svn commit: r201658 - head/sbin/geom/class/stripe

[Alexander Motin]

Bruce Evans wrote:
> On Thu, 7 Jan 2010, Alexander Motin wrote:
>> One thing IMHO would be nice to see there is the alignment of the
>> read-ahead requests to the array stripe size/offset. Dirty hack I've
>> tried there, reduced number of requests to the array components by 30%.
> 
> ffs thinks that bsize alignment is adequate. 

Alignment definitely should be adequate, but it is a different task. I
say that bsize and stripe size are not needed to be equal, as they serve
different purposes. Even with perfectly aligned usual 16K bsize and 64K
stripe size we will have 75% chance of misaligned I/O.

> It doesn't try to align
> files any more than that.  Then for sequential reads from the beginning
> of the file, vfs read clustering tries to read MAXPHYS bytes at a time,
> so it perfectly preserves any initial misalignment.

I think we would benefit, if vfs could shrink first request of long
read-ahead session a bit, to get all further MAXPHYS-sized reads to not
cross more stripe boundaries then really required. In worst case it can
generate one extra request to array, in best case it can twice reduce
number of requests to the array components.

-- 
Alexander Motin