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: http://en.wikipedia.org/wiki/Page_cache Jon On Tue, Jul 30, 2013 at 10:51 PM, Jan Algermissen < jan.algermis...@nordsc.com> 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 http://www.rustyrazorblade.com skype: rustyrazorblade
