OOM : key caches, mmap ?

[Alain RODRIGUEZ <ar...@gmail.com>]

Hi,

I faced a similar issue as described there :
http://comments.gmane.org/gmane.comp.db.cassandra.user/11184.

I was running Cassandra 1.0.0 with a 3 node cluster on 3 t1.micro from
Amazon EC2.

I have no error in cassandra logs, but an OOM in /var/log/kern.log which
put one of my nodes down.

After some search I have seen two possible reason for it :

1 - I may be keeping too much keys cached (200 000 which is default). I
read that now this cache is out of the heap, my heap is about 300 M, the
HEAP_NEWSIZE is about 75M, the system use about 150M all this out of 600M

What happens when I try to cache a row key and my memory is already full ?
How to choose the number of keys cached ? I saw that it is hard to
calculate the amount of memory used by this cache. Should I start by
caching 2,000 rows per CF and update my schema increasing progresively the
amount of key cached if I see that it works fine ?

2 - I read a lot about mmap without understanding clearly if I should keep
it "auto" or "mmap_index_only". Anyways, I didn't find the
"disk_access_mode" option in my cassandra.yaml, which let me think that it
was removed and I have no other choice to keep "auto" value.

Thank you,

Alain