Limiting Disk Bottlenecks

Disk bandwidth on an NFS server has the greatest effect on NFS client performance. Providing sufficient bandwidth and memory for file system caching is crucial to providing the best possible file server performance. Note that read/write latency is also important. For example, each NFSop may involve one or more disk accesses. Disk service times add to the NFSop latency, so slow disks mean a slow NFS server.

Follow these guidelines to ease disk bottlenecks:

• Balance the I/O load across all disks on the system.

If one disk is heavily loaded and others are operating at the low end of their capacity, shuffle directories or frequently accessed files to less busy disks.

• Partition the file system(s) on the heavily used disk and spread the file system(s) over several disks.

Adding disks provides additional disk capacity and disk I/O bandwidth.

• Replicate the file system to provide more network-to-disk bandwidth for the clients if the file system used is read-only by the NFS clients, and contains data that doesn't change constantly.

See the following section, "Replicating File Systems"."

• Size the operating system caches correctly, so that frequently needed file system data may be found in memory.

Caches for inodes (file information nodes), file system metadata such as cylinder group information, and name-to-inode translations must be sufficiently large, or additional disk traffic is created on cache misses. For example, if an NFS client opens a file, that operation generates several name-to-inode translations on the NFS server.

If an operation misses the Directory Name Lookup Cache (DNLC), the server must search the disk-based directory entries to locate the appropriate entry name. What would nominally be a memory-based operation degrades into several disk operations. Also, cached pages will not be associated with the file.