# Software Options

Consider these examples of commonly-sold software options:

- Cluster Server (CS)

  A cluster server monitors the EMS server processes and their host computers, and ensures that exactly one server process is running at all times. If that server fails, the CS restarts it; if the CS fails to restart it, it starts the other server instead.

- Clustered File System (CFS)

  A clustered file system lets the two EMS server processes run simultaneously. It even lets both servers mount the shared file system simultaneously. However, the CFS assigns the lock to only one server process at a time. The CFS also manages operating system caching of file data, so the standby server has an up-to-date view of the file system (instead of a stale cache).

With dual-port SCSI or SAN hardware, either a CS or a CFS might satisfy the Distributed File Locking criterion. With NAS hardware, only a CS can satisfy this criterion (CFS software generally does not). Of course, you must confirm all four requirements with your vendors.

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