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From Janne Jalkanen <>
Subject Re: Cassandra JVM heap sizes on EC2
Date Sat, 24 Aug 2013 10:24:28 GMT

We've been trying to keep the heap as small as possible; the disk access penalty on EC2 is
big enough - even on instance store - that you want to give as much memory to disk caches
as you can.  Of course, then you will need to keep extra vigilant on your garbage collection
and tune various things like bloom filters, cache sizes (if using on-heap cache) and sstable
size for LCS accordingly.

YMMV of course; we're running on m1.xlarge, so we have less RAM to play with than you. It
all depends on your data size, the size of the hot portion, etc.  Currently we use 3.5GB for
Cassandra 1.2.8, which seems like a good tradeoff for our usage patterns.  I tend to bump
the heap up and down in .5 GB intervals just to see what happens; let it run for a few hours
or a day and then check Munin graphs to see what the effect was compared to other nodes.


On Aug 24, 2013, at 01:12 , David Laube <> wrote:

> Hi All,
> We are evaluating our JVM heap size configuration on Cassandra 1.2.8 and would like to
get some feedback from the community as to what the proper JVM heap size should be for cassandra
nodes deployed on to Amazon EC2. We are running m2.4xlarge EC2 instances (64GB RAM, 8 core,
2 x 840GB disks) --so we will have plenty of RAM. I've already consulted the docs at
but would love to hear what is working or not working for you in the wild. Since Datastax
cautions against using more than 8GB, I'm wondering if it is even advantageous to use even
slightly more.
> Thanks,
> -David Laube

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