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From Tyler Hobbs <ty...@datastax.com>
Subject Re: need some clarification on recommended memory size
Date Tue, 15 May 2012 22:26:02 GMT
On Tue, May 15, 2012 at 3:19 PM, Yiming Sun <yiming.sun@gmail.com> wrote:

> Hello,
>
> I was reading the Apache Cassandra 1.0 Documentation PDF dated May 10,
> 2012, and had some questions on what the recommended memory size is.
>
> Below is the snippet from the PDF.  Bullet 1 suggests to have 16-32GB of
> RAM, yet Bullet 2 suggests to limit Java heap size to no more than 8GB.  My
> understanding is that Cassandra is implemented purely in Java, so all
> memory it sees and uses is the JVM Heap.
>

The main way that additional RAM helps is through the OS page cache, which
will store hot portions of SSTables in memory. Additionally, Cassandra can
now do off-heap caching.



>  So can someone help me understand the discrepancy between 16-32GB of RAM
> and 8GB of heap?  Thanks.
>
> == snippet ==
> Memory
> The more memory a Cassandra node has, the better read performance. More
> RAM allows for larger cache sizes and
> reduces disk I/O for reads. More RAM also allows memory tables (memtables)
> to hold more recently written data. Larger
> memtables lead to a fewer number of SSTables being flushed to disk and
> fewer files to scan during a read. The ideal
> amount of RAM depends on the anticipated size of your hot data.
>
> • For dedicated hardware, a minimum of than 8GB of RAM is needed. DataStax
> recommends 16GB - 32GB.
>
> • Java heap space should be set to a maximum of 8GB or half of your total
> RAM, whichever is lower. (A greater
> heap size has more intense garbage collection periods.)
>
> • For a virtual environment use a minimum of 4GB, such as Amazon EC2 Large
> instances. For production clusters
> with a healthy amount of traffic, 8GB is more common.
>



-- 
Tyler Hobbs
DataStax <http://datastax.com/>

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