> My plan was to have 8-10 cheap drives, most of them IDE drives from
> 120 gig and up to 320 gig. Does that mean that I can get 7-9 drives
> with data plus full redundancy from the last drive? It sounds almost
> like magic to me to be able to have the data on maybe 1 TB of drives
> and have one drive that gives redundancy.
RAID does indeed seem almost magical, but it really does work.
The basic idea is simple. Suppose you have 4 data drives, A, B, C, D.
You want to be able to survive the loss of any drive, so you add a
fifth drive, E, and generate the data for it using this equation:
E = A ^ B ^ C ^ D
where ^ is XOR. You can do it with addition too, but XOR is nicer
because there's no carry bit to worry about. It also has the nice
property that A ^ A = 0, so any number is its own negative with
respect to XOR. In particular, XOR'ing E with both sides, the
equation above can be written in a pleasingly symmetic form:
A ^ B ^ C ^ D ^ E = 0
The key is that you can reconstruct any drive from this equation.
For example, suppose you lose drive B. No problem:
B = A ^ C ^ D ^ E
and similarly for all the others.
Pretty cool, eh? FYI, RAID was invented by David Patterson in 1988.
It's hard to imagine life without it, yet it's only 19 years old --
still a relatively new idea in the grand scheme of things.
Jeff
_______________________________________________
zfs-discuss mailing list
zfs-discuss@opensolaris.org
http://mail.opensolaris.org/mailman/listinfo/zfs-discuss
