Yes, you're going to have a problem.. :(
If you add 36GB drives to an existing RAID group of 18GB disks, the 36GB disks will be reported and used as 18GB drives.
Once a RAID group is established with a parity drive of xxGB, any disks added to that RAID group will only be used to the same capacity (xxGB) as the parity drive.
If you reverse the situation, and add 18GB drives to an existing 36GB drive RAID group, all drives will be used to their full capacity, but you will run into performance troubles as the volume fills up and the system is forced to write only to the larger disks, thereby reducing the number of physical spindles working for you.
You will definitely want to create a new RAID group (or a new volume) for your 36GB drives to take advantage of the higher capacity.
--- Justin Sullivan Network Appliance, Technical Support Engineer Network Appliance Certified Associate justins@netapp.com Get answers NOW! - NetApp On the Web - http://now.netapp.com Tech Support Hotline: 1-888-4NETAPP
-----Original Message----- From: Jeff Bryer [mailto:bryer@sfu.ca] Sent: Wednesday, April 25, 2001 6:26 PM To: toasters@mathworks.com Subject: Adding a 36gb shelf to F740 with 18gb shelves
We have an F740 with 3 shelves of 18GB disks. It's config'd as two volumes (two raid sets). We've almost outgrown the capacity (we're down to one hot spare and a free disk).
We want to add a new disk shelf to the system, but we're wondering if we're going to lose anything by going to a shelf of 36gb disks. We don't have any intention of adding new volumes (or new raid sets), just growing the existing volumes.
Can we add a shelf of 36gb disks and then switch our 2 existing 18gb parity disks over to 36gb disks? We'd keep a 36gb disk as a hot spare and use all of the 18gb disks in the volumes (including the 2 freed up parity disks and the freed up hot spare). Are we going to run into problems with this? Or do we need to keep an 18gb disk around as a hot spare as well (ie when an 18gb disks dies and uses a 36gb disks as a hot spare, can we force it to go back to the 18gb disk once the faulty 18gb disk has been replaced?).
Are the 36gb disks going to be used at full capacity in this situation (when mixed with the 18gb)? Or is there a better way to do this?
Thanks,
Thanks Justin. So we can't change the parity disk from an 18gb disk to a 36gb disk kind of on the fly. We'd have to create a new volume and do a vol copy or some such thing. Not sure where we got the idea we could 'upgrade the parity disk'.
So we either have to buy 18gb shelves or re-architect our layout. (or waste half a disk adding a 36gb to an 18gb set but that's not a great option as our volume is growing quickly).
Yes, you're going to have a problem.. :(
If you add 36GB drives to an existing RAID group of 18GB disks, the 36GB disks will be reported and used as 18GB drives.
Once a RAID group is established with a parity drive of xxGB, any disks added to that RAID group will only be used to the same capacity (xxGB) as the parity drive.
If you reverse the situation, and add 18GB drives to an existing 36GB drive RAID group, all drives will be used to their full capacity, but you will run into performance troubles as the volume fills up and the system is forced to write only to the larger disks, thereby reducing the number of physical spindles working for you.
You will definitely want to create a new RAID group (or a new volume) for your 36GB drives to take advantage of the higher capacity.
Justin Sullivan Network Appliance, Technical Support Engineer Network Appliance Certified Associate justins@netapp.com Get answers NOW! - NetApp On the Web - http://now.netapp.com Tech Support Hotline: 1-888-4NETAPP
Hold on a second. I think that you can add a larger disk to a raid group of smaller disks and the larger disk becomes the parity drive automatically. Then if you add more large disks, all the space on them is used.
I think the confusion here is a disk failure. If the hot spare is larger than the failed drive, then you waste space on the spare drive because only stripes from the failed drive are reconstructed on the spare. Extra space on the spare is not used.
So if you have a raid group with different size drives, its best to also have hot spares in those sizes. ONTAP is smart enough to replace a failed drive with a spare of the same size if possible. Otherwise it has to use a bigger replacement and that does indeed waste space.
Has this policy changed? If so, when?
Thanks Justin. So we can't change the parity disk from an 18gb disk to a 36gb disk kind of on the fly. We'd have to create a new volume and do a vol copy or some such thing. Not sure where we got the idea we could 'upgrade the parity disk'.
So we either have to buy 18gb shelves or re-architect our layout. (or waste half a disk adding a 36gb to an 18gb set but that's not a great option as our volume is growing quickly).
Yes, you're going to have a problem.. :(
If you add 36GB drives to an existing RAID group of 18GB disks, the 36GB disks will be reported and used as 18GB drives.
Once a RAID group is established with a parity drive of xxGB, any disks added to that RAID group will only be used to the same capacity (xxGB) as the parity drive.
If you reverse the situation, and add 18GB drives to an existing 36GB drive RAID group, all drives will be used to their full capacity, but you will run into performance troubles as the volume fills up and the system is forced to write only to the larger disks, thereby reducing the number of physical spindles working for you.
You will definitely want to create a new RAID group (or a new volume) for your 36GB drives to take advantage of the higher capacity.
Justin Sullivan Network Appliance, Technical Support Engineer Network Appliance Certified Associate justins@netapp.com Get answers NOW! - NetApp On the Web - http://now.netapp.com Tech Support Hotline: 1-888-4NETAPP
-- Jeff Bryer bryer@sfu.ca Systems Administrator (604) 291-4935 Academic Computing, Simon Fraser University
Steve Losen scl@virginia.edu phone: 804-924-0640
University of Virginia ITC Unix Support
----- Original Message ----- From: "Sullivan, Justin" Justin.Sullivan@netapp.com To: "'Jeff Bryer'" bryer@sfu.ca; toasters@mathworks.com Sent: Wednesday, April 25, 2001 5:55 PM Subject: RE: Adding a 36gb shelf to F740 with 18gb shelves
Yes, you're going to have a problem.. :(
If you add 36GB drives to an existing RAID group of 18GB disks, the 36GB
disks will be reported and used as 18GB drives.
Once a RAID group is established with a parity drive of xxGB, any disks
added to that RAID group will only be used to the same capacity (xxGB) as the parity drive.
When did this happen? Netapp used to have a nifty feature whereby the first larger disk drive added to a RAID group became the new parity drive, the old smaller drive became a new data drive, and additional new larger drives would then be used at their full capacity.
Unlike in the situation where a smaller driver fails and the contents must be reconstructed on to a larger drive.
Bruce
This used to be the case. Back in the sad days of bug 13880.
http://now.netapp.com/NOW/cgi-bin/bol?Type=Detail&Display=13880
This issue is resolved with the whiz-bang spacemap as implemented in 6.0 and later, where we have a much better idea of where all the space is, and won't paint ourselves into a corner if we don't have to.
As to the parity disk question, to quote our own dogfood:
http://now.netapp.com/NOW/knowledge/docs/ontap/rel601r3/html/sag/disks4.htm
"Parity disk determination: If an added disk is larger than the existing parity disk, the added disk becomes the parity disk, and the former parity disk becomes a data disk."
We'll bring Justin back to the lab for reprogramming.
Now, a reasonable follow up question would be: can I make a new raid group out of all 36G drives, and not waste any 36G drives as parity disks on 18G raid groups? My understanding is that, if you change the volume's raid group size to whatever the size of the 'last' 18G disk raid group is, we'll make a new raid group out of the 36G drives. I'm going to test this now.
A funny thing happened to me on the way to the lab. I went to ask the lab manager if I could do some vol destroys on a few filers to get some mixed disk sizes. Our lab manager, Jeff Mohler, is a reformed PSE, and he'd done the very experiment I was about to conduct at customer sites, many times.
Rest assured, you don't need to create a new volume.
Look at your sysconfig -r output presently. Since everything's in one volume, the last raid group listed (raid group 1, if you have two raid groups like you say you do) is the raid group that's subject to change in size. This is controlled by the vol option raidsize. Set the raid size to whatever this raid group size is presently by 'vol options <volname> raidsize <n>'. Add your disks. If the number of disks you are adding is greater than <n>, add some number of 36G drives less than n, and change the vol option to the number of 36G disks you intend to add.
I had fears that the raid group size would be stored in the raid group by some pernicious machinations, but Jeff assuaged any such trepidations. I'll bet Jeff's life on it. If you have one 18G spare, and one 36G spare, ONTAP now has the smarts to pick the right sized spare. If you are using mixed raid group sizes, I recommend you run ONTAP 6.x to take advantage of the spacemap, and avoid some unpleasant endgame scenarios if you do manage to fill up your disks.
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Bruce Sterling Woodcock -
Jeff Bryer -
Sam Rafter -
Steve Losen -
Sullivan, Justin