29.06.2011 18:50, Christoph Hellwig wrote: > On Wed, Jun 29, 2011 at 06:17:02PM +0400, Michael Tokarev wrote: >>> Honestly, I don't know. Usually the problem is resolved with setting a >>> different cache option, so nobody bothers to ask for details. I'd guess >>> that it's ext4 in most cases. >> >> Extremly poor performance also happens on raw devices -- >> be it lvm volumes or plain partitions, so that's w/o >> any filesystem. > > Depends on your setup. If you have ATA devices that have WCE=1 > it will suck due to the non-queueable FLUSH command. If you have > an older kernel with the draining barriers semantics it will to. > > If you run on a SAS disk or FC array with WCE=0 performance will > be quite good and close to native performance on these. > > That's all assuming you use O_DIRECT. using cache=writhrough > as-is will suck everywher.
The whole question (sorry I cut a bit more details than needed) was about cache=writhrough :) O_DIRECT on consumer drives with WCE=1 works fairy well too. /mjt
