[glue-wg] Datastore proposal

[Paul Millar]

Hi all,

Some quick comments, as its Friday and late...

On Friday 11 April 2008 18:45:22 Felix Nikolaus Ehm wrote:
> >   Also it seems to me that something in the system must know
> > what the occupancy is, otherwise how can it decide whether a
> > new file can be written to a given tape?
>
> For CASTOR this information is available and kept in the "VolumeManager"
> tables.


[obtaining tape storage information, such as totalSize]
> I strongly assume that this information can be obtained somehow 
> from storage systems which have a tape backend.
[...]
> Also, HSM with tape backend do have monitoring tools to see how much
> tape space is left (I don't think that e.g. CASTOR Tape Operations
> considers the lost space as 'theoretically free'). This number can then
> be published into GLUE.

Yes, the problem here is not that the numbers aren't recorded somewhere: I 
believe they are (but, of course, could be wrong!).  The problem is more that 
there might be no tools to extract the information, so no way of pulling this 
information out  (short of hacking proprietary database formats, which isn't 
fun).

This is what concerns me most about publishing information about tape systems.  
My impression is that some operate like a black-box and obtaining meaningful 
numbers will be either difficult, very difficult or "impossible".

Of course, I'm happy if others can publish this information, and maybe 
obtaining this information isn't such a problem so we, too, can provide this 
information.  But I feel these attributes, if included, should be marked as 
optional so people don't rely on them.

Cheers,

Paul.