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From Jonathan Ellis <jbel...@gmail.com>
Subject Re: Cassandra JVM heap size
Date Tue, 04 Oct 2011 04:57:29 GMT
Sure, other things being equal.

Of course, other things are not truly equal and in practice I think
dual-quad-core, 32GB servers are at a good sweet spot for a lot of
applications.

As a rule of thumb, inserts will be cpu-bound and reads will be ram/io bound.

On Mon, Oct 3, 2011 at 11:10 PM, Ramesh Natarajan <ramesh25@gmail.com> wrote:
> Thanks. We are not planning to use row cache because we don't anticipate
> requests for the same row coming in often and we would better let the OS do
> the caching..  So does this mean in my case instead of running 6 servers
> with 100 GB each, I can run 75 servers with 8 GB RAM and set the Xms/Xmx to
> 4GB.
> thanks
> Ramesh
>
> On Mon, Oct 3, 2011 at 10:25 PM, Jonathan Ellis <jbellis@gmail.com> wrote:
>>
>> That's misleading, because you don't necessarily need to give the
>> memory to the JVM for Cassandra to make use of it.  (See, for example,
>>
>> http://www.datastax.com/dev/blog/whats-new-in-cassandra-1-0-improved-memory-and-disk-space-management.)
>>  In fact it's counterproductive to increase heap size past the point
>> where it can handle the bloom filters + memtables for your data set.
>>
>>  I suspect that the vast majority of deployments will not benefit from
>> heaps larger than 4GB, and there is a ticket open to make this the
>> default for 1.0: https://issues.apache.org/jira/browse/CASSANDRA-3295
>>
>> That said, if you have the choice it's generally better to choose
>> more, smaller servers than fewer, larger ones, primarily because it's
>> easier to deal with failures.  If you had 12 nodes half as expensive,
>> for instance, losing one would be 1/12 of your capacity instead of
>> 1/6.
>>
>> On Mon, Oct 3, 2011 at 9:47 PM, Ramesh Natarajan <ramesh25@gmail.com>
>> wrote:
>> > I was reading an article
>> > @ http://www.acunu.com/products/choosing-cassandra/
>> > and it mentions cassandra cannot benefit from more than 8GB allocated to
>> > JVM
>> > heap.  Is this true?  Are these cassandra installations with larger heap
>> > sizes? We are planning to have a cluster of 6 nodes with each node
>> > running
>> > with about 100 GB or so RAM. Will this be a problem?
>> > thanks
>> > Ramesh
>> >
>> > from http://www.acunu.com/products/choosing-cassandra/
>> > Memory Ceiling
>> >
>> > Cassandra typically cannot benefit from more than 8GB of RAM allocated
>> > to
>> > the Java heap, imposing a hard limit on data size. Taking advantage of
>> > big
>> > servers with lots of memory or many disks is no problem for Acunu.
>> > Thereʼs
>> > no memory ceiling for Acunu and as a result, no data ceiling either.
>> > Need to
>> > use larger servers? Go ahead.
>>
>>
>>
>> --
>> Jonathan Ellis
>> Project Chair, Apache Cassandra
>> co-founder of DataStax, the source for professional Cassandra support
>> http://www.datastax.com
>
>



-- 
Jonathan Ellis
Project Chair, Apache Cassandra
co-founder of DataStax, the source for professional Cassandra support
http://www.datastax.com

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