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