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.



------------------------------------------------------------------------------
BlackBerry® DevCon Americas, Oct. 18-20, San Francisco, CA
The must-attend event for mobile developers. Connect with experts. 
Get tools for creating Super Apps. See the latest technologies.
Sessions, hands-on labs, demos & much more. Register early & save!
http://p.sf.net/sfu/rim-blackberry-1
_______________________________________________
Bacula-users mailing list
Bacula-users AT lists.sourceforge DOT net
https://lists.sourceforge.net/lists/listinfo/bacula-users