It is always important to separate database data and log files. The write patterns for these types of object are very different and so mixing them on the same device will adversely affect performance. Log writes are always sequential and high bandwidth, whereas writes to data tablespaces can range from random to large and sequential. It is important to ensure that log writes are fast and do not suffer from device latency in order to provide the highest performing database environment.

When using SMS tablespaces, it is not possible to separate data and indexes onto different devices. This means that there is no way to reduce contention for I/O and memory between these two types of database object. However, when using DMS devices, it is possible to place the data and indexes of tables into different tablespaces. This can provide much improved performance in environments which have very heavy usage of indexes and/or constrained memory.