Will MMapping data files be detrimental for reads, in this case?
No. 

In general, when should we opt for MMap data files and what are the factors that need special attention when enabling the same?
mmapping is the default, so I would say use it until you have a reason not to. 

mmapping will map the entire file, but pages of data are read into memory on demand and purged when space is needed. 

Cheers

-----------------
Aaron Morton
Freelance Cassandra Developer
New Zealand

@aaronmorton

On 4/12/2012, at 11:59 PM, Ravikumar Govindarajan <ravikumar.govindarajan@gmail.com> wrote:

Our current SSTable sizes are far greater than RAM. {150 Gigs of data, 32GB RAM}. Currently we run with mlockall and mmap_index_only options and don't experience swapping at all.

We use wide rows and size-tiered-compaction, so a given key will definitely be spread across multiple sstables. Will MMapping data files be detrimental for reads, in this case?

In general, when should we opt for MMap data files and what are the factors that need special attention when enabling the same?

--
Ravi