On Tue, 29 Dec 2009, Ross Walker wrote:
Some important points to consider are that every write to a raidz vdev must
be synchronous. In other words, the write needs to complete on all the
drives in the stripe before the write may return as complete. This is also
true of "RAID 1" (mirrors) which specifies that the drives are perfect
duplicates of each other.
I believe mirrored vdevs can do this in parallel though, while raidz vdevs
need to do this serially due to the ordered nature of the transaction which
makes the sync writes faster on the mirrors.
I don't think that the raidz needs to write the stripe serially, but
it does need to ensure that the data is committed to the drives before
considering the write to be completed. This is due to the nature of
the RAID5 stripe, which needs to be completely written. It seems that
mirrors are more sloppy in that writing and committing to one mirror
is enough.
Bob, an interesting question was brought up to me about how copies may affect
random read performance. I didn't know the answer, but if ZFS knows there are
additional copies would it not also spread the load across those as well to
make sure the wait queues on each spindle are as even as possible?
Previously we were told that zfs uses a semi-random algorithm to
select which mirror side (or copy) to read from. Presumably more
(double) performance would be available if zfs was able to precisely
schedule and interleave reads from the mirror devices, but perfecting
that could be quite challenging. With mirrors, we do see more read
performance than one device can provide, but we don't see double.
Bob
--
Bob Friesenhahn
bfrie...@simple.dallas.tx.us, http://www.simplesystems.org/users/bfriesen/
GraphicsMagick Maintainer, http://www.GraphicsMagick.org/
_______________________________________________
zfs-discuss mailing list
zfs-discuss@opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
