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

Martin Steigerwald
2008-12-14T17:49:38Z

Am Sonntag 14 Dezember 2008 schrieb 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
> :)

Exactly that. My expectation from my technical understanding of the write
barrier feature is from most performant to least performant:

1) Write cache + no barrier, but NVRAM ;)
2) Write cache + barrier
3) No write cache, where is shouldn't matter whether barrier was enabled
or not

With 1 write requests are unordered, thus meta data changes could be
applied in place before landing into the journal for example, thus NVRAM
is a must. With 2 write requests are unordered except for certain
markers, the barriers that say: Anything before the barrier goes before
and anything after the barrier goes after it. This leaves room for
optimizing the write requests before and after - either in-kernel by an
IO scheduler or in firmware by NCQ, TCQ, FUA. And with 3 write requests
would always be ordered... and if the filesystems places a marker - a
sync in this case - any write requests that are in flight till then have
to land on disk before the filesystem can proceed.

>From that understanding, which I explained in detail in my Linux-Magazin
article[1] I always thought that write cache + barrier has to be faster
than no write cache.

Well I am ready to learn more. But for me until now that was the whole
point of the effort with write barriers. Seems I completely misunderstood
their purpose if thats not what they where meant for.

[1] Only in german, it had een translated to english but never published:
http://www.linux-magazin.de/online_artikel/beschraenktes_schreiben

Ciao,
--
Martin 'Helios' Steigerwald - http://www.Lichtvoll.de
GPG: 03B0 0D6C 0040 0710 4AFA B82F 991B EAAC A599 84C7