This process is an exercise of trading synchronous I/Os for GBP access to obtain better  performance. The number of synchronous I/Os per transaction hovers from the high-20s to  the mid-30s for GBP sizes ranging 5 GB - 151 GB. During those measurements, there is no  clear relationship between CPU time and increased GBP size, as shown in Figure 3-24.  However, when a 566 GB group buffer pool is used, synchronous I/Os are down to less than 5  per transaction, and there is clear performance advantage in terms of class 2 CPU time. At  this point, a large group buffer pool clearly benefits this workload.  Large group buffer pools can help to minimize the number of synchronous reads for  GBP-dependent objects. The access time to a group buffer pool is about three orders of  magnitude faster than access to disk. This difference shortens the transaction’s class 3  suspension time, which results in faster transaction response time and improves overall  workload throughput. The reduction in the number of synchronous reads for GBP-dependent objects should also  benefit transaction CPU time. However, in our observation, access shifting from DASD I/O to  CF requests results in longer CF service times in our environment and negatively affects  transaction CPU time. It is not until synchronous I/Os are drastically reduced (566 GB  measurement) that we begin to observe improved transaction class 2 CPU time. 3.6.2 Summary 120  IBM Db2 12 for z/OS Performance Topics Figure 3-24 Db2 Class 2 CPU time with increasing GBP size