Client-Side Failover

Failover is process of selecting a server from a list of servers that support a replicated file system. Normally, the next server in the sorted list is used unless it fails to respond. By using client-side failover, an NFS client can detect when multiple servers are making the same data available and can switch to an alternative server when the current server is unavailable. This switch is known as remapping. Through normal use, the clients store the path name for each active file on a remote file system. During the remapping, these path names are evaluated to locate the files on the new server.

The file system can become unavailable if one of the following conditions occurs:

• If the file system is connected to a server that crashes

• If the server is overloaded

• If a network fault occurs

The failover under these conditions is normally transparent to the user. It can occur at any time without disrupting the processes that are running on the client.

For failover to occur, the file systems must be mounted read-only. The file systems must be identical for the failover to occur successfully. For information about identical file systems, see What Is a Replicated File System?. A static file system or a file system that is not changed often is the best candidate for failover.

You cannot use CacheFS and client-side failover on the same NFS mount. Extra information is stored for each CacheFS file system. This information cannot be updated during failover, so you can use only one of these two features when mounting a file system.

The number of replicas that must be established for every file system depends on many factors. Ideally, you have a minimum of two servers. Each server supports multiple subnets. This setup is better than having a unique server on each subnet. The process requires the checking of each listed server. Therefore, if more servers are listed, each mount is slower.