Disaster Recovery

Software vs. hardware RAID: Tips on striping, mirroring, and striping with parity

Scott Lowe responds to a TechRepublic discussion and one member's RAID dilemma. Here are some tips on RAID levels and some feedback on the software vs. hardware RAID question.

In reading through TechRepublic discussion forums, I stumbled across a very recent discussion where member mccoygreg asked questions about software RAID capacity, and some other members chimed in with their comments regarding software RAID. As a result of this discussion, I thought I'd put together a quick RAID primer and offer some of my own opinions about the issue of hardware vs. software RAID solutions.

If you're interested in some pretty in-depth information about how RAID works, take a look at the article I originally wrote for TechProGuild. While that article went in-depth about a bunch of different RAID levels, I'm going to focus on only RAID levels 0 (striping), 1 (mirroring), and 5 (striping with parity) in this article.

First, I want to address the storage dilemma posed by mccoygreg, who is setting up a lab server so he can play to his heart's content. He indicated that he purchased a new 80-GB drive that he wanted to use in conjunction with his existing 6-GB and 8-GB drives. Unfortunately, while he can create a RAID-5 array, because of the way RAID-5 works, he will lose 74 GB capacity on his shiny new drive. Why? RAID-5 requires drives of equal capacity. For situations in which the selected drives differ in size, RAID-5 defaults to the least common denominator. In this case, the smallest drive is 6 GB in size, so the RAID-5 configuration will use only 6 GB from the other two drives as well, and the remaining space is ignored.

If mccoygreg wants to use the total capacity of 80 GB, he would need to purchase two more 80-GB drives, since RAID-5 requires three drives in order to operate. Mccoygreg's other alternative is to forgo RAID-5 and use RAID-0 instead, which provides the ability to use the maximum capacity of each individual disk. In this case, a RAID-0 array would result in a total of 92 GB of capacity. I would strongly recommend against this approach, however. In a RAID-0 configuration, the loss of a single disk results in the loss of all data on the array. By using RAID-0 on three disks, mccoygreg would triple his chances for complete data loss.

RAID-1-mirroring requires two drives of equal size, so the 6-GB and 8-GB drives might be good, but the member would still lose 2 GB from the 8-GB drive, since RAID-1 also uses the smallest disk size in cases where the drives sizes differ.

I've said a couple of times that mismatched drive sizes default to the smallest of the individual drives. Note, however, that this doesn’t always work. Some RAID controllers really require identical drives.

Finally, let's address the issue of software vs. hardware RAID. As one member pointed out, software RAID is like playing with fire. If you have a server hiccup, you could have some data loss issues. However, even though RAID hardware has come down in price, it's still not always feasible for organizations to incur the additional expense, or perhaps you're using older servers that don't have RAID hardware. In these cases, if you want to further protect your data, use software RAID. It's definitely better than having no RAID at all! And, Windows 2000 and Windows Server 2003's handling of these kinds of scenarios is really pretty good, though not as reliable as a hardware solution.

These days, all new servers in my environment use either RAID-1 or RAID-5, and, with the exception of a couple of older servers that will soon be virtualized, all servers use hardware RAID controllers, rather than relying on the Windows RAID feature.

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