12x performance drop on md/linux+sw raid1 due to barriers [xfs]

[Eric Sandeen]

Martin Steigerwald wrote:

> At the moment it appears to me that disabling write cache may often give 
> more performance than using barriers. And this doesn't match my 
> expectation of write barriers as a feature that enhances performance. 

Why do you have that expectation?  I've never seen barriers advertised
as enhancing performance.  :)

I do wonder why barriers on, write cache off is so slow; I'd have
thought the barriers were a no-op.  Maybe I'm missing something.

> Right now a "nowcache" option and having this as default appears to make 
> more sense than defaulting to barriers. 

I don't think that turning off write cache is something the filesystem
can do; you have to take that as an administrative step on your block
devices.

> But I think this needs more 
> testing than just those simple high meta data load tests. Anyway I am 
> happy cause I have a way to speed up XFS ;-).

My only hand-wavy concern is whether this has any adverse physical
effect on the drive (no cache == lots more head movement etc?) but then
barriers are constantly flushing/invalidating that cache, so it's
probably a wash.  And really, I have no idea.  :)

-Eric