I agree with Tom. Your best bet is to add 6 disks to a new raid group. No sense in having all those spares sitting around doing nothing.
I would try to avoid expanding a 36GB-disk raid group past 14 disks for reliability.

Moshe Linzer
IT Manager

National Semiconductor, Israel
Phone: 972-9-970-2247
Fax: 972-9-970-2001

"Yarmas, Tom" <Tom.Yarmas@netapp.com>
Sent by: owner-toasters@mathworks.com

04/06/2004 04:19

To: "Stewart, John" <johns@artesyncp.com>, toasters@mathworks.com
cc: "IT Technical Services" <ITTech@artesyncp.com>
Subject: RE: Best practices for new RAID group

The paper you reference is a little dated unless you are still running
5.x of Data ONTAP. Data ONTAP version 6.x included some significant
changes to the WAFL and ONTAP RAID.

The number of RAID groups is much less relevant than the total number of
data drives in the volume. However, adding 1 or two drives to a volume
(or RAID Group) is still not the best approach.

Your best bet is to add a new RAID group of 6 disks to the volume. You
will be adding the disks anyway (over time), and you are using qtrees to
divvy up the space, so just add them all at one time and not worry about
it.

As Chris Thompson pointed out you can use RAID-DP (assuming you are
running DOT 6.5), and add 2 disks to the existing RAID group. But that
would likely not be optimal because you would still have a small number
of "empty" disks in the RAID group and they will be utilized more than
the other "more full" disks until space gets more evenly distributed
across all of the drives.

-tom

-----Original Message-----
From: Stewart, John [mailto:johns@artesyncp.com]
Sent: Thursday, June 03, 2004 5:44 PM
To: 'toasters@mathworks.com'
Cc: IT Technical Services
Subject: Best practices for new RAID group

We've got an F740 with three shelves of 36GB disks (the 3rd recently
added).

We've only got one volume, vol0, with 14 disks in it now (13 data and
one parity). We use both CIFS and NFS. We have 7 spare disks (6 of which
we'd like to eventually use for data, with one spare of course).

I just went to add one of the spare disks to vol0 (running low on space)
and got the error:

vol add: starting a new RAID group; at least two disks are needed

So, as I (now) understand it, we've got to either:

- Override the 14 disk limit for a RAID group

or

- Add a new RAID group to the current volume

or

- Add a new volume

The 3rd choice is unattractive because we like the idea of One Huge
Filesystem, that we can divvy up as needed into different qtrees.

The 2nd choice sounds like a Bad Idea in that according to this
document, section 3.4: http://www.netapp.com/tech_library/3027.html,
that the filer will try to distribute writes evenly between the old and
new RAID group... if the new RAID group only contains two disks (we
don't want to add more disk space than is needed now), this could be
disatrous to performance

So is the 1st choice, increasing the size of the RAID group, a viable
option? What is the downsize? Only increased risk of a double disk
failure taking out the whole system? Is it possible to add a second
parity disk to the same RAID group?

Am I missing something here? Any clue-by-fours quite welcome.

thanks

johnS