Don't get burned by RAID Zero
To a computer nerd like myself, RAID refers not to a bug spray but to various ways of hooking together multiple hard disks. The various approaches are referred to as levels. Raid levels one through five are designed to decrease the chances that a hard disk failure will result in lost data. Typically RAID configurations are used in server machines as opposed to personal computers.
Raid level zero, however, is the black sheep of the RAID family. It's goal is performance rather than reliability. I'm writing this posting because two of my clients have been burned by their inadvertent use of RAID level zero. Consider this a word to the wise.
Client one purchased an external hard disk from LaCie. This person needed lots of storage space and, at the time, this particular model was top of the line, offering the most storage capacity. The reason it offered more storage than cheaper competing models is that internally there were two 3.5-inch hard disks instead of the usual one. While it looked to the outside world like one chunk of 500 gigabytes, the internal reality was that there were two 250 GB hard disks.
Client two owned a high end Dell XPS tower with two internal hard disks. This person wanted the latest and greatest and fastest computer. Thus, Dell configured the two internal hard disks for speed--RAID level zero. Like the old saying goes, be careful what you wish for, you may get it.
The hard disk is significantly slower than the processor and RAM. Thus to get the fastest read/write performance, RAID level zero stores half of a file on one hard disk and half on the other. Using both hard disks in parallel should reduce the total time needed to write a file.
To someone designing a computer system, the term single point of failure is like kryptonite to Superman. The term refers to a single point in the system which should it fail, would bring down the whole house of cards (so to speak). In an automobile, each tire is a single point of failure, thus they all have a spare in the trunk.
In a personal computer the hard disk is a single point of failure. However, in a RAID level zero configuration, there are three separate single points of failure involving the hard disks. Should either hard disk fail, all is lost because each disk contains half of every file. Oops.
In addition, both of my clients were also dependent on the RAID hard disk controller, the thing with the intelligence to split files as they are written out to the hard disks and re-assemble them back together when read. If the hard disk controller fails, the files may still be alive and well and happy on the hard disks, but you can't read them. Recovering from a RAID level zero controller error might cost thousands of dollars, as its a job for the most sophisticated data recovery companies.
RAID Zero with a failed disk. Click for full-size.
In the case of the client with the Dell computer, it was fairly easy to determine that the problem was with one of the hard disks. Replacing it, re-establishing the Raid zero environment and then restoring a disk image backup got the machine up and running. Disk image backups will be the subject of an upcoming posting on this blog.
The client with the external hard disk lost everything. Not that it mattered, but I couldn't even determine if the problem was with one of the hard disks or the RAID controller.
Update: July 8, 2007. Originally I had said the hard drive was the slowest thing inside the computer, except for the fan. Someone pointed out that optical drives are even slower, so that sentence now says the hard disk is slower than the processor and ram, which was the point I was trying to make.
Note: See
the comments on the original posting.
Update: July 13, 2007. See Following up on RAID Level Zero
Michael Horowitz is an independent computer consultant and the author of several classes on Defensive Computing. He is a member of the CNET Blog Network, and is not an employee of CNET. Disclosure. 
I have two SATA 160 gb drives in a Raid 0 configuration, and a third 160 that's part of my backup strategy. The biggest pain is the time it takes. Hard drive to hard drive (internal) is the fastest, and can be scheduled for off-hours or when you are going away from it for a period of time (like lunch for example). So, approximately once per week, I backup the Raid drive(s) to the third standalone drive. The fact that the Raid configuration has a total of 320 gb capacity is not a problem since I backup using compression which approaches a 50% compression ratio. (If one has lots of images or videos etc. which don't compress very much, then get a larger third drive). Then, every 2-4 weeks, I'll burn the compressed backup to DVD. This results in effectively 3 levels of backup ... the primary data exists on the Raid configuration disks, there is a backup on a separate third hard drive, and then, the DVD's.
While backup software that runs under Windows omits a small number of allegedly non-esssntial files that are open and being continuously updated while Windows is running, I avoid that risk by using a product like Norton Ghost, which runs at the DOS level to back up my entire hard drive with no Windows operating system or Windows dependent programs running.
Another relatively painless method that can be used either in conjuction with or by itself, involves backing up important files one at a time as you create or update them. For example, with a "backup" flash drive in place, whenever you create or update a Word doc, spreadsheet, .pdf file, or important photos etc., make a copy on the backup flash drive. Then, periodically either burn those contents, or keep until backing up your entire hard drive, or both. Even emails can be backed up in this manner by exporting your inbox, or if it's huge, then selected emails to a temporary email folder, then just export that folder.
Bottom line, if you have lots of data flying around, in and out, Raid 0 is a good solution to maximizing disk read and write throughput, and a sound backup strategy is even more important than normal.
... Jack