If performance is not a issue software vs hardware raid

Question

If performance is not a issue what makes hardware raid better then software raid. so far i have drawn the following conclusions.

Hardware raid is limited to the raid controller if that pacs up you need another of the exact raid controllers, Where as if your server or OS pacs up with software raid you can switch all the disks to another computer and use the exact same OS.

What are your guys view on the matter since so far thats the only benefit of hardware raid that i have found out.

Answer 1

There are two major benefits to hardware RAID:

1. Performance. If that's not a consideration, don't consider it.
2. Battery-Backed Cache.

2 really goes hand-in-hand with 1, but it also plays a role from a data integrity standpoint: Let's consider two identical machines, one with Software RAID, one with a battery-backed hardware RAID, both with write cacheing enabled.
If we write a whole bunch of data to these two machines, then rip the plug out of the wall before the data is sync'd to disk (by the OS on the software RAID box, or the controller on the hardware RAID box), we now have two different scenarios:

The Hardware RAID machine will, when power is restored, see that there are writes in the cache, spin up the drives, sync the data to disk and move on with its life.

The Software RAID machine will, when power is restored, reboot normally. Since the system RAM lost power the writes that were stacked up waiting to be flushed out to disk are gone forever. The results here could range from inconvenient (a little data loss) to catastrophic (the partition table has been eaten, vital irreplaceable data has been left in a half-written, corrupted state, etc.).

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Now that I have portrayed the nightmare scenario, consider the likelihood of it happening. Presumably your server is on a UPS, ideally with redundant power supplies fed from different UPS circuits. The chance of a catastrophic failure in that scenario is relatively low, and you may be fine trusting software RAID here.
In exchange for taking that (theoretically small) risk you aren't beholden to a particular brand of controller and revision of the controller firmware in the event something goes horribly wrong, and you have the flexibility afforded you by software RAID (which often includes mirroring only part of a drive, or being able to mirror across drives of different sizes, spindle speeds, etc. -- Possibly a consideration if you have Frankenstein hardware).

Answer 2

Your question basically encapsulates my philosophy.

I prefer hardware RAID controllers in "enterprise" deployments. If I know I'm going to have warranty, spares availability, and vendor support I'd typically rather have the performance and features of hardware RAID.

In small business scenarios where the likelihood is that the Customer is going to let support lapse I'd rather take the (increasingly very slight) performance "hit" and go with software RAID (typically RAID-1 in small businesses I work in, anyway). This is typically cheaper than a hardware solution. In the end, I'd rather have some kind of protection against disk failure than none at all.

As you say, with software RAID I know I'll be able to read the disks using the same software RAID implementation on any other computer without a special controller. Typically the Customer isn't pushing the performance envelope of their server and doesn't need fancy features so they end up losing nothing and gain some ability to recover in the event of catastrophic failure of the server computer.

Software RAID performance has gotten quite good in the last few years, but I've been using it in small deployments all the way back to Windows NT 4.0 with good results. It's a cheap insurance policy.