Place transaction logs on a separate segment, preventing the logs from competing with other objects for disk space. Placing the logs on a separate physical disk:
Improves performance by reducing I/O contention
Ensures full recovery in the event of hard disk failures on the data device
Speeds recovery, since simultaneous asynchronous prefetch requests can read ahead on both the log device and the data device without contention
Both create database and alter database require you to use with override before you can place the transaction log on the same device as the data.
The log device can experience significant I/O on systems with heavy update activity. Adaptive Server writes log pages to disk when transactions commit, and may need to read log pages into memory to replace deferred updates with deferred operations.
When log and data are on the same database devices, the extents allocated to store log pages are not contiguous; log extents and data extents are mixed. When the log is on its own device, Adaptive Server allocates the extents sequentially, thus reducing disk head travel and seeks, and maintaining a higher I/O rate.
Adaptive Server buffers log records for each user in a user log cache, which reduces contention for writing to the log page in memory. If log and data are on the same devices, user log cache buffering is disabled, which results in serious performance degradation on SMP systems.
See Chapter 6, “Overview of Disk Resource Issues,” in the System Administration Guide: Volume 1.
