Digital BW, The Print

David Gumbrell wrote:
> Hi,
> 
> I learned the hard way, it is not safe to rely on RAID to protect your data,
> you must have other backups.
> 
> If you buy a pair of identical drives to build your array, most likely at
> the same time, you have a higher chance of a double failure because of batch
> failure probabilities. In addition, after a single drive failure the act of
> rebuilding onto the replacement drive can tip the second drive "over the
> edge" due to the extra activity involved in rebuilding the array.

Here's a very interesting Google research paper about disk failures:

http://labs.google.com/papers/disk_failures.pdf

One of their findings is a lack of correlation between utilization
and failure rate.  In other words, just because one drive fails in a RAID
array, it's not a given that the increased activity of re-silvering a
mirror would push another drive over the edge.

If I had multiple failures in a single box in a short period of time,
I'd be looking at the power supply quality first, or some other common
environmental factor.  The other major factor that is largely overlooked
is that the disk drive is just one of the items involved; the system controller,
system memory, cables, etc. can all induce errors.  In particular, an
undetected memory error written out to a disk can create corruption that
is blamed on the disk.

> Also, RAID
> cannot protect you against accidental deletion or rogue software corrupting
> files. Another potential gotcha with RAID is what happens if your
> motherboard dies - can you be sure that the replacement will recognise your
> disks from the previous machine as a RAID set ?

Perfectly valid points, of course.

> As I said, I learned the hard way - one drive in my RAID array failed, and
> the other failed 3 hours later, before I had time to get a replacement drive
> and rebuild the array.

That would make me think something else was wrong with the cabinet
those drives are in.  What was the nature of the failures?  I assume this
was a RAID 1 mirror?

> The only answer is backups. All backup technologies have their limitations,
> I back up to external hard disks regularly and archive important stuff to
> DVDs slightly less regularly. I also have a ReadyNAS device which is a RAID
> device with upto 4 disks that acts as one large network drive - I still use
> RAID because it can reduce downtime in the case of a disk failure. There is
> also the option of offsite web-based backups. I have only just started
> looking at this and can't make any recommendations.

Managed off-site backup is really the right solution for most SOHO users,
as long as there's adequate up-stream network connection bandwidth to effectively
use it.

Dana