dump locking up system

[Robert Huff]

	(While the system involved is -CURRENT, this doesn't seem to
have anything CURRENT-related.)
	On a system running:

FreeBSD 10.0-CURRENT #0: Sun Dec 30 12:52:09 EST 2012  amd64 

	running dump causes the system to lock up ... sometimes.
	Specifics:
	I have a cron job which runs at 0200 local; it dumps three
filesystems - /, /var, and /usr - to an external hard drive attached
by eSATA.  (Dump is incremental Tuesday through Sunday, full on
Monday.)
	After some time of working transparently, this now semi-
reliably causes the system to lock up requiring power-off to fix.
	a) According to dumpdates, the dump of / always completes.
Only dumping /var or /usr cause the lock-up.
	b) There's nothing else in cron running about that time.
	c) "Top" doesn't show any suspicious processes or activity.
	d) When doing "fsck" on re-boot, the only thing suspicious is
a file - caught in fsck phase 1 - large enough to be the usused
space on the disk.
	e) The dump is run in snapshot mode; this has not previously
been a problem.
	f) The exact command used is:

	dump $DUMP_LEVEL -D $DUMPDATES_FILE -C $DUMP_CACHE -b 64 -Lau -f $DUMP_DATE.var.dump /var 

	where all of the $VARs are appropriately defined elsewhere.
	g) When run outside the cron environment, the script always
runs to completion.

	Two possibilities come to mind: some kind of hardware failure,
or a subtle corruption of the file system.
	Please - someone out there hav a better idea.

	ResEpoectfully,


		Robert Huff