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From Jack Krupansky <jack.krupan...@gmail.com>
Subject Re: cassandra reads are unbalanced
Date Fri, 04 Dec 2015 16:45:41 GMT
Thanks for the elaboration. A few more questions...

Is there only a single thread in each client or are there multiple threads
doing reading in parallel? IOW, does a read need to complete before the
next read is issued.

What client Cassandra driver are you using? Java?

What does your connection code look like, say compared to the example in
the doc:
http://docs.datastax.com/en/developer/java-driver/2.0/java-driver/quick_start/qsSimpleClientCreate_t.html

Just to make sure it really is connecting only to the local cluster and
using round robin and whether it is token aware.


-- Jack Krupansky

On Fri, Dec 4, 2015 at 10:51 AM, Walsh, Stephen <Stephen.Walsh@aspect.com>
wrote:

> Thanks for your input, but I think I’ve already answered most of your
> questions.
>
>
>
>
>
> How many clients do you have performing reads?
>
>
>
> ------------------
>
> On Wed, Dec 2, 2015 at 6:44 PM, Walsh, Stephen <Stephen.Walsh@aspect.com>
> wrote
>
> ….
>
> There are 2 application (1 for each DC) who read and write at the same
> rate to their local DC
>
> ….
>
> --------------------
>
>
>
>
>
>
>
>
>
>
>
>
>
> Is your load balancer in front of your clients or between your clients and
> Cassandra?
>
>
>
> ------------------
>
> On Thu, Dec 3, 2015 at 4:58 AM, Walsh, Stephen <Stephen.Walsh@aspect.com>
> wrote:
>
> …
>
> our production applications are behind a round robin load balancer
>
> …
>
> ------------------
>
>
>
> No Load Balancers talk to cassandra – I’m only mentioning this to show
> that the writes / read are evenly distributed over the 2 DC’s
>
>
>
>
>
>
>
>
>
>
>
>
>
> Does Node1 of DC2 have the exact same configuration of hardware of the
> other nodes
>
> Yes
>
>
>
>
>
>
>
>
>
>
>
> Is it in the same rack
>
> It’s in AWS – but we have it configured via the GossipProperytFileSnitch
> that they are all on unique racks
>
>
>
>
>
>
>
>
>
>
>
>
>
> Maybe your load balancer thinks that node is more capable and handles
> requests faster so that it looks less loaded than the other two nodes
>
> Unlikely, it’s all TCP SSL pass though connections. It doesn’t balance on
> load, it just round robins each request
>
>
>
>
>
>
>
>
>
>
>
> You might also check the read counts after a very short interval of time
> to see if Node1 is uniformly getting more requests or just occasionally
>
> ------------------
>
> On Wed, Dec 2, 2015 at 3:36 PM, Walsh, Stephen <Stephen.Walsh@aspect.com>
> wrote …
>
> We monitor the number of reads / writes of every table via the cassandra
> JMX metrics. (cassandra.db.read_count)
>
> …
>
> ------------------
>
> We can only monitor in 1 hour moving window
>
>
>
>
>
>
>
>
>
> Maybe the other two nodes are in a different rack that occasionally has
> net connectivity issues
>
> Unlikely seems its AWS
>
>
>
>
>
>
>
>
>
>
>
>
>
> *From:* Jack Krupansky [mailto:jack.krupansky@gmail.com]
> *Sent:* 03 December 2015 16:11
>
> *To:* user@cassandra.apache.org
> *Subject:* Re: cassandra reads are unbalanced
>
>
>
> How many clients do you have performing reads?
>
>
>
> Is your load balancer in front of your clients or between your clients and
> Cassandra?
>
>
>
> Does Node1 of DC2 have the exact same configuration of hardware of the
> other nodes? Is it in the same rack? Maybe your load balancer thinks that
> node is more capable and handles requests faster so that it looks less
> loaded than the other two nodes.
>
>
>
> You might also check the read counts after a very short interval of time
> to see if Node1 is uniformly getting more requests or just occasionally.
> Maybe the other two nodes are in a different rack that occasionally has net
> connectivity issues so that the requests get diverted by the client/load
> balancer to Node1 during those times.
>
>
>
>
> -- Jack Krupansky
>
>
>
> On Thu, Dec 3, 2015 at 4:58 AM, Walsh, Stephen <Stephen.Walsh@aspect.com>
> wrote:
>
> Thanks but keep in mind that both DC should be getting the same load, our
> production applications are behind a round robin load balancer – so each
> one our local application talk to its local Cassandra DataCenter.
>
>
>
> It took about 4 hours but the nodetool cleanup eventually balanced all
> nodes
>
>
>
> *From:* DuyHai Doan [mailto:doanduyhai@gmail.com]
> *Sent:* 02 December 2015 16:27
>
>
> *To:* user@cassandra.apache.org
> *Subject:* Re: cassandra reads are unbalanced
>
>
>
> If you're using the Java driver with LOCAL_ONE and the default load
> balancing strategy (TokenAware wrapped on DCAwareRoundRobin), the
> driver will always select the primary replica. To change this behavior and
> introduce some randomness so that non primary replicas get a chance to
> serve a read:
>
>
>
> new TokenAwarePolicy(new DCAwareRoundRobinPolicy("local_DC"), true).
>
>
>
> The second parameter (true) asks the TokenAware policy to "shuffle"
> replica on each request to avoid always returning the primary replica.
>
>
>
> On Wed, Dec 2, 2015 at 6:44 PM, Walsh, Stephen <Stephen.Walsh@aspect.com>
> wrote:
>
> 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>
> 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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>
> This email (including any attachments) is proprietary to Aspect Software,
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> Please notify the sender immediately, delete it from your system and
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>
>
> This email (including any attachments) is proprietary to Aspect Software,
> Inc. and may contain information that is confidential. If you have received
> this message in error, please do not read, copy or forward this message.
> Please notify the sender immediately, delete it from your system and
> destroy any copies. You may not further disclose or distribute this email
> or its attachments.
>
>
> This email (including any attachments) is proprietary to Aspect Software,
> Inc. and may contain information that is confidential. If you have received
> this message in error, please do not read, copy or forward this message.
> Please notify the sender immediately, delete it from your system and
> destroy any copies. You may not further disclose or distribute this email
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