RAID array advice/help please

On my pc I run a 3 disk raid 5 array comprising 3 WD Caviar Blue 640 GB disks.

A friend of mine keen on using a raid array as an easy way of backing up data wants my advice on what he can do to create an array. Thing is my experience and knowledge is pretty limited when it comes to RAID so help with the following situation would be appreciated.

My friend has 640GB, 250 GB and 500 GB disks (though the 500 is not in use at the moment and I think he planned to put it in an enclosure to use as a portable external). What he asked me was would he be able to create a Raid 1 array with an extra 1TB hard drive, the idea being that the 250 GB and 640 GB are mirrorred to partitions on the terabyte disk (with a little extra wasted space). My answer was 'probably not' since I thought you could only mirror a physical disk not a partition, but having said that and done some reading online I'm fairly sure I'm wrong.

Next thing is I wouldn't know how to implement the array which is the next question. So, is it possible, and if so how would you go about creating it? And finally, is it worth it? Are there obvious better setups that I've missed? Not totally restricted to the current drives since there is some money available to buy new ones, though not a lot!

You can raid partitions - if the partition is a logical drive and the RAID software/hardware/firmware supports it. However I think (and I'd have to research it) he would have to set up some sort of logical volume with his three drives and then mirror that to his 1T drive so he then has the risk of the part of the array consisting of three drives failing (because there are three disks, and RAID 1 is not a backup strategy anyway - there is still a single point of failure if the controller fails, and if a fault occurs that corrupts disk writes, both halves of the mirror will be corrupted equally well! And as for the complexity of setting up...!

So no, IMHO, it isn't worth it - he is better off getting another 1T drive and mirroring that (if he really wants RAID1, partitioning the array into manageable pieces say (for example) sizes that match the other drives he has, and backing up to the smaller drives, which he can mount in external enclosures. But unless he has a compelling reason for RAID 1 (resilience and uptime being the most compelling) don't bother.

Remember two things - back up is in case of system failure and should be offline, Hard drives are not the best backup media - they are mechanical and you cannot guarantee that they will work when you need them, even if they have been stuck on a shelf doing nothing.

Sounds sensible, the motivation behind the idea was data backup. A better idea is probably to use backup software for the data on the drives currently used with some sort of non hard disk based backup for the vital stuff.

Definitely don't even *dream* of using Raid as a backup replacement. It's not. He'd be much better off using the spare 500GB drive as an external backup drive. While hard drives *can* fail even when they're just sitting around (although it's pretty unlikely that both your computer *and* your external backup will die at the same time!), they make a good option in terms of cost / GB for backup storage - it's a good idea to have two and rotate them though, so if one fails you still have a relatively recent backup of your data to fall back on.

The other option is to use the hard drive for general backup and *also* back up the more important files to DVD / BD regularly. Of course, DVDs can degrade over time as well; there's not really any such thing as permanent backup media; but DVDs are probably the best bet for small amounts of data as they can be preserved pretty well in jewel cases with careful storage.

Whatever you advise your friend to do though, make sure you spell out that Raid is *not* a replacement for regular backups!

RAID is VERY nice for resilience but you still need backups for "disaster recovery".....because nasty things can still happen (multiple failures or human error)

If you do create a raid you want to keep the drives the same, same platter sizes and density etc if possible.

You could add multiple odd drives together using a controller that support JBOD (just a bunch of disks) but JBOD gives you none of the performance advantages of raid, while still giving you the added risk of losing the entire "array" when just 1 drive fails.