Disk mirroring is a secure and high availability feature that allows Adaptive Server to duplicate the contents of an entire database device.
See the System Administration Guide for more information on mirroring.
If you do not use mirroring, or use operating system mirroring, set the configuration parameter disable disk mirroring to 1. This may yield slight performance improvements.
Mirroring can slow the time taken to complete disk writes, since writes go to both disks, either serially or simultaneously. Reads always come from the primary side. Disk mirroring has no effect on the time required to read data.
Mirrored devices use one of two modes for disk writes:
Nonserial mode can require more time to complete a write than an unmirrored write requires. In nonserial mode, both writes are started at the same time, and Adaptive Server waits for both to complete. The time to complete nonserial writes is max(W1 ,W2) – the greater of the two I/O times.
Serial mode increases the time required to write data even more than nonserial mode. Adaptive Server starts the first write and waits for it to complete before starting the second write. The time required is W1+W2 – the sum of the two I/O times.
