RAID: Hardware vs Software?

Hardware or Software RAID?

This page got me thinking about some of the biases we carry around when thinking about RAID.

The question is usually framed as Is Hardware RAID better than software RAID? Well actually it's usually framed as a statement: Hardware RAID is better than software RAID.

The theory is usually two-fold:

• RAID operations are offloaded to a secondary processor, freeing your main processor to do more useful work; and
• since it appears as a "hard disk" to the OS, it runs from pre-boot on.

While the former argument may have held water back in the old days, CPUs are cheap and fast and can saturate a disk without thinking; I/O is more expensive than processing. On the other hand, a RAID card is likely to come with a better controller than a non-RAID card, or at the very least be a break-even in terms of operations. And with cache-battery-backup which increases the likely hood that operations committed to the card will be coherently written to the disk(s), it makes sense to pursue this option if you are spending the money on high-quality disks and buses anyway.

The latter is a better argument in my mind. The good ones (and by that I mean real RAID cards, not these pseudo-RAID chipsets that require special drivers installed for them) handle the physical disks themselves for you. If the computer has hot-swap capability, you can have a drive fail, be removed, a replacement added, and resync happen all more or less automatically.

That said, software RAID can run almost anywhere. If you are doing something cheap and cheerful -- which most of us do at home, and many of us are forced to do in edge situations at work.

For most situations, any RAID is better than no RAID. Even if you are performance-crazy, a RAID-0 array is faster than a non-RAID'd disk. For redundancy, RAID-1, or -1+0, or -5, or -6 can be the ticket.

With software RAID you usually have a choice as to how to group disks. Hardware controllers frequently have their own narrow views of the world and are limited in their configurability.

It is also easier to upgrade the software controlling your RAID if it is software.

Of course, splitting hairs would tell you that the RAID controller runs a firmware, which is software.

All that said, the answer generally depends on your circumstance. I'll usually accept software RAID unless there is a compelling reason why not -- for example, ESXi.