Automatic bad sector mapping

[Dick Dunn]

john at jwt.UUCP (John Temples) writes about automatic remapping:
> The ESIX implementation catches errors while they're still "soft,"
> i.e., the error is recoverable.  So remapping occurs with no data loss,
> as long as the first time a sector has an error it isn't a hard error.

I don't believe this is a common failure characteristic.  Assuming that
(1) you're running the drive in-spec, and (2) you've mapped out all the bad
sectors determined by the drive manufacturer [N.B.: This is *NOT* the same
as bad sectors found by running a r/w test], you shouldn't expect soft
failures because you're not using any marginal sectors.  A drive which is
about to Bite the Big One may show a few soft errors before the disaster
happens, but that's an omen that Something Bad Is About to Happen, so you
want to know about it right away.

For example, a tiny particle can get loose somehow.  If it's just the right
size to get under the head, it'll take a tiny ding out of the coating on a
platter...and there's a good chance it'll be small enough to leave you with
a soft error.  However, you now have at least *two* tiny particles cruising
around, possibly many more (the original and whatever got dug up).  You can
see how that one degenerates.  It's only one hypothetical situation; the
point is that if you start out using only the good sectors of a good disk
and run in-spec, the sorts of things that can go wrong to produce soft
errors are almost always (by that I mean something > 90%) precursors to
a disastrous failure.

If you run out-of-spec (e.g., non-RLL drives on an RLL controller), you're
much more likely to see soft errors that stay soft.
-- 
Dick Dunn     rcd at ico.isc.com -or- ico!rcd       Boulder, CO   (303)449-2870
   ...Simpler is better.