text: **2.5.5 Cache**
  
  The primary benefit of storage cache is to improve I/O response time. Reads and writes to a 
  magnetic disk drive experience seek time and latency time at the drive level, which can result 
  in 1 ms - 10 ms of response time (for an enterprise-class disk). 
  
  Cache is allocated in 4 KiB segments. A*segment*holds part of one track. A*track*is the unit of 
  locking and destaging granularity in the cache. The cache virtual track size is 32 KiB (eight 
  segments). A track might be only partially populated with valid pages. The IBM Storwize 
  V7000 combines writes up to a 256 KiB track size if the writes are in the same tracks before 
  destage. For example, if 4 KiB is written into a track, another 4 KiB is written to another 
  location in the same track. 
  
  Therefore, the blocks that are written from the IBM Storwize V7000 to the disk subsystem can 
  be any size between 512 bytes - 256 KiB. The large cache and advanced cache management 
  algorithms allow it to improve on the performance of many types of underlying disk 
  technologies. The system’s capability to manage, in the background, the destaging 
  operations that are incurred by writes (in addition to still supporting full data integrity) assists 
  with system’s capability in achieving good database performance. 