traffic when the outage began, zookeeper goes cpu bound, and
> runs our of file descriptors.
>
> Clients are primarily a couple thousand C clients using default
> connection parameters, and a couple thousand python clients using
> default connection parameters.
>
> Digging through Jira we see two issues that probably contributed to this outage:
>
>      https://issues.apache.org/jira/browse/ZOOKEEPER-662
>      https://issues.apache.org/jira/browse/ZOOKEEPER-517
>
> Both are tagged for the 3.4.0 release. Anyone know if that's still the
> case, and when 3.4.0 is roughly scheduled to ship?
>
> Thanks!
> Travis

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