Software RAID on top of RAID?

system · 06-23-2003, 09:58 PM

I've got a 5 disk 40MB per disk software RAID 5 array. I'd like to migrate from 40MB disks, but not all at once. What would the performance consequences be if I created a new RAID 0 across the 5 disks (/dev/md0) which was 200MB, and bought another 200MB drive and created a RAID 1 array using md0 and the 200MB hdd? Thus, one device of the RAID 1 array would actually be the 40MB drives in RAID 0. Any esitimates on how this would compare in read/write throughput and latency to the current pure RAID 5 array?

Also, another thought. To upgrade, I'll end up buying a second 200MB drive and making a 3 element RAID 5 array. Two 200MB drives, and the RAID 0 array. That's not for sure, but may happen depending on my ability to sell off the 40MB drives. I'd also like to know theoretical performance of that too...

crokett · 06-24-2003, 09:18 AM

So you are talking about a software RAID 1 with one of the drives being a RAID 0 hardware array? The 5 disk RAID 0 will be slightly faster than a single disk now writing to 5 disks instead of 1 and the latentcy is less, but on the other disk you are still only writing to one disk. Any disk system is only as fast as the slowest component, so you haven't gained much. Once you add a third disk for a RAID 5 it will also be a little slower than the RAID 1, especially if Linux has to calculate all the parity. Unless you need the capacity, and if performance is a premium when you get the second 200 gig drive, replace theRAID 0 array with it and just run a mirrored set. That will be faster than the RAID 5.

system · 06-25-2003, 08:51 PM

I didn't explain clearly.. here:

Code:

Current config:

/dev/md0
     software RAID 5
     devices:
     5 x 40 GB

Proposed Cheap Upgrade:            Alternative Upgrade Path:

/dev/md0                            /dev/md0
     software RAID 0                   software RAID 1
     devices:                          devices:
     5 x 40 GB drive                   2 x 200 MB drive

/dev/md1
     software RAID 1
     devices:
     /dev/md0
     1 x 200 GB drive

Proposed Future Upgrade Path:

/dev/md0
     software RAID 0
     devices:
     5 x 40 GB drive

/dev/md1
     software RAID 5
     devices:
     /dev/md0
     2 x 200 GB drive

All RAID devices will be software. I'm currently running RAID 5. I want to maintain redundancy.

The machine running the RAID arrays is a K6-2 500MHz, 100MHz bus with 418 MB ram.. It is dedicated and only runs samba and software RAID. Basically, I'm using the K6 proc just to run the array.

My question is whether layering multiple software RAID arrays on top of each other, in this case a RAID 1 array on top of a RAID 0 array, will cause a large change in latency/throughput compared to a pure 2 disk RAID 1 array.