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 -- To unsubscribe from this list: send the line "unsubscribe linux-raid" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
