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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. 

<img src='content_image/1010693.jpg'>

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. 

## Disk mirroring 

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. 

## Hardware configuration considerations 

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. 

