Glen wrote:I agree with your comments MOz on Raid 0 (is it 0 which is just striping)
Yeah:
0=striping, full capacity of all disks
1=mirroring, half total capacity (2 disks only)
3&5=checksummming, capacity=(N-1)*disk capacity (min 3 disks)
6=double checksums, capacity=(N-2)*disk capacity (min 4 disks)
10=1+0, half capacity but lots of speed (min 4 disks)
RAID2 and 4 are not generally used any more, technology has passed them by. There's also a simple concatenation option in most controllers which lets you use randomly sized disks. Most of the above require the disks to be all the same size (or more accurately, the accessible capacity of all disks is equal to the smallest capacity of any disk - a mirror of a 10GB and 100Gb drive gives 10GB usable space, RAID5 with disks of 50,100,100,200 gives 3*50GB=150GB. Most controllers will not let you access the extra space either).
Note that some of the NAS units out there will let you use odd-sized disks without losing space, but that's not mainstream yet so I would want to see extensive test results before I put anything important on there. So they let you shove in your 80GB + 100GB + 250GB disks for a total of 430GB and get some significant fraction of that of that as protected storage.
My preference is just to swap all four drives at the same time, which works as long as I triple the capacity at each upgrade (copy the whole array to one new disk, build a new array with three disks, copy the data, "live upgrade" the array with the 4th disk once I'm convinced it all works). That way I always have at least two copies of the data as well as my backups. It means I sell a lot of second hand disks and those are worth very little, but I tend not to lose data very often.
FWIW, I'm going to buy a 1TB external disk soon, and swap my 2x500GB disks to being the second backup, so I will still have two backups (one at work, one at home) but one of those backups will be split over two disks. Then I'll swap one of those for a 2TB disk and buy a second 1TB disk to continue that pattern. (yes, I know you can't buy 2TB disks today... but they will be).