Mirroring devices - strategies

Mirroring in Sybase ASE allows to avoid failures of databases. So the answer to question "what to mirror" is the same as "what is the most important"?

• master database and device is surely one of the most important componentes of server. Failure in master device will cause the entire server to stop. In case of master database additional step should be done apart from just mirroring devices.
  When Sybase ASE starts it needs to load master database. If the device would be destroyed when server was offline, it will not be able to start. Remember that information about mirroring is stored in sysdevices table in... master. Server without master is not aware of exiting mirror. To solve this issue parameter -r can be added to RUN file pointing to a mirrored master device.
• Important databases only. This is good idea to mirror all the important databases' devices. In case of IO failure on one device users may not be aware of the fault at all, as Server will switch to mirrored device automatically
• Logs only. This Allows to recover in case of failure on a device containg the database. Using backups and mirrored log of database, during the restore operation no one transaction will be lost, however users will need to wait until recovery is finished.
• All databases. Very good idea, however the most expensive. To mirror all the data double so much disk space will be required and also performance on all databases may be affected specially if serial write option has been used