Path: utzoo!attcan!telly!lethe!torsqnt!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!ukc!dcl-cs!aber-cs!athene!pcg From: pcg@cs.aber.ac.uk (Piercarlo Grandi) Newsgroups: comp.arch Subject: Undetected error rates, high capacity storage Message-ID: Date: 21 Dec 90 11:13:44 GMT References: <11357@pt.cs.cmu.edu> <11393@pt.cs.cmu.edu> Sender: pcg@aber-cs.UUCP Organization: Coleg Prifysgol Cymru Lines: 33 Nntp-Posting-Host: odin In-reply-to: lindsay@gandalf.cs.cmu.edu's message of 14 Dec 90 00:08:30 GMT X-Old-Subject: Re: IO buses On 14 Dec 90 00:08:30 GMT, lindsay@gandalf.cs.cmu.edu (Donald Lindsay) said: lindsay> In article lindsay> pcg@cs.aber.ac.uk (Piercarlo Grandi) writes: pcg> There are even drives, magnetic or not, whose mean undetected error pcg> rate is of the same order as their capacity, so virtually pcg> guaranteeing that you get an undetected error every time you make a pcg> copy of them. After all even a fairly respectable undetected error pcg> rate of 1 in 10^12 is usually expressed in bits. lindsay> Good point! Creo's 1 TB optical tape holds 10^12 bytes and has lindsay> "fewer than 1 in 10^12" bit errors. The pessimistic reading is lindsay> as "fewer than 8 mistakes per reel". No, the pessimistic say that every time you try to backup a reel you will get *at least* :-) eight undetected errors (the idea is that since they are undetected, you don't know). In other words you cannot make a duplicate of a reel. Also, how long does it take to duplicate a reel holding 1TB? :-(. lindsay> Rather than expecting perfection, we should probably expect lindsay> systems to have selectable, adjustable amounts of protection. I am starting to think that ECC will become more and more of an end-to-end issue. If I had a tape unit like above I would make quite sure that I used some heavvily ECC'ing modification of 'tar' to write and read to it. -- Piercarlo Grandi | ARPA: pcg%uk.ac.aber.cs@nsfnet-relay.ac.uk Dept of CS, UCW Aberystwyth | UUCP: ...!mcsun!ukc!aber-cs!pcg Penglais, Aberystwyth SY23 3BZ, UK | INET: pcg@cs.aber.ac.uk