If a disk mechanism fails, the application and therefore the user does not see any outage. Many failures cause of the outage and what can be done to minimize the duration. After understanding all failure scenarios which might result in unplanned downtime, the next step is to identify all planned downtime procedures from replacing a failed component to upgrading an entire system. Unplanned disk events include disks failures, cable failures, host card failures, and disk controller failures. In the figure, the disks are protected using a RAID device.
All failures are transparent to the user with no outage.
When using disk mirroring, it is mportant to mirror together two disks that are on separate IO cards and connected via separate cables. Disk failures are protected by mirroring to multiple disks mechanisms. Again, these failures are transparent to the user with no outage to the application. Planned disk events include replacing a failed component and adding or removing a component. A disk mechanism can be hot-swapped and the data resynced automatically by the hardware.