Linearly growing number of writes in Cassandra

[Vladi Feigin]

Hi All,

After moving to Java Datastax driver 3.0.1 we're experiencing linear and constant grow of writes 

(metric of nodetool WriteCount). It's growing constantly. 

Our configuration is:

Our Cassandra version is 2.0.14

Consistency Level : LOCAL_QUORUM

LoadBalancing : DCAwareRoundRobinPolicy wrapped by TokenAware

RetryPolicy : ExponentialReconnectionPolicy

Default connection pool configuration

What can be done to pinpoint the problem? 

Did someone experience similar behaviour ? 

Please advice and help 

Thank you in advance!!!

Vladi Feigin

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[Kevin Gallardo]

Hi, 

I'm a bit puzzled by what you mean by " grow of writes ". You updated the driver to 3.0.1 and did not change the Statements you execute, and you noticed more requests being sent?

I'm looking at the nodetool documentation, and it sounds like the Write Count represents the number or times data is written in C* (any INSERT or UPDATE query), if that's correct, it sounds normal that you experience this behaviour on the Write Count if you execute INSERT or UPDATE statement, or am I missing something? Maybe a bit more information on what your application code does would help.

BTW, to monitor closely what are the requests being sent to C*, you can take a look on our documentation on how to log requests sent, responses received, and others.

Thanks.

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Kévin Gallardo.

kgdo.me

Software Engineer in Drivers and Tools Team at DataStax.

[Vladi Feigin]

Thank you for the fast response!

Eventually we've found a query, which is a root cause

The query is :

 select count(*) from our_table 

 which is running periodically,  every few minutes

This query is causing the constant linear growth of writes 

We suspect it is related to Cassandra read-repair but it's still unclear how this can cause such constant writes growth

Where should we look to understand the real reason ? 

Thank you 

Vladi

 

 

Vladi Feigin

R&D System Architect

T: +972-74-700-4376

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