## 12.3.6 Throughput terminology 

A TS7700 disk-cache-only cluster has a fairly consistent workload throughout. 

Because the TS7700T contains physical tapes to which the cache data is periodically written, recalls from tape to cache occur, and Copy Export and reclaim activities occur. The TS7700T exhibits the following basic throughput rates: 

GLYPH<SM590000> Peak write 

GLYPH<SM590000> Sustained write 

GLYPH<SM590000> Read-hit 

GLYPH<SM590000> Recall 

These rates are described next. 

## Peak and sustained write throughput 

For the TS7700T, a measurement is not begun until all previously written data is copied or premigrated from the disk cache to physical tape. Starting with this initial condition, data from the host is first written into the TS7700T disk cache with little (if any) premigration activity occurring. This approach allows for a higher initial data rate, and is known as the peak  data rate. 

After a pre-established threshold of non-premigrated data is reached, premigration starts, which can reduce the host write data rate. This threshold is called the premigration priority threshold , and has a default value of 1600 GB. When another threshold of non-premigrated data is reached, the incoming host activity is actively throttled to allow for increased premigration activity. 

This throttling mechanism operates to achieve a balance between the amount of data that is coming in from the host and the amount of data that is copied to physical tape. The resulting data rate for this mode of behavior is called the sustained data rate. Theoretically, it can continue forever, given a constant supply of logical and physical scratch tapes. 

This second threshold is called the premigration throttling threshold ,  and has a default value of 2000 GB. These two thresholds can be used with the peak data rate to project the duration of the peak period. The priority and throttling thresholds can be increased by using a host command-line request, which is described in 12.6.2, “Premigration and premigration throttling values” on page 720. 

## Read-hit and recall throughput 

Similar to write activity, the following types of TS7700T read performance are available: 

GLYPH<SM590000> Read-hit  (also referred to as peak ) occurs when the data that is requested by the host is in the disk cache. 

GLYPH<SM590000> Recall  (also referred to as read-miss ) occurs when the requested data is no longer in the disk cache and must be first read in from physical tape. 

Read-hit data rates are typically higher than recall data rates. 

## Summary 

The two read performance metrics, along with peak and sustained write performance, are sometimes referred to as the four corners  of virtual tape performance. Performance depends on several factors that can vary greatly from installation to installation, such as number of physical tape drives, spread of requested logical volumes over physical volumes, location of the logical volumes on the physical volumes, and length of the physical media. 

