512 sectors for read-ahead.  Are your new fancy SSD drives using large sectors?  If your read-ahead is really reading 512 x 4KB per random IO, then that 2 MB per read seems like a lot of extra overhead.

-Bryan




On Thu, May 16, 2013 at 12:35 PM, Keith Wright <kwright@nanigans.com> wrote:
We actually have it set to 512.  I have tried decreasing my SSTable size to 5 MB and changing the chunk size to 8 kb 

From: Igor <igor@4friends.od.ua>
Reply-To: "user@cassandra.apache.org" <user@cassandra.apache.org>
Date: Thursday, May 16, 2013 1:55 PM

To: "user@cassandra.apache.org" <user@cassandra.apache.org>
Subject: Re: SSTable size versus read performance

My 5 cents: I'd check blockdev --getra for data drives - too high values for readahead (default to 256 for debian) can hurt read performance.