ABCs of z/OS System Programming Volume 3 - IBM Redbooks

ack-end traffic. The ESS Model 800 has a maximum cache size of 64 GB, and the NVS
standard size is 2 GB.
The ESS manages its cache in 4 KB segments, so for small data blocks (4 KB and 8 KB are
common database block sizes), minimum cache is wasted. In contrast, large cache segments
can exhaust cache capacity when filling up with small random reads. Thus the ESS, having
smaller cache segments, is able to avoid wasting cache space for situations of small record
sizes that are common in interactive applications.
This efficient cache management, together with the ESS Model 800 powerful back-end
implementation that integrates new (optional) 15,000 rpm drives, enhanced SSA device
adapters, and twice the bandwidth (as compared to previous models) to access the larger
NVS (2 GB) and the larger cache option (64 GB), all integrate to give greater throughput while
sustaining cache speed response times.
FICON host adapters
FICON extends the IBM TotalStorage Enterprise Storage Server Model 800’s ability to deliver
bandwidth potential to the volumes that need it, when they need it.
Performance enhanced channel command words (CCWs)
For z/OS environments, the ESS supports channel command words (CCWs) that reduce the
characteristic overhead associated to the previous (3990) CCW chains. Basically, with these
CCWs, the ESS can read or write more data with fewer CCWs. CCW chains using the old
CCWs are converted to the new CCWs whenever possible. The cooperation of z/OS software
and the ESS provides the most significant benefits for the application’s performance. In other
words, in ESS there is less overhead associated with CCW chains by combining tasks into
fewer CCWs, introducing Read Track Data and Write Track Data CCWs. They allow reading
and writing more data with fewer CCWs. It will be used by z/OS to reduce ESCON protocol for
multiple record transfer chains. Measurements on 4 KB records using an EXCP channel
program showed a 15% reduction in channel overhead for the Read Track Data CCW.
Chapter 8. Storage management hardware 473