On Tue, Jan 24, 2012 at 10:15:04AM -0500, Chris Mason wrote:

https://lkml.org/lkml/2011/12/13/326

This patch is another example, although for a slight different reason.
I really have no idea yet what the right answer is in a generic sense,
but you don't need a 512K request to see higher latencies from merging.

That assumes the 512k requests is created by merging.  We have enough
workloads that create large I/O from the get go, and not splitting them
and eventually merging them again would be a big win.  E.g. I'm
currently looking at a distributed block device which uses internal 4MB
chunks, and increasing the maximum request size to that dramatically
increases the read performance.