On Thu, Jan 27, 2011 at 08:56:54AM -0500, Phil Stracchino wrote:
> On 01/27/11 07:33, Graham Keeling wrote:
> > On Thu, Jan 27, 2011 at 06:26:04AM -0500, Phil Stracchino wrote:
> >> On 01/27/11 06:12, Graham Keeling wrote:
> >>> I think this last problem is what Phil is trying to solve by setting 
> >>> either
> >>> Maximum Volume Jobs or Volume Use Duration. But these solutions seem
> >>> unsatisfactory for disks (I can't comment on tapes because I don't know 
> >>> enough
> >>> about them).
> >>> You are wasting space if the volume is not full up by the time Volume Use
> >>> Duration expires.
> >>
> >> Define "full".  Disk volumes aren't like packing crates.  They're more
> >> like balloons.  They grow as you add data to them.
> > 
> > The idea is to split the disk up into equal-sized chunks. 
> > In this scenario, you have specified a number of volumes, and a maximum size
> > for each volume. For example, on a terabyte disk, you might define 100 
> > volumes,
> > 10GB each.
> > 
> > If you don't use the "full" 10GB in each volume, the space that you don't 
> > use
> > is wasted.
> 
> No, it isn't.  Because it isn't used.  If you set maximum volume size to
> 10GB, and you create a new volume and write a 5KB job to it, you have
> 5KB of data in a 5KB volume, not 5KB of data in a 10GB volume.
> 
> Now, if that volume fills, and gets purged, and you *keep the purged
> volume around consuming 10GB of disk space* until it gets reused, well,
> then you're wasting disk space, yes.  But that has absolutely nothing to
> do with what's governing the size of the volume.  It is always going to
> be the case with any purged disk volume.
> 
> 5.0.3 has a feature to truncate purged disk volumes which gets around
> this.  But the problem can equally easily be addressed by only using any
> given volume once - by whatever means you decide it's "full" - deleting
> used volumes as you purge them.

The truncate on purge feature does not work automatically - you have to "do it
by hand" - which makes it almost useless. See here:
http://sourceforge.net/apps/wordpress/bacula/2010/02/01/new-actiononpurge-feature/

> > You can't use the unused space for some other application, because the 
> > volume
> > might get purged, recycled, and then the next job that writes to it wants to
> > use all 10GB - you will then find that it can't.
> 
> This simply neither reflects reality nor makes any sense.  I can't even
> understand what you're trying to say here.


I shall summarise what I thought I had already said, because it is quite clear
to me.


You have a terabyte disk that you want to use for backups.

You split it into 100 Volumes, set 10GB max volume size each, and 1 job
per volume.

All your backup jobs are 5KB.
You can then only use 500KB of disk space before you run out of volumes.


You have three obvious options:
a) Make more volumes, reduce the max sizes.
b) Make more volumes, keep the max sizes the same.
c) Increase the number of jobs per volumes.


Problems:
(a): If you make the volumes too small, you get overhead/maintenance problems.

(b): You can easily run out of disk space because you have allocated more than
the size of the disk.

(c): It becomes very difficult to work out which volumes you can purge/recycle.


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