30 TB RAID6 + XFS slow write performance

[Dave Chinner]

On Wed, Jul 20, 2011 at 04:04:31PM +0200, Michael Monnerie wrote:
> On Mittwoch, 20. Juli 2011 Stan Hoeppner wrote:
> > I thought this was packing multiple small files into
> > a single stripe write, which you just explained XFS does not do.
> 
> This is interesting, I jump in here. Does that mean that if I have a XFS 
> volume with sw=14,su=64k (14*64=896KiB) that when I write 10 small files 
> in the same dir with 2KB each, each file would be placed at a 896KiB 
> boundary?

No, they'll get sunit aligned but default, which would be on 64k
boundaries.

> That way, all stripes of a 1GB partition would be full when 
> there are roughly 1170 files (1170*896KiB ~ 1GB). What would happen when 
> I create other files - is XFS "full" then, or would it start using sub-
> stripes? If sub-stripes, would they start at su (=64KiB) distances, or 
> at single block (e.g. 4KiB) distances?

It starts packing files tightly into remaining free space when no
free aligned extents are availble for allocation in the AG.

Cheers,

Dave.
-- 
Dave Chinner
david at fromorbit.com