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redbook SG248025-02 http://www.redbooks.ibm.com/

Rule: All virtual cartridges get an equal amount of space up to either the limit size that is configured when the virtual cartridge was created (maximum cartridge size), or the calculated amount of Nominal Space/Number of Cartridges, whatever comes first. You can estimate the nominal size of a repository by multiplying the physical size of the repository by the HyperFactor ratio.

My VTL space:

• total physical space = 5.4GB (or 5.7GiB)
• total nominal space = 31 TiB (deduplication ratio ~5.8) with available nominal space = 4.6Tib (26.4TiB is used)

All libraries cartridges:

• AS400 : 74
• TSM : 288
• shelf (=replication): 618 (only 13 of those for AS400)

→ almost 1000 media on My VTL with <100 media for AS400

By applying the rule mentioned above: nominal media size (as seen by backup application) =~ 30TiB/1000media ~ 30GiB/media: just what we have (30.5GB for backup of tonight)