4.8 Stable storage

NFS version 3 protocol servers must be able to recover without data loss from multiple power failures (including cascading power failures, that is, several power failures in quick succession), operating system failures, and hardware failure of components other than the storage medium itself (for example, disk, nonvolatile RAM).

Some examples of stable storage that are allowable for an NFS server include:

1. Media commit of data, that is, the modified data has been successfully written to the disk media, for example, the disk platter.

2. An immediate reply disk drive with battery-backed on-drive intermediate storage or uninterruptible power system (UPS).

3. Server commit of data with battery-backed intermediate storage and recovery software.

4. Cache commit with uninterruptible power system (UPS) and recovery software.

Conversely, the following are not examples of stable storage:

1. An immediate reply disk drive without battery-backed on-drive intermediate storage or uninterruptible power system (UPS).

2. Cache commit without both uninterruptible power system (UPS) and recovery software.

The only exception to this (introduced in this protocol revision) is as described under the WRITE procedure on the handling of the stable bit, and the use of the COMMIT procedure. It is the use of the synchronous COMMIT procedure that provides the necessary semantic support in the NFS version 3 protocol.