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 maximum mounts per hour for a Diamondback tape library.
|Library configuration||Robotic accessor maximum mounts per hour|
|Diamondback tape library||270|
|Specifications were produced from models, but are consistent with test results.|
The mount performance for a tape library is dependent on the library configuration and cartridge usage. For most configurations and usage, the impact of HD slots is not noticeable. For configurations with high accessor utilization (as measured in mounts per hour), the overall reduction in maximum mounts per hour compared to tier 1 performance can range from no impact to a worst-case 60% reduction in mounts per hour for a 98% full library. This reduction in maximum mounts per hour is generally determined by the distribution of tiers that cartridges are mounted from. The library configuration and cartridge usage influences the reduction in maximum mounts per hour 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 Tier 1
- When cartridge cache hits are expected to be low, application usage of the "no unshuffle" option for the SCSI Move Medium command (HD Control parameter) eliminates unshuffle operations