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From Yiming Sun <>
Subject Re: need some clarification on recommended memory size
Date Tue, 15 May 2012 23:12:57 GMT
Thanks Tyler... so my understanding is, even if Cassandra doesn't do
off-heap caching, by having a large-enough memory, it minimize the chance
of swapping the java heap to a disk.  Is that correct?

-- Y.

On Tue, May 15, 2012 at 6:26 PM, Tyler Hobbs <> wrote:

> On Tue, May 15, 2012 at 3:19 PM, Yiming Sun <> 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 <>

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