Hi there
We too are in the throes of a debate about virtual vs. physical tape
libraries.
On the VTL side, much is made of the ability to over-provision the disk
capacity - eg a 100Gb virtual tape will only occupy as much space on disk
as has been written to it. As a result, so the theory goes, we need only
consider the occupancy when sizing the VTL. In a TSM environment, this
seems to be wrong on a number of counts.
- We still need to take into account the overhead of the reclaimable space
on a virtual tape. This can be managed by varying the reclamation
thresholds, but not eliminated.
- A pending delete volume will still occupy an underlying disk capacity
equivalent to its size.
- Since the conversion of a pending delete volume to a scratch tape takes
place purely in the TSM database, a virtual scratch tape will also occupy
the full disk space on the VTL. until it is re-used.
So am I correct in thinking that in the whole scratch, filling, full,
reclaim, pending, delete, scratch lifecycle of a storage pool volume, the
only time that we get the benefit of the over-provisioning is when it's
filling? In our current physical tape environment (with collocation at the
node level), only about 20% of volumes are filling. Ignoring de-dupe for
now, does it seem reasonable to base the sizing for a replacement on the
total physical tape capacity of the existing library and some estimates of
expected growth.
Regards
Neil Schofield
Yorkshire Water Services Ltd.
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