General question about RAID configurations, specifically nested RAID 50

Question

I understand about the different types of RAID configurations:

1. RAID 1 -- Mirror [Redundant]
2. RAID 0 -- Stripe [Performant]
3. RAID 5 -- Hot Spare [Redundant]

From what I understand, RAID 1 gives you redundancy with no performance hit, RAID 0 gives you performance at the cost of redundancy, and RAID 5 is good in the senes that you gain extra storage space and still have redundancy, at the expence of performance.

I also understand a bit about Nested RAID configurations, specifically RAID 10 (1+0). This provides the benifit of both RAID 1 + RAID 0. Giving you the best of both worlds.

My question is this: (And a SAN is out of the question here) Would a RAID 50 or a RAID 10 be better suited for a virtualization environment where most VMs are very under used in terms of disk IO and CPU use. I'm trying to maximize the number of VMs I can put on a host machine, and my limiting factor is drive space.

So, I'm trying to put as many VMs on a Virtual Host as possible, most of the IO on any of the VMs will be OS related and on occasion user requests will trigger a VM to do some IO, but this is rare and relativly evenly distributed across VMs.

My thought is that the RAID 50 could prove better because A) better use (2/3 total size of all disks in the array instead of 1/2) and B) potentially better better random read/write performance since there are more drives in play for storage, so read/write seeks are more likely to be on different physical drives.

update

Virtual Hosts - Windows Server 2008 R2

Virtual Machine - Windows 2000 Server - Windows Server 2008 R2

update 2

Single hardware RAID controller. Which I assume would do parity calculations?

Two RAID 5 arrays < 10 total drives

Raw performance is not an issue in most situations, the main limiting factor is physical drive space. Most of the IO is just OS chatter. Due to limitations in our database software we need a 1-server::1-system, even if its only 5 users. So most of our database servers sit idle. The overhead is just running that many OSes on the drive array.

update 3

I am currently using a RAID 10, and it is working just fine; but I'm finding that more drive space would be useful, (thus the inquiry about RAID 50 for better use of physical disks). As suggested below, I'm hearing that RAID 6 could offer similar performance, redundancy, and storage capacity as RAID 50 -- is that true or am I not understanding the responses?

Answer 1

Honestly, the answer to your question depends, in part, on what the rest of your installation looks like. You're asking questions about redundancy levels on local RAID controllers when you're talking about storage that itself has a single point of failure, the host machine that it's connected to.

Unless you go to some sort of shared array (which might be much simpler and lower-end than a SAN) you'll find that failure of your physical compute host will take down all your workloads. This will always be the case unless you move to a shared array that doesn't use your local RAID controller. This fact may lead you to a strategy where you back up your VMs pretty often, more so than you might if you clustered two or more hosts with a shared back end.

And if you do those backups so often that you can survive a host failure, you might conclude that you just want to use that local RAID controller in a striped (RAID 0) mode for pure performance.

Honestly, though, in your situation I'd probably be looking for a single RAID6 array, if your controller supports it. As "freiheit" said, though, it depends very much on the type of storage traffic that your VMs are going to generate. A lot of random writes from several VMs simultaneously might overwhelm your RAID array. If so, you'd want to optimize for performance. If not, you might want to optimize for reliability, depending less on backups.

Answer 2

How many drives are you talking about? How much storage do you need? Are you talking about 2 RAID5s in a RAID0, or are you talking 20 RAID5s in a RAID0? What's the hardware? Fundamentally, the performance difference between RAID50 and RAID10 depends on the hardware.

RAID 1 gives you no performance change on write, but may double your read performance (especially with multiple simultaneous read requests; they can be spread across both drives)

RAID 5 isn't "hot spare", it's more like "striped parity". You can lose any single drive in a RAID5 and it continues to operate, but in a "degraded" fashion. Write performance to a RAID 5 can be limited by how fast you can calculate parity; a given RAID card will have a maximum speed that it can calculate parity and therefore a maximum write speed, regardless of how many drives you have.

A RAID 50 (RAID 0 of RAID 5 arrays) could work well. It's similar in performance to a single large RAID 5 or a JBOD of RAID 5. However, a RAID10 has no parity calculation limit. Unless you've got some beefy hardware, it's likely that parity calculation on the multiple underlying RAID5 drives will be a limiting factor on writes. It really depends on what's doing the RAID5 parity calculations.

Whether you do RAID 50 or RAID 10, with that many drives you want to be sure to keep a hot spare (an unused drive ready to be moved into any of the underlying arrays in case of drive failure). With that many drives, it's inevitable that one will fail and a hot spare can buy you time. You might also consider RAID6 (RAID5 double parity) instead, since it's more fault tolerant. Performance, space usage and fault tolerance of a single RAID6 should be very similar to a RAID 0 made up of two RAID 5 arrays.