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From Derek Andree <dand...@lacunasystems.com>
Subject Re: Disk usage for CommitLog
Date Wed, 31 Aug 2011 00:54:52 GMT
Okay I figured this out, the default for MemtableFlushAfterMins is not 60 minutes like some
here said and what datastax docs say (http://www.datastax.com/docs/0.8/configuration/storage_configuration),
it's 24 hours (1440).  I changed them all to 60 for every CF and now commit logs only hang
around for an hour, which is way better for me.

So, is the value of 1440 correct and the docs are wrong, or is 60 correct and 1440 made it
into 0.8.4 somehow?

Am I correct in assuming that if even one CF has it's MemtableFlushAfterMins set high (like
1440) and if data from that CF makes it into any commitlog segment then that commitlog segment
will be on disk for 24 hours even if all the other CFs have MemtableFlushAfterMins set to
60?  If so then maybe this should be a global setting, or alternatively  each CF could have
it's own CommitLog.  I don't know enough about cassandra internals to know if there is a reason
commitlogs need to contain data for all CFs (performance, too many files, etc).

Thanks,
-Derek


> > 86GB in commitlog and 42GB in data
>  
> Whoa, that seems really wrong, particularly given your data spans 13 months. Have you
changed any of the default cassandra.yaml setting? What is the maximum memtable_flush_after
across all your CFs? Any warnings/errors in the Cassandra log?


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