Raid 6 and different size HDDs

[zootielolo]

So i have lurked here for a while and finally had a question I couldn't find with a search. I recently switched from a LSI 9650se controller to a LSI 9750 and a HP sas expander. My previous array was (4) 1 tb drives in raid 5. I was running low on space, so I decided to increase my capacity and bought (4) 2tb Hitachi drives. I also changed to raid 6. I contacted LSI to ask if different size drives could be fully utilized if they were larger than the current drive size. I was told that when I migrated the array, the drives would not be fully utilized, but that if I sent in a log file an engineer would write a script that would allow the full space of the drives to be utilized. However, I was having a bit of trouble generating the log file, and when I contacted LSI again (new agent) I was told that it would not be possible to utilize the space on the drive fully. Has anyone else ran into this with an LSI controller? Also, I'm sure many others have tried to use larger HDDs to increase capacity, have you been successful in fully using the new drives, and if so how did you accomplish this?

Forgot to include: I am running Windows Server 2008 R2 Standard

 

[john4200]

I'm not an LSI user, but I think their software will let you create a second RAID volume with your 4 new 2TB drives. I think you were wanting to combine them with your 4 1TB drives to create an 8 drive RAID, but I do not think that is feasible. Is there any reason why you cannot just have two separate RAID volumes? Depending on your OS, you can probably span the two volumes together at a higher level than the RAID.

 

[zootielolo]

You are correct, I was wanting to combine the 4 1tb drives with the 4 2 tb drives in a single raid. I was wanting to do this because of the economies of scale you get from using a larger number of drives in a single raid6 rather than the two separate arrays. Would it be possible to combine the 4 1 tb drives into 2 tb raid0 arrays, and then build a single raid6 with the 2 tb drives and these two raid0 arrays?

 

[john4200]

Would it be possible to combine the 4 1 tb drives into 2 tb raid0 arrays, and then build a single raid6 with the 2 tb drives and these two raid0 arrays?

Theoretically, I think it should be possible. I'm pretty sure it could be done with linux software RAID (mdadm). My guess is the LSI controller and software probably does not support it -- it would be a kind of hybrid RAID 0+6 which is not something you hear about much.

But even if it is possible, I advise against it. When it comes to RAID, I prefer to keep things as simple as possible. Added complexity often causes trouble down the road.

I suggest two separate RAID volumes, and then span them at the OS level (volume manager).

Or you might consider copying your old volume to a new 4 x 2TB RAID 6 volume, and then expanding that with more 2 TB drives in the future. And keep the old 4 x 1 TB RAID 5 as a backup.

 

[JeffBlair]

I know with mdadm you can do a liner array with different sized disks, but as far as I know it's not redundant. The best, and easiest, way is like john4200 said. Create a new RAID5 array with the new 2T drives, and then copy the data over. You could see if you can find a cheap external array for the 4 1T drive's you have left over. They would make either a nice backup, or storage for files that you're not trying to stream. That's what I'm trying to find now for my old 300gig IDE drives I just retired.

 

[drescherjm]

You can definitely do a raid5 or 6 in mdadm with different sized disks however it will use the smallest disk as the size so its best to partition the drives instead of giving mdadm the full disk and then with partitions you can use the leftover space for a different array. Then use lvm to extend these arrays.

 

[zootielolo]

So I ended up doing what john4200 suggested, but my drive performance seems to have suffered dramatically. I rebuilt the drives into two separate raid5 arrays, then I created a spanned disk out of them in partdisk. However, when i transferred my files back to the array, the maximum write speed is only around 24 mb/sec. When I had the drives in a single raid6, they were writing closer to 100-150 mb/sec. Is this typical when creating a spanned disk? Thanks to all those who have answered!