Take a look at the key cache hit rate in nodetool cfstats. 

One approach is to increase the cache size until you do not see a matching increase in the hit rate. 

Is there a limit to key cache size? I know that is all taken from heap but how much max we can go with setting the key cache sizes?
It's pretty much a memory thing. 

Each entry maps a description of the SStable and the key to the offset in the index file. (off the top of my head) The SSTable description is shared, the row key, row token (16 bytes) and offset (offset) will take up space.

Cheers
 
-----------------
Aaron Morton
Freelance Developer
@aaronmorton

On 3/04/2012, at 11:24 AM, Shoaib Mir wrote:

Hi guys,

We are calculating key cache size right now. There is this column family with ~ 100 million columns and right now we have the cache size set at 2 million.

I suspect that the active data we got is not all fitting in the 2 million cache size and we at times are getting query execution time way higher then the normal. Is there a limit to key cache size? I know that is all taken from heap but how much max we can go with setting the key cache sizes?

cheers,
Shoaib