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From Vladimir Yudovin <vla...@winguzone.com>
Subject Re: Can a Select Count(*) Affect Writes in Cassandra?
Date Thu, 10 Nov 2016 11:31:45 GMT
As I said I'm not sure about it, but it will be interesting to check memory heap state with
any JMX tool, e.g. https://github.com/patric-r/jvmtop



By a way, why Cassandra 2.0.14? It's quit old and unsupported version. Even in 2.0 branch
there is 2.0.17 available.


Best regards, Vladimir Yudovin, 

Winguzone - Hosted Cloud Cassandra
Launch your cluster in minutes.





---- On Thu, 10 Nov 2016 05:47:37 -0500Shalom Sagges &lt;shaloms@liveperson.com&gt;
wrote ----




Thanks for the quick reply Vladimir. 

Is it really possible that ~12,500 writes per second (per node in a 12 nodes DC) are caused
by memory flushes?










 


 
Shalom Sagges
 
DBA
 
T: +972-74-700-4035
 

 
 
 
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On Thu, Nov 10, 2016 at 11:02 AM, Vladimir Yudovin &lt;vladyu@winguzone.com&gt; wrote:









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Hi Shalom,



so not sure, but probably excessive memory consumption by this SELECT causes C* to flush tables
to free memory. 



Best regards, Vladimir Yudovin, 

Winguzone - Hosted Cloud Cassandra
Launch your cluster in minutes.





---- On Thu, 10 Nov 2016 03:36:59 -0500Shalom Sagges &lt;shaloms@liveperson.com&gt;
wrote ----




Hi There!



I'm using C* 2.0.14. 

I experienced a scenario where a "select count(*)" that ran every minute on a table with practically
no results limit (yes, this should definitely be avoided), caused a huge increase in Cassandra
writes to around 150 thousand writes per second for that particular table.



Can anyone explain this behavior? Why would a Select query significantly increase write count
in Cassandra?



Thanks!




 
Shalom Sagges
 

 

 
 
 
 We Create Meaningful Connections
 
 

 













This message may contain confidential and/or privileged information. 

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email and delete this message. Thank you.












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