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From Jonathan Haddad <>
Subject Re: VM dimensions for running Cassandra and Hadoop
Date Wed, 31 Jul 2013 06:15:13 GMT
Having just enough RAM to hold the JVM's heap generally isn't a good idea
unless you're not planning on doing much with the machine.

Any memory not allocated to a process will generally be put to good use
serving as page cache. See here:


On Tue, Jul 30, 2013 at 10:51 PM, Jan Algermissen <> wrote:

> Hi,
> thanks for the helpful replies last week.
> It looks as if I will deploy Cassandra on a bunch of VMs and I am now in
> the process of understanding what the dimensions of the VMS should be.
> So far, I understand the following:
> - I need at least 3 VMs for a minimal Cassandra setup
> - I should get another VM to run the Hadoop job controller or
>   can that run on one of the Cassandra VMs
> - there is no point in giving the Cassandra JVMs more than
>   8-12 GB heap space because of GC, so it seems going beyond 16GB
>   RAM per VM makes no sense
> - Each VM needs two disks, to separate commit log from data storage
> - I must make sure the disks are directly attached, to prevent
>   problems when multiple nodes flush the commit log at the
>   same time
> - I'll be having rather few writes and intend to hold most of the
>   data in memory, so spinning disks are fine for the moment
> Does that seem reasonable?
> How should I plan the disk sizes and number of CPU cores?
> Are there any other configuration mistakes to avoid?
> Is there online documentation that discusses such VM sizing questions in
> more detail?
> Jan

Jon Haddad
skype: rustyrazorblade

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