Mount performance
Mounts per hour is a measure of the overall capability of the cartridge accessor and tape drives. It is defined as the number of cartridges that the tape library can mount in one hour.
A mount, often called the demount and mount cycle, involves the following steps: (1) removing the cartridge from a drive; (2) returning it to a storage slot; (3) collecting another cartridge from a random storage slot; (4) moving it to the drive; and (5) loading the cartridge into the drive. Table 1 shows the mount performance for a tape library with all mounts from Tier 0, which are the single-deep slots on the door side of the HD frame.
| Library configuration | Maximum mounts per hour | |
|---|---|---|
| Single accessor | Dual accessor | |
| 4 frames | 360 | 730 |
| 6 frames | 315 | 720 |
| 8 frames | 270 | 680 |
| 12 frames | 220 | 620 |
| 18 frames | 180 | 550 |
| Specifications were produced from models, but are consistent with test results. The dual accessor data assumes that each cartridge is mounted in a drive that is located within the same library zone. | ||
The mount performance for a TS4500 tape library is dependent on the library configuration and
cartridge usage. For most configurations, the impact of HD slots is not noticeable. For
configurations with high accessor utilization (as measured in mounts per hour), the overall
reduction in mount performance compared to tier 0 performance can range from no impact to a
worst-case 50% reduction in mounts per hour (for a 99% full library). This reduction in mount
performance is determined by the distribution of tiers that cartridges are mounted from and the need
to destage least-recently-used (LRU) cartridges from the cartridge cache. The library configuration
and cartridge usage influences the reduction in mount performance in the following ways:
- Unlicensed capacity decreases the mounts from the highest tiers and decreases the time required for shuffle operations (load balancing ensures the highest tiers are the last ones utilized)
- Lower capacity utilization decreases the mounts from the highest tiers and decreases the time required for shuffle operations (load balancing ensures the highest tiers are the last ones utilized)
- A higher ratio of cartridges that are mounted more than once (cartridge cache hits) increases the mounts from Tiers 0 and 1
- Cartridge eject operations prior to mount operations reduce the need to destage LRU cartridges (this is because eject operations empty some Tier 0 slots)