Disk mirroring is a high availability feature that allows Adaptive Server to duplicate the contents of an entire database device.
See Chapter 2, “Disk Mirroring,” in the System Administration Guide, Volume 2.
If you mirror data, put the mirror on a separate physical disk from the device that it mirrors, minimizing mirroring’s performance impact. Disk hardware failure often results in whole physical disks being lost or unavailable
If you do not use mirroring, or use operating system mirroring, you may see slight performance improvements by setting disable disk mirroring configuration paramter to 1.
Mirroring can increase the time taken to complete disk writes, since the writes are executed on both disks, either serially or simultaneously. 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 start at the same time, and Adaptive Server waits for both to complete. The time to complete nonserial writes is 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 the sum of the two I/O times.
