HD frame performance considerations
This topic explains how mount performance, cartridge move time, and insert and eject
performance are affected in HD libraries.
In HD frames, the cartridge accessor performs a
shuffle
operation in order to access the
cartridges stored in Tier 2 and beyond. A shuffle is the process of moving cartridges in lower
tiers into the gripper, or other available slots, in order to access cartridges in higher tiers
(Tier 2 or greater). In order to reduce the occurrence of shuffle operations, and to take
advantage of repeated accesses of certain cartridges, the role of
cartridge cache
is assigned
to all single-deep (Tier 0) slots in an HD library. In order to maintain efficient shuffle
operations, the library performs a
load balancing
of the tiers, which stores cartridges across
all HD slots in the library string. In other words, all HD slots are filled to a minimum tier
level until that tier is full across the library. This reduces the need to access cartridges in
higher tiers.
The mount performance for a TS3500 Tape Library that includes HD frames is dependent on
the library configuration and cartridge usage. For most configurations, the impact is not
noticeable. For configurations with high accessor utilization (as measured in mounts per
hour), the overall reduction in mount performance compared to a TS3500 Tape Library
without HD frames can range from no impact to a worst-case 50% reduction in mounts per
hour (for a 99% full library in which all expansion frames are HD frames). 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:
•
Each non-HD frame in the configuration increases the mounts from Tier 0
•
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)
Eject and insert performance can also be impacted for a TS3500 Tape Library that includes
HD frames. The impact to eject performance is determined by factors similar to mount
performance (distribution of tiers from which cartridges are ejected). The impact to insert
performance is different because inserts tend to go to HD frames since Tier 0 slots are
