Am Sonntag 14 Dezember 2008 schrieben Sie: > Am Sonntag 14 Dezember 2008 schrieb Peter Grandi: > > But talking about barriers in the context of metadata, and for a > > "benchmark" which has a metadata barrier every 11KB, and without > > knowing whether the storage subsystem can queue multiple barrier > > operations seems to be pretty crass and meangingless, if not > > misleading. A waste of time at best. > > Hmmm, as far as I understood it would be that the IO scheduler would > handle barrier requests itself if the device was not capable for > queuing and ordering requests. > > Only thing that occurs to me know, that with barriers off it has more > freedom to order requests and that might matter for that metadata > intensive workload. With barriers it can only order 11 KB of requests. > Without it could order as much as it wants... but even then the > filesystem would have to make sure that metadata changes land in the > journal first and then in-place. And this would involve a sync, if no > barrier request was possible. No it hasn't. As I do not think XFS or any other filesystem would be keen to see the IO scheduler reorder a journal write after a corresponding meta data in-place write. So either the filesystem uses sync... > So I still don't get why even that metadata intense workload of tar -xf > linux-2.6.27.tar.bz2 - or may better bzip2 -d the tar before - should > be slower with barriers + write cache on than with no barriers and > write cache off. ... or it tells the scheduler that this journal write should come prior to the later writes. This is what a barrier would do - except for that it cannot utilize any additional in-hardware / in-firmware support. So why on earth can write cache off + barrier off be faster than write cache on + barrier on in *any workload*? There must be some technical detail that I miss. 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
