[Gllug] re: backups

[Bernard Peek]

In message <tim-040905.09273361.04040 at seacon.co.uk>, t.clarke 
<tim at seacon.co.uk> writes
>What, realistically, is the safe life of a DVD-R disc?
>I understand that DVD-RW discs naturally have a shorter life, but I got the
>impression from various web pages that DVD-Rs are rated at 100 years archive
>life.   Whilst this may seem optimistic, it would seem that DVD-Rs should be
>a good choice for archiving large amounts of 'static' data.  They are also
>dirt-cheap, if you stock the the 4Gig variety.
>
>Am I correct in understanding that to write a DVD-R you need to first assemble
>the complete disc-image on a 'filesystem-file' on disc and then splat the whole
>thing at the DVD in one fell swoop?  In which case, one small constraint on
>using DVDs would be the need to have spare hard-disc space.

That shouldn't be a constraint. The best strategy is, I think, to make a 
backup to disk which is fast and cheap then take a copy of that on 
removable media, which you can do at your leisure.

>
>With regard to compression of mail archives, I guess if you were to tar off
>a modest number of files indvidually and then compress, prior to then taring
>the compressed tars, you would at least wouldn't lose the lot if an area of
>archive media goes bad (assuming you can skip it!).  Having said that, I was
>under the impression that modern tape systems (including DAT) have pretty
>good error-correction techniques to avoid the problems of occasionally bad
>media patches.  I guess if the data is that valuable you back it up several
>times on different media by different methods, and then keep the backups in
>different places!

Defence in depth is what the military strategists call it. Try to avoid 
the situation where any single failure can beat you. If I was setting up 
a backup system for commercial data I would think of all of the possible 
options then decide which ones were justified. Bear in mind that for 
lots of organisations losing the data means losing the organisation. 
About a third of organisations that suffer catastrophic data loss will 
not survive.

Think of:

Hot standby systems in another building, holding a mirrored copy of the 
live data. Possibly with standby operators to run them.

Insurance to replace all of the hardware without worrying about whether 
hardware suppliers will extend enough credit.

RAID arrays to protect against disk failures.

Shadow copies to protect against accidental file deletion

Backups to local hard disks, at least one generation and preferably more

Backups to removable media and at least one set of spare drives so that 
I can't be caught out by using obsolete hardware.

Historical backups to restore files that were accidentally deleted six 
months or five years ago

Regularly restoring data from the backups using the spare drives.

Test backup procedures by walking into the server room and switching off 
a server at a random time and with no prior warning. (This is 
controversial.)



-- 
Bernard Peek
London, UK. DBA, Manager, Trainer & Author. Will work for money.

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