RAID 10 vs RAID 5 (12 disk)

In the real world you wouldn't calculate what you can do with the storage you just bought, but you'd buy what you need for your applications!

So if your applications need 2000 IOps with a R/W ratio of 80%, this will generate the following:

0.8 x 2000 = 1600 Read IOps and 0.2 x 2000 = 400 write IOps

In RAID5 this means you'll be generating 1600 + 4x400 = 1600 + 1600 = 3200 IOps

In a (4+1) set up you'll need 3200 / 180 = 17.77 drives, so rounded up to the next set of 5 disks = 20 disks

In RAID10 this means you'll be generating 1600 + 2x400 = 1600 + 800 = 2400 IOps

In any RAID10 set up you'll need 3200/180 = 17.77 drives, so rounded up to the next even number (RAID10 is always an even number) this gives you 18 disks.

If the capacity you need can be delivered by the 18 disks in RAID10 set up, meaning 9 disks, this would be the cheaper solution!

If the capacity you need is more than 9, but max (4/5)x20=16 drives, you need the RAID5 set up.

a) How unreliable will be RAID10 without spares compared to RAID5 with 2 spares? We have Premium Support for VNXe, but only business hours availability from our side.

Hot Spares are global, so not dedicated to a single Raid Goup. If you have 1 HS for the entire array, you should be fine. If you're unconfortable with just 1, use 2, it's whatever you want.

b) Does VNXe RAID10 implementation reads from both disks on a mirror, or just from one?

Both, whichever one is faster serves the I/O

c) What would be the read IOPS from both configs?

180 IOps per disk, it doesn't matter if you use RAID10 or 5 or whatever. The thing you're looking for is the write penalty: With RAID10 the WP is 2, since each write has to go to 2 drives. With RAID5 the WP is 4. But that's only for WRITES! With Reads it doesn't matter what Raid level you have.

c.1) Will RAID10 give me 12x(read IOPS of 1 disk) or just 6x(IOPS of 1 disk)?

If there's 12 disks in the Raid Group, it'll give you 12 x 180 IOps. It makes no difference if you're reading or writing. It's the write penalty that takes care of the math. Well actually, you need to do the math. I'll give you an example at the end of this post.

c.2) Will RAID5 give me 10x(read IOPS of 1 disk) or just 8x(IOPS of 1 disk)?

If you have 2 RAID5(4+1) Raid Groups, this means you have 10 disks, so that adds up to 10 x 180 IOps.

d) What would be the expected ballpark random write IOPS from both configs, considering the VNXe 3100 8GB config (~1GB cache)?

I don't understand your question.

If you have 10 disks in a RAID5 set up this will give you 10 x 180 = 1800 IOps. If you're doing 100% writes, this will give you 1800 / 4 = 450 random host writes per second. Since most applications do about 90% reads and only 10% writes the math is as follows: suppose your application does 1400IOps and 90% of those are reads, this means that 0.9x1400=1260 IOps are reads and 0.1x1400=140 IOps are writes. 1260 + 4x140 = 1260 + 560 = 1820 IOps.

In a RAID10 config (5+5) with a WP of 2 and 90% reads and 10% writes the math is as follows: 1650 IOps from the host means 0.9 x 1650 = 1485 reads and 0.1 x 1650 = 165 writes per second. 1485 + 2 x 165 = 1815 IOps.

But a mojor thing to consider is Little's law which states that at around 66% of the performance the response times will significantly increase. A small increase in IOps will invoke a large increase in response times. Depending on the application this could be a burden or not. I can imagine that for important database applications this should be avoided.

Another consideration is available space. A RAID5 config of 2 x (4+1) gives you the capacity of 8 drives while a RAID10 (5+5) gives you the capacity of only 5 drives.

e) What config would you prefer, and why?

It depends on your applications. Do the math! What do your applications generate? How much % is reads and how much is writes? Do you need to handle spikes in performance without any problems or is it ok if at top performance it takes a little longer to process all IOs ? In this case you could hammer the array a bit harder or you could choose a less performant configuration, giving you more capacity. It's your choice!

A wise choice is to do the VNX performance workshop.

Thanks for your insights.

First some remarks:

- Both RAID10 and RAID5 configurations fit my current baseline performance needs. Both fit my capacity needs as well.

- As this was the smallest VNXe configuration I could buy that would match my baseline performance needs, "best fitting" was never an option.

- Because this has 12 disks, my choices are either two RAID10 groups without spares, or two RAID5 groups with two spares. RAID5 should be more unreliable, but I wonder what the lack of a spare on the RAID 10 will do. I should be able to replace a disk in 4 hours during business time, or 8 hours outside business hours. Still looks like a smaller window than a RAID 5 rebuild.

About the IOPs, are you sure RAID10 will provide me the read IOPs of all the disks? Regular RAID controllers only read from one side of the mirror.

Thanks.

I would never go for a set up without any hot spares. Suppose a disk fails on a Friday right after you left and EMC isn't somehow able to reach you, and for some silly reason a second disk fails on Sunday.

You come in on Monday seeing 2 disks dead. Are you willing to take the risk of data loss?

I haven't seen 2 disks failing at the same time in the same Raid Group yet, but I did see 3 failed disks in separate Raid Groups in a 300+ disks array! I'm not willing to take the chance. It's not my data, but it's my job to protect that data as if it were mine!

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