8464ch12_Performance.fm Draft Document for Review February 7, 2020 4:43 pm 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:  These rates are described next.  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.  12.3.6 Throughput terminology Peak and sustained write throughput GLYPH<SM590000> Peak write GLYPH<SM590000> Sustained write GLYPH<SM590000> Read-hit  GLYPH<SM590000> Recall 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. 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.  Similar to write activity, the following types of TS7700T read performance are available:  Read-hit data rates are typically higher than recall data rates.  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. Read-hit and recall throughput  Summary Chapter 12. Performance considerations  711