Mirroring Database Devices

Disk mirroring can provide nonstop recovery in the event of media failure.

The disk mirror command duplicates an SAP ASE database device, that is, all writes to the device are copied to a separate physical device. If one device fails, the other contains an up-to-date copy of all transactions.

When a read or write to a mirrored device fails, SAP ASE “unmirrors” the bad device and displays error messages. SAP ASE continues to run unmirrored.

• Determining Which Devices to Mirror
  When deciding to mirror a device, you must weigh such factors as the costs of system downtime, possible reduction in performance, and the cost of storage media. Considering these issues can help you decide whether to mirror only the transaction logs, all devices on a server, or selected devices.
• Conditions That Do Not Disable Mirroring
  SAP ASE disables a mirror only when it encounters an I/O error on a mirrored device.
• Disk Mirroring Commands
  The disk mirror, disk unmirror, and disk remirror commands control disk mirroring. You can issues any of these commands while the devices are in use. In other words, you need not stop databases to set up or modify disk mirroring.
• Disk Mirroring Tutorial
  The steps for disk mirrorming include intializing, mirroring, unmirroring, remirroring, disabling, and then removing a test device.
• Disk Resizing and Mirroring
  Use disk resize only when mirroring is permanently disabled.