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. ----------------------------------------- Are you a cactus or a sponge? Go to the on-line quiz at http://www.yorkshirewater.com/becool to find out how water efficient you are. Only available in Yorkshire. YORKSHIRE WATER - WINNER OF THE UTILITY OF THE YEAR AWARD 2004, 2005 AND 2006 The information in this e-mail is confidential and may also be legally privileged. The contents are intended for recipient only and are subject to the legal notice available at http://www.keldagroup.com/email.htm Yorkshire Water Services Limited Registered Office Western House Halifax Road Bradford BD6 2SZ Registered in England and Wales No 2366682
