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IBM Tivoli Storage Manager Implementation Guide
2.9 Tape drives and sizing
In this section we discuss the calculations for estimating the size of a tape library
to support a Tivoli Storage Manager solution. Similar calculations can be used to
determine the total disk storage needed to support an environment.
Most Tivoli Storage Manager designs incorporate tape and tape drives as the
ultimate long-term storage locations for backups and archives. Therefore, it is
important to understand the performance characteristics of tape drives and tapes
when designing a Tivoli Storage Manager solution.
When using tape storage pools, a Tivoli Storage Manager server should have
access to no less than two drives. For architecture calculation purposes, assume
only 80% of maximum, uncompressed throughput for a tape drive. Be prepared
to do restores while other administrative operations are happening on the system
or when drives are broken.
Carefully consider card and bus throughput when attaching tape drives to
systems. Most protocol/tape combinations can accommodate two or three tape
drives per card. When using Fibre Channel/SAN attached tape drives and disks,
do not mix disk and tape traffic on the same HBA.
When planning for tape drive throughput and tape capacity, it is best to be
conservative. Although drives can perform close to and better than their
compressed rating, typically their performance is far less than this in reality. We
use 80% of native uncompressed throughput ratings and 150% of native capacity
ratings for theoretical calculations of drive throughput and tape capacity.
Table 2-14 summarizes those calculations for some popular tape drives.
Table 2-14 Tape device comparisons
Tape drive
Native speed
(MBps)
Native capacity
(GB)
Assumed speed
(GB/HR)
Assumed
capacity (GB)
IBM LTO Gen 2
35
200
98
300
IBM LTO Gen 3
80
400
224
600
IBM 3590-E1A
14
40
39
60
IBM 3590-H1A
14
60
39
90
IBM 3592
40
300
112
450
STK T9840C
30
40
84
60
STK T9940C
30
200
84
300
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