On Fri, 31 Dec 2010, Spelic wrote: > Ok never mind, I found the problem: > LVM probably uses O_DIRECT, right? > Well it's absymally slow on MD parity raid (I checked with dd on the bare MD > device just now) and I don't know why it's so slow. It's not because of the > rmw because it's slow even the second time I try, when it does not read > anything anymore because all reads are in cache already. The point of O_DIRECT is to *not* use the cache. Although a write-through cache would seem to be OK, you have to make sure that ALL writes write-through the cache, or the data on parity raid will be corrupted. The R/M/W problem afflicts every level of parity raid in subtle ways. That's why I don't like it. -- Stuart D. Gathman <stuart@bmsi.com> Business Management Systems Inc. Phone: 703 591-0911 Fax: 703 591-6154 "Confutatis maledictis, flammis acribus addictis" - background song for a Microsoft sponsored "Where do you want to go from here?" commercial. _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/