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From "Pawel Matras (JIRA)" <j...@apache.org>
Subject [jira] [Created] (CASSANDRA-10792) Make warning level for count of partitions in unlogged BatchStatement configurable.
Date Tue, 01 Dec 2015 10:10:10 GMT
Pawel Matras created CASSANDRA-10792:
----------------------------------------

             Summary: Make warning level for count of partitions in unlogged BatchStatement
configurable.
                 Key: CASSANDRA-10792
                 URL: https://issues.apache.org/jira/browse/CASSANDRA-10792
             Project: Cassandra
          Issue Type: Improvement
          Components: Coordination
         Environment: CentOS,  JDK 1.7, cluster of servers with 32 CPU each.
            Reporter: Pawel Matras
            Priority: Minor


Currently mutations for only one partition are allowed in BatchStatement. This amount is hard
coded and not configurable as e.g. batch size.

General suggestion looks to be to consider multi partition unlogged batch statements as distributed
anti pattern. As cure async inserts are proposed. Proposal might be OK if one consider only
cassandra side of the system.

If a complete system have to share the same hardware things does not look so obvious any more.
When cassandra shares the same hardware with other also clustered components lots of small
async inserts become another well known distributed anti pattern. 

In our case changing from unlogged batches to async inserts destabilize the system as async
inserts require amounts of interrupts per second and context switches per second beyond what
hardware can handle. With replica aware unlogged batches these parameters are at 40% of hardware
limits and the system as a whole runs stable without visible hotspots (cassandra metrics looks
almost the same on all nodes).

Unfortunately latest versions (we switched from 2.0.6 to 2.2.1) of cassandra log lots of warnings
of type "Unlogged batch covering NN partitions detected against table XXX...". I found two
workarounds to avoid this warning. The one is to use logged batches. But they generate 20%
more interrupts and context switches and 400% more network load. The other way is more a hack
and uses filtering features of logging framework and suppresses the warning just before it
get logged.

So my suggestion is to allow users of cassandra to decide by configuration if they consider
unlogged batches antipattern or not. This is partially done with the size of the batch, where
size is configurable and not hardcoded to 5 kB.
It would be fine to stay consistent on this and let the user configure mutations for how many
partitions to allow.

Of course there are several other solutions possible here, probably more costly. E.g warning
could be produced only if batch is not token/replica aware.




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