The performance gain from striping active log data sets can be larger when there is hardware  contention. However, the benefit can be minimal when there is no disk contention, such as  when dedicated control units are used for the Db2 logs. By means of demonstration, an OLTP  brokerage workload was run with the Db2 logs and data that was allocated on separate  storage control units, and with the Db2 log and data allocated on the same storage control  unit for non-striped and striped active log data sets. The results are shown in Figure 3-26. When the Db2 log and the data are on separate control units, the log I/O wait time is small for  the non-striped case because no contention with the data exists. Therefore, the benefit of  striping when the Db2 log and the data are on separate control units is miniscule.  However, when the Db2 log and the data are sharing the same control unit, we observed a  48% reduction in log I/O wait time with striping. This issue occurs because when the log and  the data are on the same control unit, hardware contention resulted without striping. With  striping, hardware contention was considerably reduced by spreading the log data sets  across multiple stripes. Whether log striping is enabled, log I/O is susceptible to problems that are caused by remote  DASD replication. Each I/O introduces a risk of a potential delay because log write I/O is  synchronous to the Db2 transaction. Also, because striping creates more I/Os, striping might  increase the risk of a delay.  If possible, each log data stripe should be on a different RAID rank to avoid potential  contention. If logs and data are on the same RAID rank, avoid sharing RAID ranks with  update intensive tables and indexes that cause (many) asynchronous writes, such as Db2  deferred writes and castout writes. To reduce the disk constraint, allocate Db2 active logs on  the fastest disk available, such as 15 KB RPM drives or even better, allocate them on SSDs. Hardware configuration considerations Disk mirroring  128  IBM Db2 12 for z/OS Performance Topics Figure 3-26 Effect of location of active logs on non-striped versus striped log data sets 0 0.5 1 1.5 2 2.5 Log and Data separate CU Log and Data same CU Log write I/O wait time  Log and Data allocation impact on Log Write  Response Time for Log Write I/O Size ~10K  Non Stripe 2 Stripes - -