8464ch12_Performance.fm Draft Document for Review February 7, 2020 4:43 pm Consider the following points when you configure or monitor a TS7700T resource usage: The physical tape drives are managed by the TS7700T internal management software, and  cannot be accessed from any other attached host. These drives are used exclusively by the  TS7700T for the mounts that are required for copying virtual volumes to stacked volumes,  which recalls virtual volumes into the cache and reclaims stacked volume space. The availability of TS7700T physical tape drives for certain functions can significantly affect  TS7700T performance.  The two major maintenance or “housekeeping” tasks at work are the premigration of data  from cache to tape, and deferred copies to and from other clusters. The TS7700T delays  these housekeeping tasks to preference host I/O while no thresholds are reached  (PMTHLVL). The TS7700T manages the internal allocation of these drives as required for various  functions, but it usually reserves at least one physical drive for recall and one drive for  premigration. TVC management algorithms also influence the allocation of physical tape drives, as  described in the following examples: GLYPH<SM590000> Workloads that create data (host I/O, remote writes, or copy processes from other  clusters) in the CP0 use resources of the cache bandwidth (write to cache).  GLYPH<SM590000> After PMTHLVL is crossed for the CPx, the Host I/O creating data in the CPx is throttled,  but no throttling occurs to the jobs that are creating data in CP0. Therefore, resources for  premigration might be limited even after PMTHLVL is reached, especially in small  configurations (for example, up to four drawers) where the jobs running to CP0 are  exhausting the cache bandwidth resources.  GLYPH<SM590000> If the unpremigrated amount of data still increases, throttling the workload into the CPx  also increases. This issue might cause several delays to the jobs that are creating data in  CPx.  TS7700T tape drives usage considerations GLYPH<SM590000> Cache freespace low: The TS7700T increases the number of drives that are available to  the premigration function. It also reduces the number of drives available for recalls. GLYPH<SM590000> Premigration threshold crossed: The TS7700T reduces the number of drives available for  recall down to a minimum of one drive to make drives available for the premigration  function. The number of drives that is available for recall or copy is also reduced during reclamation. The number of drives for premigration can be restricted on a physical pool base. If the number  of drives available for premigration is restricted, or the physical drives are used by other  processes, it can lead to limiting the number of virtual volumes in the cache to be  premigrated. This limited premigration might lead to premigration throttling (host I/O is  throttled), and later it can lead to free space or copy queue throttling. If no physical drive is available when a recall is requested, elongated virtual mount times for  logical volumes that are being recalled can result.  Recall performance is highly dependent on the  placement  of the recalled logical volumes on  stacked volumes and the  order  in which the logical volumes are recalled. To minimize the  effects of volume pooling on sustained write performance, volumes are premigrated by using  a different distribution algorithm. Chapter 12. Performance considerations  713