TCP/IP Tutorial and Technical Overview - IBM Redbooks

► Compound commands for performance, which execute lookup and read
commands in one operation
► Server delegation of status and locks to a given client, improving caching and
performance
► Internationalization in the form of 16/32-bit character support for file names by
means of the UTF-8 encoding
14.4.6 Cache File System
14.4.7 WebNFS
The Cache File System (CacheFS) provides the ability to cache one file
system on another. CacheFS accomplishes caching by mounting remote
directories on the local system. Whenever the client needs a mounted file, it first
refers to local cache. If the requested file exists, it is accessed locally. Otherwise,
the file is retrieved from the server using NFS. If reading the file sequentially, the
future data is retrieved for future access to the cache, increasing the file access
speed. Whether or not the cache is stored in random access memory (RAM) or
disk is implementation specific. However, if the size the cache is not prohibitive,
storing the cache on RAM is preferred because it provides much faster access.
CacheFS periodically checks the cached data for the data accuracy, sweeps out
the inaccurate data from the cache, and retrieves the current data from the
server to the cache. Although this reduces server and network loads, improving
performance for clients on slow links, CacheFS is only proper for the cases in
which the files are not changed frequently.
WebNFS is an enhanced version of the standard NFS. WebNFS enables
clients to access files over wide area networks (WANs). Because, as a transport
protocol, UDP is fast for local area networks (LANs), many implementations use
UDP for the standard NFS. However, a TCP connection is much more reliable for
wide area networks. Most of the new NFS implementations support TCP and
UDP. WebNFS clients first attempt to use a TCP connection. If it fails or is
refused, it then uses a UDP connection. WebNFS can easily recover from
dropped lines and recover the lost data.
Path name evaluation
The standard NFS version 3 is designed for LANs. The amount of time for each
LOOKUP request is not significant. As a result of this, the NFS protocol permits
only one single path name request at a time. When we consider a WAN or the
Internet, to request the path name and evaluate the path name might cause
significant delays. This process is very expensive, especially for files that are
Chapter 14. File-related protocols 545