Hey Frank,
Frank Cusack wrote:
Patrick Bachmann:
IMHO it is sufficient to just document this best-practice.
I disagree. The documentation has to AT LEAST state that more than 9
disks gives poor performance. I did read that raidz should use 3-9 disks
in the docs but it doesn't say WHY, so of course I went ahead and used
12 disks.
The man pages says "The recommended number [of devices] is between 3
and 9". At the time of the discussion the "ZFS Admin Guide" didn't
mention this at all and the result of this discussion is, that now
it says: "If you are creating a RAID-Z configuration with many
disks, as an example, a RAID-Z configuration with 14 disks is better
split up into a two 7-disk groupings. RAID-Z configurations with
single-digit groupings of disks should perform better."
When I say I disagree, I mean this has to be documented in the standard
docs (man pages) not some best-practices guide on some wiki.
To me "documenting a best-practice" does not imply that it is just
spelled out in some random wiki or a BluePrint but rather that it is
written down "somewhere" and that this "best-practice" is not just
documented in the mailing-lists archives, which in some other
communities seem to be the only documentation available.
But really I disagree that this needs documentation. So much of zfs is
meant to be automatic, now we're back to worrying about stripe width?
(Or maybe that's not the problem but it sure is the same type of manual
administration.) I may have 12 disks and it simply does not make sense
(for my theoretical configuration) to split them up into two pools. I
would then have to worry about sizing each pool correctly. zfs is supposed
to fix that problem.
Richard already pointed out that you should split the devices into a
number of vdevs and not pools.
How is ZFS going to know what gives the best performance on your
systems config? There are a lot of things you know better off-hand
about your system, otherwise you need to do some benchmarking, which
ZFS would have to do too, if it was to give you the best performing
config.
Oh, and performance isn't the only objective. See the threads
started by Richard Elling in the past couple of weeks on this
mailinglist.
If no one has done it yet, I'll file a bug against the zpool(1M) man
page to get the performance concern included. But to me the "ZFS
Admin Guide" seemed to be a required reading. You might want to
check it out.
Greetings,
Patrick
_______________________________________________
zfs-discuss mailing list
zfs-discuss@opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
