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

Redeeman
2008-12-14T03:31:25Z

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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