Is it possible to use a sparsebundle file on a locally attached disk?

Question

I'm using a basic Drobo to store all my media (iTunes, iMove Events etc.) and would also like to use it for Time Machine, without Time Machine claiming all of the available space therein.
Partitioning the Drobo is pretty much out of the question, I tried it, but it breaks a good deal of Drobo's useful features (namely its ability to warn me when the drive is getting full).
I been playing around creating a 1TB sparsebundle file, which I then copied to the Drobo volume, but whether I mount it or not, Time Machine cannot (will not) see it. I've tried selecting it in Time Machine preferences and also by Option-clicking on the Time Machine icon in the menu bar. Is this because Time Machine will only recognise sparsebundle files mounted over a network?
Is there something else I could try, besides sparsebundles and partitioning, to limit the space Time Machine consumes? Is there a way, for example, to automatically delete old backups?

Answer 1

m0thr4 wrote:
I wonder how the manual delete facility in the Time Machine UI works then... and if it's possible to write a shell script that does the same thing?
I have a feeling that's far more involved than it sounds. The backups have a complex structure, with hard links at both the file and folder level. Just how Time Machine manages to delete the right things is, I suspect, rather complex, too.
I don't know exactly how it's done, but I suspect there are circumstances where more has to be done than just delete a dated backup folder and all it's hard links. Some hard links that were at the folder level may have to be re-constructed for the remaining backups, at the file level.
Then there's the issue of permissions, extended attributes, and other metadata. The permissions are stored differently than on a "real" file, so the actual backup files (hard links) can have those "deny everybody everything" permissions, presumably so us mere mortals won't try to fool with them.
While I doubt deleting that way would corrupt the backups, you might well leave "loose" files containing permissions and possibly other things as well, which might eventually cause problems and take up a fair amount of space on the drive.
Last, they do something fancy with the dated folders themselves; when the time zone changes, all the backup folder names change accordingly. What's more curious is, if you start a full restore, without OSX on your internal HD, so Time Machine doesn't know what time zone it's in, the list of backups will be shown in *U.S. Pacific Time.*
Those are just the possible "gotchas" I can think of offhand . . .