how to set up a mirrored RAID card

May 5, 2004 5:22 PM

Some months ago I bought a RAID card for my desktop PC, and a second hard drive. I'd like to set up a mirrored RAID array just to decrease my chances of ever losing any data. My problem is that the instructions are clear on how to install the card, but not on what to do if I ever experience trouble. Can I take one drive from a mirrored array and put it in another machine? If one drive fails, will the other just keep trucking and boot the PC? Any experience getting a mirrored RAID set up, or recovering one, would be appreciated.
posted by scarabic to Computers & Internet (7 answers total)

well, you defintely want a RAID 1 setup, then, since you want an exact mirror of your partitions and not just striping. how exactly it functions depends on the controller and os you have, i believe. I managed to score an old SCSI RAID controller off the tech guys here at work that has it's own CPU and RAM, and it half the size of my main motherboard. That way, the error checking and stuff doesn't degrade system performance. Most consumer RAID cards rely on the main CPU for this and therefore can slow your whole computer down.

As far as getting back up after a crash is concerned, that's a good question. Never had to do that before. Someone else can probably take it from here.
posted by Hackworth at 5:33 PM on May 5, 2004

Yup, if one drive fails, the other will just keep trucking. You could easily test this by unplugging one of the two mirrorred drives. (Of course, if the data is important, make sure you have another backup, for the tiny chance that the RAID isn't working properly.) Although you can move a drive to a second machine, it might not boot smoothly if the second machine's hardware configuration is sufficiently different from the original.
posted by blue mustard at 7:25 PM on May 5, 2004

That's the reason for RAID1 mirroring. It will run fine on one drive. There should be instructions with the card on how to replace the bad drive, and have it copy the good disk to the replacement. Usually you can do the copy while you are running, after shutting down the machine to replace the disk. I see that the Promise card has a gui program to handle these tasks.

I use SCSI RAID1 on servers. Each brand seems to have it's own way of setting up and repairing the arrays.

The ones I use will read from both disks, so it can be reading from different areas of the filesystem at the same time, making reads faster than with one disk. Writes are a bit slower, but reads are usually much more common.
posted by jjj606 at 7:31 PM on May 5, 2004

Response by poster: There should be instructions with the card on how to replace the bad drive, and have it copy the good disk to the replacemen

Yes, this is more or less the installation instructions for putting in the card and second drive. They don't explicity state that this is also the recovery procedure!

A follow up question. Once I've got this covered, how proof am I against data loss? Can a fried mobo still screw me? I understand that there are no guarantees in life, but do I still need to backup all 80GB to DVD-R as often once I have a mirror set up, or can I relax a bit? (just a bit!)
posted by scarabic at 8:57 PM on May 5, 2004

RAID only protects against hardware failure, which is actually the least likely thing to happen to your data. More common are viruses, badly written software, pilot error, and power that happens to go out right in the middle of a save. Yes, you still need regular backups. But don't backup to DVD-R; get another 80GB hard disk (a singleton) and synchronize your main drive to it periodcally.
posted by kindall at 9:34 PM on May 5, 2004

Response by poster: Interesting point, kindall. Rather than double up on disks against hardware failure, I should double up on my data storage, and use my second hard drive in lieu of removable media...? Fairly convincing. Hm.
posted by scarabic at 11:51 PM on May 5, 2004

RAID only protects against hardware failure

More specifically, RAID only protects against disk failure. One particularly annoying scenario it does not protect against is failure of the RAID card. This should be relatively rare, but it can happen (as I learned to my chagrin).
posted by blue mustard at 4:21 PM on May 6, 2004