On Fri, Aug 5, 2011 at 5:10 AM, Paul Mather <paul AT gromit.dlib.vt DOT edu>
wrote:
> On Aug 4, 2011, at 8:46 PM, Joseph L. Casale wrote:
>
>>> Why do we use volumes? It sounds like a silly question, but it's
>>> genuine. Is it so a backup can span several media types? Tape, file,
>>> disk, Pandora's box, what? Why do I care about volumes and how long
>>> they're retained, how often they're pruned, recycled, or what type
>>> they are?
>>
>> You have to associate where the data was placed, literally as tapes are
>> finite in size and files shouldn't be infinite. It's a way to manage the
>> correlation. Don't think that files are any different than tapes, they
>> have much the same limitations except for the obvious like wear.
>
>
> Well, there are some very important differences between tape and disk that
> directly influence the implementation of things like volume recycling.
> Specifically, if you want Bacula to be somewhat media agnostic, then you
> have to pander to the more restrictive tape media, which makes some decisions
> seem illogical when applied to disk. With tape, file marks explicitly
> delimit files on the linear tape; random addressing is difficult, though
> appending is natural. This explains why volumes are recycled wholly and not
> partially. For many, this doesn't make sense for the more flexible disk
> media, which doesn't require its blocks to be written contiguously. But, for
> an abstracted storage model, it makes sense to let the lowest common
> denominator dictate to more flexible media types.
>
> Many Bacula concepts make a lot more sense if you think in terms of managing
> tape.
>
> Cheers,
>
> Paul.
Hi Terry, Joseph, Paul -
Thank you for your detailed responses. Like I said, I was looking
more for procedural information, not so much technical, and all three
of you hit the nail on the head. Thank you for your explanations.
One more thing that I'm not 100% on is how volume retention and
recycling works. I believe I understand that of all three retentions
(File, Job, Volume), Volume always has the last word. Is this
correct? Is it normal (allowed?) to add additional volumes to my
configuration, or is it more common to add another Pool? I think I'd
be wanting to add volumes to each pool, as it's been pointed out that
I should reserve Pool creation to their specific role (daily, weekly,
monthly, archive). I know that's a loaded question as well and it's
highly site-specific, but I wanted to discuss it for a moment if you
wouldn't mind.
Does only a Volume get recycled? How about pools? If pools as well,
why? I know that volumes (assumed to be tape) are recycled because
they're always appended (if not always, then most-always since it's
good practice to prevent overwriting existing data).
What can I do to ensure that a volume that, say, contains a years'
worth of data doesn't get recycled too soon and potentially
overwritten? What's my stop-gap to ensure that doesn't happen? I'd
much rather be bothered with some administration tasks because Bacula
complained about not having any other place to store backups, rather
than having Bacula overwrite my long lost archives. Are retention
periods (on volumes) hard rules that will guarantee that this does not
happen?
Is there any direct correlation between a File and Volume retention
period, and if so, between the two, which takes precedence (can I
think of it that way?)? Of the two, which should I be more concerned
or interested with/in?
I hope I'm not wasting anyone's time by asking these kinds of
questions. Having them answered is certainly helping me out, and I
really hope it helps a lot of other people get over their case of cold
feet, as well.
Again, thanks for the time.
Thanks
-dant
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