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From Aaron Blew <>
Subject Re: Cassandra Scaling Questions
Date Tue, 03 Aug 2010 00:11:42 GMT
1.) 16 to 24GB out of how much total system memory?  Is this 50% of
available system RAM or 90%?

Thanks for the reply!

On Mon, Aug 2, 2010 at 2:24 PM, Aaron Morton <>wrote:

> Will answer as best I can, others will know more.
> 1) Most people seem to lean towards more memory for the JVM, around 16 to
> 24gb. Memory is also used by the MemTables and I assume during the
> compaction processes.
> 2) Cannot say for sure, but I assume so. Think I've seen the cache with
> data in it when I have only done writes.
> 3) I've noticed large differences between nodes when using the RP and
> automatic token assignments, such as the last node with very little data.
> Try setting tokens at start up, see
> 3.5) Yes load balance restores things, I suggest you run it on one node at
> a time. Start with the node with the lowest load. Watching the progress by
> watching the streams via JMX or nodetool.
> Hope that helps.
> Aaron
> On 03 Aug, 2010,at 07:28 AM, Aaron Blew <> wrote:
> Hi All,
> I've got a couple questions that have come up about how Cassandra works and
> what others are seeing in their environments.  Here goes:
> 1.) What have you found to be the best ratio of Cassandra row cache to
> memory free on the system for filesystem cache?  Are you tuning it like an
> RDBMS so Cassandra has the vast majority of the RAM in the system or are you
> letting the filesystem cache do some of the work?
> 2.) Is the Cassandra cache write-through (ie are new records held in the
> row cache as they're written to disk?
> 3.) When using the random partitioner how much difference should be
> expected (or has been observed) between nodes?  2%? 10%?
> 3.5) Can a load balance be expected to bring the data distribution pretty
> close to even among all nodes in the ring?  Is the correct process for a
> loadbalance to run the loadbalance operation on each node in the ring?
> Thanks!  I'm curious to hear what other's have observed.
> -Aaron

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