[Gllug] Tape backups

[Martin A. Brooks]

John Winters wrote:
> Bruce Richardson wrote:
>   
>> On Sun, Feb 15, 2009 at 01:43:15PM +0000, John wrote:
>>     
>>> If you put aside the sarcastic, unhelpful replies Adrian, you ask a
>>> very good question.
>>>
>>> I've put in a few systems which have mirrored data to RAID setups
>>> using Dirvish, which I would recommend as being easy to install and
>>> understand.
>>> http://www.dirvish.org/
>>>       
>> A dirvish system doesn't give you backups, it gives you a mirror with
>> history (on a single logical device).  While this is useful, people
>> *really* have to get their head round the difference.
>>     
>
> What then would be your definition of a backup?
>   


I've clearly been watching too much Yes (Prime) Minister :)



I refer the honourable gentleman to my reply to the list on 03/03/2006 
at 21:08 where I said:

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A copy on disk is a copy.

A backup is an indexed archive that guarantees a point-in-time recovery 
of data to a time resolution decided by local policy. A backup is 
distributed, is stored off or near line, and is non-trivial to corrupt 
or delete.  A backup is on non-fragile media - a hard disk drive will 
almost certainly not survive dropping 1 meter onto concrete, a DLT tape 
almost certainly will.  A backup lets you see what changed, when (within 
the above set time resolution), and preferably whom it was changed by.

A backup is what you use when your data centre has burned down, all the 
hardware is beyond recovery and you somehow have to get your production 
systems operational again.

In this scenario if I turned round and said "Oh sorry, I copied all that 
important stuff onto a RAID5 set, well, that's sort of a backup, isn't 
it? And would you know it, it's corrupted on me!"  I'd expect to be 
stared at in wild-eyed disbelief followed by laughter as they realise 
that no-one, no-one who has ever dealt with mission critical data, ever, 
could possibly confuse a backup system for copying stuff onto a bunch of 
hard disks.

And then, as I have done mercifully few times before, I call up Hayes, 
who's number has been in my mobile phone for a long time, and say 
"Please send our last full backup set, and all the dailies since, on a 
bike as soon as possible please."
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I also refer the honourable gentleman to his own reply to my reply on 
05/03/2006 at 08:52 where he said:


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I don't think I've worked on any serious development which *didn't* have
a backup system much as described above (although I did start work at
one place that didn't, I very quickly changed things so that it did).

As ever, a formal description can sound very daunting, but setting up a
system as described above is not nearly as bad as it sounds.  Trying to
work without one would give me the willies.
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