Determining if an NFS Server Is Memory Bound

Paging file system data from the disk into memory is a more common NFS server performance problem.

To Determine if the Server Is Memory Bound

1. Watch the scan rate reported by vmstat 30.

   If the scan rate (sr, the number of pages scanned) is often over 200 pages/second, then the system is short of memory (RAM). The system is trying to find unused pages to be reused and may be reusing pages that should be cached for rereading by NFS clients.

2. Add memory.

   Adding memory eliminates repeated reads of the same data and enables the NFS requests to be satisfied out of the page cache of the server. To calculate the memory required for your NFS server, see "Calculating Memory," which follows.

The memory capacity required for optimal performance depends on the average working set size of files used on that server. The memory acts as a cache for recently read files. The most efficient cache matches the current working set size as closely as possible.

Because of this memory caching feature, it is not unusual for the free memory in NFS servers to be between 0.5 Mbytes to 1.0 Mbytes if the server has been active for a long time. Such activity is normal and desirable. Having enough memory allows you to service multiple requests without blocking.

The actual files in the working set may change over time. However, the size of the working set may remain relatively constant. NFS creates a sliding window of active files, with many files entering and leaving the working set throughout a typical monitoring period.