Replication NFS

Multiple location support in the automounter implements a simple network load-balancing scheme for replicated filesystems. At first glance, this seems to be a bit of overkill; after all, you don't need or want replication for read-write filesystems. However, serving large, read-only filesystems such as the manpages may add to an NFS server's request load. Having multiple servers share this load improves performance by reducing the total load placed on the most heavily used servers. Ideally, you want clients that are "close" to each server to mount its filesystems, reducing the amount of traffic that must go through bridges or routers.



For example, if you have four NFS servers that each export the manpages, the best client mounting scheme is probably not to have one-quarter of the clients mount/usr/man from each server. Instead, clients should mount the manpages from the server that is closest to them. Replicated filesystems are included in automounter maps simply by listing all possible servers in the map:


/usr/man -ro wahoo:/usr/man mahimahi:/usr/man \ thud:/usr/man onaga:/usr/man

The backslash at the end of the first line continues this indirect map entry onto the next line. If more than one server:directory pair is listed in an automounter map, the automounter pings all servers by sending a request to the null procedure of all NFS servers. From the set that responds, the automounter picks one that is "closest" by comparing the address of the servers with that of the clients. Ties are broken by using the server that responded to the ping first. The selected server is used by the automounter to serve the mount point.