bit level bit-level verify backup

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From: Tony Lawrence <tony@aplawrence.com>
Subject: Re: system backup
Date: Fri, 18 Oct 2002 06:38:43 -0400
References: <3da3fad6$1@eumel.hag.hilti.com> <d1f73d90.0210170126.55fd0c99@posting.google.com> <0hwr9.75402$NW3.19077@sccrnsc03> <d1f73d90.0210170641.7b9d4fa7@posting.google.com> <H44wpC.3vD@wjv.com> <d1f73d90.0210171934.46e5ad14@posting.google.com> 

James Szabadics wrote:
> bv@wjv.comREMOVE (Bill Vermillion) wrote in message news:<H44wpC.3vD@wjv.com>...

>>And if a bit or two were flipped,
>>does your system check every record for accurate data?  I'm curious
>>on this point. I can envision scenarios where some data in a record
>>could be corrupt and not be found unless you perform some summing
>>routine on each record?   This is a question of curiosity - not a
>>criticism. 
> 
> 
> Good point - that might be a problem.  The risk is probably small but
> still exists.

You'd probably be surprised how often a bit level verify proves useful.

As Bill does, I receive backup status email for several of my clients, 
so I can tell you that bit-level verification "failures" ( failure of 
one or more files to match the hard drive- not a complete backup 
failure) are not all that uncommon.  Once in a while it's due to 
incipient tape failure, and it's very helpful to see a problem 
developing before it gets serious- if we're lucky the really important 
files aren't the ones the tape had trouble with.

Quite often the failure is due to unexpected activity on the system 
during the backup.  Usually it's explainable, but a couple of times in 
my long history it has been an employee up to no good after hours and 
the bit-level verify helped show what they were doing.

Once or twice the bit-level verify has alerted us to incipient hard 
drive failures.  Those usually would have been spotted by other means 
too, but it never hurts to have multiple channels of alerts.  As someone 
said, if it's REALLY important that you get up at 4:00 AM tomorrow, you 
set two alarm clocks AND have the hotel desk call you.

Now and then we get "cosmic ray" failures.  It's probably really just a 
wayward piece of dust finding its way to the tape, but the point is that 
one file fails to verify and we can't see any reason why.  We always get 
suspicious of the tape of course, but sometimes it just goes away and 
the tape continues on for months.  Just because of the odds, such 
failures are usually an unimportant or easily recreated file, but once 
in a great while it hits something very important, and that's when we're 
very happy to have the knowledge that, if we should need it, Tuesday's 
tape is NOT suitable for restoration because that important file is 
suspect.  The point here of course is the KNOWLEDGE: we KNOW there is a 
problem on this tape.  We probably don't need the tape (not planning on 
trashing the system today) but if we DID need it, we know that it has a 
problem.

And yes, we often do other things too.  For convenience and redundancy I 
often have rsync or similar things tucking important bits of data here 
and there around the network.  On really important systems we have more 
than one system that does tape or dvdram backup and lots of cross 
pollenization between the systems- it doesn't cost a lot to do this kind 
of thing and the peace of mind is much improved.

But always I have Supertars.  Don't leave home without it.  Whenever I 
pick up a new customer, I insist on it.  I wish I had it for NT systems!




-- 

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Tony Lawrence
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