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From Jeremy Hanna <jeremy.hanna1...@gmail.com>
Subject Re: 4/20 nodes get disproportionate amount of mutations
Date Tue, 23 Aug 2011 11:02:16 GMT

On Aug 23, 2011, at 3:43 AM, aaron morton wrote:

> Dropped messages in ReadRepair is odd. Are you also dropping mutations ? 
> 
> There are two tasks performed on the ReadRepair stage. The digests are compared on this
stage, and secondly the repair happens on the stage. Comparing digests is quick. Doing the
repair could take a bit longer, all the cf's returned are collated, filtered and deletes removed.
 
> 
> We don't do background Read Repair on range scans, they do have foreground digest checking
though.
> 
> What CL are you using ? 

CL.ONE for hadoop writes, CL.QUORUM for hadoop reads

> 
> begin crazy theory:
> 
> 	Could there be a very big row that is out of sync ? The increased RR would be resulting
in mutations been sent back to the replicas. Which would give you a hot spot in mutations.
> 	
> 	Check max compacted row size on the hot nodes. 
> 	
> 	Turn the logging up to DEBUG on the hot machines for o.a.c.service.RowRepairResolver
and look for the "resolve:…" message it has the time taken.

The max compacted size didn't seem unreasonable - about a MB.  I turned up logging to DEBUG
for that class and I get plenty of dropped READ_REPAIR messages, but nothing coming out of
DEBUG in the logs to indicate the time taken that I can see.

> 
> Cheers
> 
> -----------------
> Aaron Morton
> Freelance Cassandra Developer
> @aaronmorton
> http://www.thelastpickle.com
> 
> On 23/08/2011, at 7:52 PM, Jeremy Hanna wrote:
> 
>> 
>> On Aug 23, 2011, at 2:25 AM, Peter Schuller wrote:
>> 
>>>> We've been having issues where as soon as we start doing heavy writes (via
hadoop) recently, it really hammers 4 nodes out of 20.  We're using random partitioner and
we've set the initial tokens for our 20 nodes according to the general spacing formula, except
for a few token offsets as we've replaced dead nodes.
>>> 
>>> Is the hadoop job iterating over keys in the cluster in token order
>>> perhaps, and you're generating writes to those keys? That would
>>> explain a "moving hotspot" along the cluster.
>> 
>> Yes - we're iterating over all the keys of particular column families, doing joins
using pig as we enrich and perform measure calculations.  When we write, we're usually writing
out for a certain small subset of keys which shouldn't have hotspots with RandomPartitioner
afaict.
>> 
>>> 
>>> -- 
>>> / Peter Schuller (@scode on twitter)
>> 
> 


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