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From Andrew Purtell <apurt...@apache.org>
Subject Re: HBase Exceptions on version 0.20.1
Date Tue, 10 Nov 2009 13:53:20 GMT
Yes I agree with you. 

Individually each HBase daemon can block clients and/or reject writes and/or defer compactions
as write latency increases. This is a second order indication of cluster stress. By the time
write latency falls outside bounds, it may be too late to shut down gracefully. But anyway,
we should do these things.

Metrics can be used for HBase and applications/writers and cluster admins to globally know
directly that the cluster is under stress in order to coordinate a response. Such a response
could happen concurrently on multiple levels. 

But getting back to your point, there are certainly opportunities for HBase to take self preserving
actions where currently it does not.

    - Andy

From: elsif <elsif.then@gmail.com>
To: hbase-user@hadoop.apache.org
Sent: Tue, November 10, 2009 1:25:20 AM
Subject: Re: HBase Exceptions on version 0.20.1

The larger issue here is that any hbase cluster will reach this tipping
point at some point in its lifetime as more and more data is added.  We
need to have a graceful method to put the cluster into safe mode until
more resources can be added or the load on the cluster has been
reduced.  We cannot allow hbase to run itself into the ground causing
data loss or corruption under any circumstances.
Andrew Purtell wrote:
> You should consider provisioning more nodes to get beyond this ceiling you encountered.

> DFS write latency spikes from 3 seconds to 6 seconds, to 15! Flushing cannot happen fast
enough to avoid an OOME. Possibly there was even insufficient CPU to GC. The log entries you
highlighted indicate the load you are exerting on your current cluster needs to be spread
out over more resources than currently allocated.
> This: 
>> 2009-11-06 09:15:37,144 WARN org.apache.hadoop.hbase.util.Sleeper: We slept 286007ms,
ten times longer than scheduled: 10000
> indicates a thread that wanted to sleep for 10 seconds was starved for CPU for 286 seconds.
Obviously Zookeeper timeouts and resulting HBase process shutdowns, missed DFS heartbeats
possibly resulting in spurious declaration of dead datanodes, and other serious problems will
result from this. 
> Did your systems start to swap? 
> When region servers shut down, the master notices this and splits their HLogs into per
region reconstruction logs. These are the "oldlogfile.log" files. The master log will shed
light on why this particular reconstruction log was botched. Would have happened at the master.
The region server probably did do a clean shutdown. I suspect DFS was in extremis due to overloading
so the split failed. The checksum error indicates incomplete write at the OS level. Did a
datanode crash? 
> HBASE-1956 is about making the DFS latency metric exportable via the
> Hadoop metrics layer, perhaps via Ganglia. Write latency above 1 or 2
> seconds is a warning. Anything above 5 seconds is an alarm.  It's a
> good indication that an overloading condition is in progress. 
> The Hadoop stack, being pre 1.0, has some rough edges. Response to overloading is one
of them. For one thing, HBase could be better about applying backpressure to writing clients
when the system is under stress. We will get there. HBASE-1956 is a start. 
>     - Andy

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