Reliability of System V 1K file system

[Vadim G. Antonov]

In article <1990Sep22.215128.16693 at virtech.uucp> cpcahil at virtech.UUCP (Conor P. Cahill) writes:
>A power failure can wreak havoc all over the system if the *hardware*
>does not correctly handle the loss of power during an otherwise normal
>operation.

	... Or if disk dirver does not handle drive's idle state
	properly. Some controllers require clearing registers when
	device is inactive. It usually is not documented - and
	requires very careful design and testing of drivers.
	(Have you ever heard about testing Unix disk drivers on
	power failures?) Practically all machines provide power fail
	interrupts - and I don't know why Unix device drivers
	have no "xxpwfail" entries. Anyway I'm quite sure *any* device
	can correctly handle power fails - if you handle device
	properly :-).

	It seems to me the best way to protect disks from accidental
	damaging by power fails is to start recalibrating or moving
	heads to landing zone - usually quite simple logic circuitry
	protects from writing while heads move.

	Hmmm... For example I've never seen any recommendations about
	such things in DEC's HW manuals, but I've seen that DEC's PDP-11
	systems clear controller after completing disk i/o. Needless to
	say when I've made a similar trick in Unix I've never got
	powerfail problems again.

	My $0.02 is: the quailty of disk drivers determines the
	reliability of a whole system. It is possible to work even
	on Soviet hardware if you wrote good drives. :-) I think
	Western software manufacturers should buy Soviet computers
	to test their systems on :-). "One who can pilot Camel can
	fly on anything" [Back In The Future, if my memory is right].

	Vadim Antonov
	DEMOS, Moscow, USSR           The cat is sleeping under sofa.
	(It is NOT a joke!)