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

[Redeeman]

On Sat, 2008-12-13 at 11:40 -0600, Eric Sandeen wrote:
> 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.  :)

My initial thoughts were that write barriers would enhance performance,
in that, you could have write cache on. So its really more of an
expectation that wc+barriers on, performs better than wc+barriers off :)

> 
> 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
> 
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