Kurt:
This violates two "rules" for good backup policy.
1. Don't muck with good data. The data on tape is presumably good.
Reading it all in and then writing it all back out again are two more
opportunities for entropy (and Murphy ) to get at your data.
2. As Gene can probably attest to, tape heads have limited lifespans
compared to the rest of your computers. At my last job, we had 4mm DAT
drives and did daily backups on our servers. On the average, our tape
drives lasted, on average, 14-18 months before failing. Your procedure
would shorten the lifespans of similar drives by at least half.
To paraphrase an earlier statement, tapes are cheap compared the cost
of losing data and the labor to restore it. Buy more tape, that's a better
use of your time and money.
Best wishes for a Happy New Year.
Donald L. (Don) Ritchey
E-mail: Donald.Ritchey AT exeloncorp DOT com
-----Original Message-----
From: Kurt Yoder [mailto:kylist AT shcorp DOT com]
Sent: Tuesday, December 30, 2003 7:44 AM
To: gene.heskett AT verizon DOT net
Cc: Andrew Hall; Joshua Baker-LePain; amanda-users AT amanda DOT org
Subject: Re: Append to active tape
Gene Heskett said:
<snipped>
> The design decision is because amanda has absolutely no guarantee
> that
> someone will not eject the tape, or otherwise mess with the tapes
> position, from one days run to the next. So amanda is designed to
> do
> 2 things:
>
> 1: rewind the tape and read the label to make sure its the right
> tape
> in the sequence to use next, doing this with each days invocation.
> This in itself destroys the location you get from an 'mt -f device
> tell' between last nights run and tonights.
>
> 2. Because not all tape drives can reliably do an 'mt -f device
> seof'
> to restore that location, and it would be a total loss to write over
> last nights backup with tonights, the chance of doing that is never
> taken. Hence, amanda will never attempt to append to a tape. It
> *must* know that it has the tapes undivided attention during the
> duration of a run. To that end, if the drive has a door lock
> command
> or ioctl, it is used to prevent you from ejecting the tape in
> mid-run.
>
> Its inconvienient for some, but infinitly safer for all this way.
Couldn't "append to tape" be simulated? That is, rewind the tape and
check the tape label as normal. Then copy all contents of the tape
to the holding disk. Then use amdump to add to the holding disk.
Then flush the holding disk back to tape. The drawback to this
solution is that it would take quite a bit longer. However, there
would be no risk of accidentally overwriting portions of the tape.
I'm just asking in theory; I have no plans to implement anything
like this at my site.
--
Kurt Yoder
Sport & Health network administrator
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