[H-GEN] Tape Drives

Frank Brand fbrand at uq.net.au
Sat Jan 5 22:27:17 EST 2002


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>
>Well, considering that the main point of backing anything up is to not
>have any data loss when, say, a disk's head decides to meet the disk
>surface, or a bearing decides to seize up, I really do not see the
>point of using another drive, especially a removable which is going to
>be subject to stresses as it is removed, inserted and transported.



The question is not whether disks give problems (they do but it is pretty
rare) but whether tapes drives and disks are any more reliable.

#include <prepare_for_jason_tirade.h>

The question of reliability is lessened as, to be a problem, both hard
drives need to fail together (ie the original hdd and the back up hdd). Two
drives failing at the one time is possible but not highly probable. The
chance of this can be minimised by a regular test regime. The biggest
problem I have with using HDD's in the same machine is when the power supply
has a massive and catastrophic failure destroying both (all) drives...this
also goes for tape drives...at least you still have yesterday's HDD but how
quickly can you get a replacement tape drive of this type (I have had such a
situation with Seagate and had to wait 3 weeks for a drive to come from
Singapore - fortunately I was able to get a borrowed drive but wit 40Gb IDE
tape drives this may not be easy)

Moral of the story...put your backup drive in another machine...not so
important with HDD's as you only lose one back up drive and you have
yesterday's drive anyway and, with anything other than rsync type updates
which will be as up to date as minutes, yesterday's backup will be the most
recent anyway.

Granted you need to be more careful with removables...unless you use laptop
HDD's...and even then too.



>That said, one thing I've done is to set up another machine---it
>doesn't need to do anything but this---and run rsyncs from the
>backed-up host to it every few hours.  It buys another copy of your
>files, and smart users can even be taught to do their own restores.
>The downside is the cost in CPU and maintenance, not to mention the
>amount of time needed to make *sure* it's working correctly.



But this copies to another hard drive OK you are still using my basic
concept of using HDD rather than tape to save but removing any small
unreliability caused by removeable drives ...yes the drives don't really
need to be removeable. This arrangement is better in that the drive is in a
separate box. Of course you could have a box with several drives in it and
copy to a different drive at the end of each day and have daily backups. I
have actually used this so that the "live master" dies then the copied disk
(exact replica of the original) is just taken from the copy box and put into
the production box...5 minutes disk change and back in operation as the
whole disk has been copied including boot files and all...just need to make
the boot partition active...of course with RAID and mirrored drives you may
not need this.

One feature this method can overlook is that (for disaster recovery) the
backups should be held somewhere else (ie not in the same building as the
original).


>I cannot stress enough how important it is to think---hard---about
>your backup scheme.  Trace through it for a month, asking at each step
>`Okay, what if this tape/disk/tape drive/whatever is corrupted, or
>lost?'.  You'll be terrified to think of just how much pain you have
>to go through to have backups that can truly be depended on.


Cannot argue with these sentiments. Backups are often not needed for years,
months or maybe never at all but it's a real bugger to find an error of
logic when it is too late.



>One thing I mentioned to Pat last night that bears repeating here is
>that *in theory* it's a good idea to have two tape units.  If the unit
>used to write your backups has a head misalignment, and the unit dies,
>then unless you can find another unit that has the same head
>misalignment you are essentially up the proverbial creek.


I made reference to this before regarding lost drives when the power supply
spews 240 volts all over your box, fusing bearings in anything that moves.


>You'll need to run regular backup tests (you'd need to do this with
>removable disks, too):  verify your backups by:



I once knew a guy who backed up on floppies...not a big backup but all his
finacial data. He didn't realise the foppies were faulty (probably 2000
saves or so) for a long while. When he had a disaster he could not back up
and had to enter all the year's data in again.


>> Maybe other people have different ideas.
>
>About backups?  You bet.  The whole topic is only slightly less
>contentious than `which editor should I use?'.


LOL. Just a bit less contentious as most people use an editor but only some
people get involved in backing up.

Backing up is time consuming and costly and unless you do it right, why
bother. The whole process is a compromise between cost and the importance of
data to the organisation. Maybe have a tape drive, oh no lets get two in
case the first one dies, no lets get three in case  two die on the same day.
Lets make two back up copies at a time in case the tape is
faulty...eventually the compromise has to end.

Frank Brand


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