I didn't know you use actual key instead its md5 (for random patitioner) in KCF.  It's good point that I'll watch hit ratio of KCF to determine whether it needs to be increased.

Thanks,
-Weijun

On Tue, Feb 16, 2010 at 5:34 PM, Jonathan Ellis <jbellis@gmail.com> wrote:
On Tue, Feb 16, 2010 at 7:27 PM, Weijun Li <weijunli@gmail.com> wrote:
> Yes my KeysCachedFraction is already 0.3 but it doesn't relief the disk i/o.
> I compacted the data to be a 60GB (took quite a while to finish and it
> increased latency as expected) one but doesn't help much either.
>
> If I set KCF to 1 (meaning to cache all sstable index), how much memory will
> it take for 50mil keys?

10/3 what 0.3 takes :)

>Is the index a straight key-offset map? I guess key
> is 16 bytes and offset is 8 bytes.

key length depends on your data, of course.

> Will KCF=1 help to reduce disk i/o?

depends.  w/ trunk you can look at your cache hit rate w/ jconsole to
see if increasing it more would help.

-Jonathan