Path: utzoo!utgpu!news-server.csri.toronto.edu!rutgers!att!tut.cis.ohio-state.edu!pt.cs.cmu.edu!gandalf.cs.cmu.edu!lindsay
From: lindsay@gandalf.cs.cmu.edu (Donald Lindsay)
Newsgroups: comp.arch
Subject: Re: Error rates (was Re: IO buses
Message-ID: <11417@pt.cs.cmu.edu>
Date: 16 Dec 90 02:44:30 GMT
References: <11393@pt.cs.cmu.edu> <3838@bnr-rsc.UUCP>
Organization: Carnegie-Mellon University, CS/RI
Lines: 46


In article <3838@bnr-rsc.UUCP> bcarh185!schow@bnr-rsc.UUCP 
	(Stanley T.H. Chow) writes:
>In article <11393@pt.cs.cmu.edu> lindsay@gandalf.cs.cmu.edu (Donald Lindsay) writes:
>>Creo's 1 TB optical tape holds 10^12 bytes and has "fewer
>>than 1 in 10^12" bit errors. The pessimistic reading is as "fewer
>>than 8 mistakes per reel".

> What does the specified error rates mean?

In general, it means that any bit has one chance in 10^12 of being
wrong. However, that's merely the standard abstraction, found in
marketing literature.  When you get down to actually building
specific devices, you deal in various error sources, and characterize
each. For example, the Creo stores 64 KB of data with 16 KB of ECC,
making an 80 KB physical record. So, you would want to know the
chance of a given one-record read having an uncorrectable error.
Since optical systems are susceptible to dust, you would also want to
know the chance that the error was soft, ie. that a reread would
succeed.

> Assuming we want systems to have "selectable, adjustable amounts of
> protection", which component of the system should handling the error
> correction code, etc.?

This stuff is best done in hardware: if not in the drive, then in
the controller. I don't see any reason why that hardware can't
allow the software to select from some limited menu.

> What facilities should the O/S provide? An error corrected file system?

Well, it would be nice if media error rates (particularly corrected-
error rates) could be logged in some coherent fashion. This is more a
management issue than an applications issue: perhaps a disk is
planning to fail, or a tape drive needs cleaning.

Arbitrarily high reliability is achieved by replication - for
instance, multiple backup tapes, kept in different buildings. How
much protection is enough? Well, it's usually figured that the chance
of a subsystem mangling data, should be [..hand wave..] less than the
chance of some other subsystem mangling it. More reliability than
that, is a waste of money. Less reliability than that, is asking to
be the goat. Does anyone have a current figure on the bit error rate
going _to_ the storage system?
-- 
Don		D.C.Lindsay .. temporarily at Carnegie Mellon