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From Aaron Morton <>
Subject Re: Cassandra Scaling Questions
Date Mon, 02 Aug 2010 21:24:51 GMT
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.

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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