On Sat, May 24, 2003 at 11:45:52AM +0900, Curt Sampson wrote:
> On Sat, 24 May 2003, Luke Mewburn wrote:
> 
> > I used to use a mirror set for this info.  Now I've put the drives into
> > two separate machines, and manually rsync the volume between them,
> > because I figure that I'm more likely to lose time over a user or
> > application error corrupting the data, than the loss of time if one of
> > the drives fails (since it's not data that must be available all the time).
> 
> Yes, that occurred to me, too. The next step in my thinking was to do
> an off-site backup by putting one of the drives in a removable bay, and
> either rsyncing it or letting the RAID resync it on a regular basis.

Backup to disk is nice, but you need to be a bit careful.  Disks are *not*
designed to spin up and run nicely after being powered on, written to,
and then powered off for a few years, so they're not suitable as long-term
backup media.  They are also significantly more mechanically complex than
tapes, and if, for example, put in a safety deposit box that is then
exposed to high heat or flooding, are a *lot* less likely to be readable
whether with normal or heroic effort.

Tapes are not only designed for these purposes, they're tested to ensure
that they're suitable for that kind of use -- accellerated and even
actual aging tests, heat and water and mechanical wear tests, etc.  Tapes 
can be re-spooled, spliced, etc; and I've successfully read 7-track
tapes that were several decades old with no trouble at all once I got
the 7-track drive working.

CDs and DVDs have their own large set of issues mostly to do with the
instability of the dyes.  People like to throw around all sorts of
longetivity numbers for them that are utter nonsense because they're
based on predicted or measured lifetimes of *stamped metal* discs,
not the laser-written photochemical sort to which you can do backups
without a CD/DVD manufacturing plant.  The actual fundamental research
on the useful lifetime of these dyes in adverse conditions is pretty
scary -- heat and UV light both break them down pretty quickly and
even stored in cold, dark conditions they are not entirely chemically
stable; you're looking at maybe a decade lifetime, max.

The bottom line?  I use RAID to protect against disk failures; I use
removable disks to protect against "oops!" failures; but to protect
against house-burns-down failures, I still use DLT tape and I expect
that I will continue to do so for quite some time.