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From "Walsh, Stephen" <Stephen.Wa...@Aspect.com>
Subject RE: cassandra reads are unbalanced
Date Wed, 02 Dec 2015 14:44:06 GMT
Very good questions.

We have reads and writes at LOCAL_ONE.
There are 2 application (1 for each DC) who read and write at the same rate to their local
DC
(All reads / writes started all perfectly even and degraded over time)

We use DCAwareRoundRobin policy

On update on the nodetool cleanup – it has help but hasn’t balanced all nodes. Node 1
on DC2 is still quite high

Node 1 (DC1)  =  1.35k    (seeder)
Node 2 (DC1)  =  1.54k
Node 3 (DC1)  =  1.45k

Node 1 (DC2)  =  2.06k   (seeder)
Node 2 (DC2)  =  1.38k
Node 3 (DC2)  =  1.43k


From: DuyHai Doan [mailto:doanduyhai@gmail.com]
Sent: 02 December 2015 14:22
To: user@cassandra.apache.org
Subject: Re: cassandra reads are unbalanced

Which Consistency level do you use for reads ? ONE ? Are you reading from only DC1 or from
both DC ?
What is the LoadBalancingStrategy you have configured for your driver ? TokenAware wrapped
on DCAwareRoundRobin ?





On Wed, Dec 2, 2015 at 3:36 PM, Walsh, Stephen <Stephen.Walsh@aspect.com<mailto:Stephen.Walsh@aspect.com>>
wrote:
Hey all,

Thanks for taking the time to help.

So we have 6 cassandra nodes in 2 Data Centers.
Both Data Centers have a replication of 3 – so all nodes have all the data.

Over the last 2 days we’ve noticed that data reads / writes has shifted from balanced to
unbalanced
(Nodetool status still shows 100% ownership on every node, with similar sizes)


For Example

We monitor the number of reads / writes of every table via the cassandra JMX metrics. (cassandra.db.read_count)
Over the last hour of this run

Reads
Node 1 (DC1)  =  1.79k    (seeder)
Node 2 (DC1)  =  1.92k
Node 3 (DC1)  =  1.97k

Node 1 (DC2)  =  2.90k   (seeder)
Node 2 (DC2)  =  1.76k
Node 3 (DC2)  =  1.19k

As you see on DC1, everything is pretty well balanced, but on DC2 the reads favour Node1 over
Node 3.
I ran a nodetool repair yesterday – ran for 6 hours and when completed didn’t change the
read balance.

Write levels are similar on  DC2, but not as bad a reads.

Anyone any suggestion on how to rebalance? I’m thinking maybe running a nodetool cleanup
in case some of the keys have shifted?

Regards
Stephen Walsh


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