On 24/06/2010 20:52, Arne Jansen wrote:
Ross Walker wrote:
Raidz is definitely made for sequential IO patterns not random. To
get good random IO with raidz you need a zpool with X raidz vdevs
where X = desired IOPS/IOPS of single drive.
I have seen statements like this repeated several times, though
I haven't been able to find an in-depth discussion of why this
is the case. From what I've gathered every block (what is the
correct term for this? zio block?) written is spread across the
whole raid-z. But in what units? will a 4k write be split into
512 byte writes? And in the opposite direction, every block needs
to be read fully, even if only parts of it are being requested,
because the checksum needs to be checked? Will the parity be
read, too?
If this is all the case, I can see why raid-z reduces the performance
of an array effectively to one device w.r.t. random reads.
http://blogs.sun.com/roch/entry/when_to_and_not_to
--
Robert Milkowski
http://milek.blogspot.com
_______________________________________________
zfs-discuss mailing list
zfs-discuss@opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
