Chapter 2. System overview 21
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.
The cache is separated into two layers: An upper cache and a lower cache.
Figure 2-6 shows the separation of the upper and lower cache.
Figure 2-6   Separation of upper and lower cache
The upper cache provides the following functionality, which enables the system to streamline 
data write performance:
/SM590000Fast write response times to the host by being as high up in the I/O stack as possible
/SM590000Partitioning
Important: The active/active architecture provides availability to process I/Os for both 
controller nodes. It enables the application to continue running smoothly, even if the server 
has only one access route or path to the storage controller. This type of architecture 
eliminates the path and LUN thrashing typical of an active/passive architecture.