Web Performance Tuning by Patrick Killelea

Author:Patrick Killelea [Killelea, Patrick]
Language: fra
Format: epub, pdf
Tags: Informatique
Publisher: O'Reilly
Published: 2001-08-26T07:09:53+00:00

This file describes a number of hostname-to-address

#

mappings for the TCP/IP subsystem. It is mostly

#

used at boot time, when no name servers are running.

#

On small systems, this file can be used instead of a

#

"named" name server. Just add the names, addresses

#

and any aliases to this file...

#

127.0.0.1 localhost

141.211.144.53 www.umich.edu

# End of hosts.

page 102

Web Performance Tuning

Unix users can easily set up a DNS server on their own client machines. The advantage here is that DNS servers cache entries they've looked up, making subsequent access much faster. Netscape Navigator's DNS helper also caches DNS entries automatically for you.

Although DNS servers generally don't see a very large load, try to be sure that your DNS server and web server are not competing for bandwidth on your Internet connection. This may mean placing a DNS server inside your organization rather than relying on your ISP's DNS server. Note that DNS servers, like most Internet servers, tend to degrade in a nonlinear way, with performance dropping off rapidly after a certain load.

See Chapter 2, for a discussion of round robin DNS.

10.2.8 NFS

The Network File System (NFS) from Sun Microsystems is a method of making a set of files on a remote system appear to be on the disk of local computer. "Remote" can mean across a WAN, but NFS is much more commonly used on LANs because its performance over WANs is usually not good. NFS is a stateless protocol. NFS Version 2 runs over UDP, and Version 3 runs over TCP. A well-tuned NFS server has much better throughput than most web servers, given the same hardware. This means it is feasible to use an NFS server to help scale web services, particularly if you are serving static content. You can set up multiple web servers and provide the content to each server from the same central NFS server to keep the content synchronized across all servers.

There is a caching mechanism for NFS called cachefs, which keeps a local copy of files served by NFS and greatly improves read performance for subsequent access. Write performance is much lower than read

performance for NFS because writes must be committed to a non-volatile medium such as disk, according to the NFS protocol. Note that each directory requires an NFS lookup, so looking up a path with many directories over NFS results in a big performance loss the first time you do it. See Managing NFS and NIS, by Hal Stern (O'Reilly

& Associates).

Poor NFS usage can hurt web server performance a lot. For example, you don't want your web server log file NFS mounted, because a block of the log will have to be appended and copied back to the NFS server for each hit. In fact, I had a similar problem when appending mail messages to my mbox. The mbox file was on the NFS

server because it was in my home directory, which was NFS mounted. I noticed that as the mbox grew bigger, appending new messages to it got slower. A quick use of snoop showed that a large piece of my mbox was being copied to my machine, appended, and copied back to the NFS server.

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