disk-block integrity after system crashes

[Rob Warnock]

Just how bad can a power failure be? Well, how about wiping out
the formatting (and therefore the data, to say the least) on several
(even "many") cylinders on the disk? (Under Unix, might as well just
reformat and hope your backup tapes are healty!)

This can happen even if the disk drive has power-fail protection,
if the drive is in an "expansion box" and powered by a separate
power supply. As the power to the disk controller (in the main box)
goes down, so does the power to the drive cable terminating resistors
(which are normally pulled to +5 volts). This can, if you are unlucky,
cause the "WRITE ENABLE L" signal to drop below the TTL threshold and
start writing on the disk BEFORE the power to the disk (in the expansion
box) drops enough to shut off the write amps in the disk. It all depends
on the relative "hold-up" time of the two power supplies in the two boxes.
Conversely, if you have TWO disks on the same controller sharing a bussed
"control" cable, if the expansion box power drops first, you can wipe the
data & formatting on the disk in the main box.

One of the nice things about large DEC systems is that they have a power-
fail line which is bussed between all of the expansion boxes (if the
installer hooked them up correctly) and which causes ALL the boxes to
panic and protect themselves if ANY of the boxes loses power. (Of course,
if you are having troubles with power supplies, this bussed line makes it
hard to figure out which box is causing the problem, so sometimes it gets
unhooked for debugging and never gets put back...)

It is possible (and not too expensive) to protect disks fairly well from
this sort of thing, but a lot of the current low-cost "desktop" computers
don't bother. (*sigh*)


Rob Warnock
Systems Architecture Consultant

UUCP:	{ihnp4,ucbvax!dual}!fortune!redwood!rpw3
DDD:	(415)572-2607
USPS:	510 Trinidad Lane, Foster City, CA  94404