I’ve been trying to convince one of our larger customers to install mod_deflate on their server for about a month. They have had concerns about compatibility with older browsers and the possibility that it will affect the PageRank, but I have finally put enough pressure on to have them let me try it. They have very few users with old browsers (and really, if somebody is running using that archaic of a browser, how likely is it that they are going to buy something on your site), and convinced them that there should be no SEO consequences with the change (if anything, the search engines will respect your site more for using less bandwidth and having a knowledgeable administrator).
Early this morning, I got it installed (took all of about 15 minutes) and it’s running great. It’s compressing HTML and Javascript files to about 20% of their original size, which equates to some significant bandwidth savings, and quicker page-load times. About 60% of the total bandwidth used on this site is for HTML and JavaScript files (the other 40% is images, movies, and a few other odds and ends). Overall, it looks like about a 30-40% drop in total bandwidth usage, which is very significant. I’ve heard of no problems with browser compatibility either, so everybody is happy.
Overall, I’d say that there is no good reason not to use mod_deflate on your site. Especially if you ever get charged for bandwidth overages.
Here are some useful resources for installing and gathering statistics on mod_deflate
Awstats – Detailed Web-Based statistics package
Perl-based mod_deflate statistics utility
Apache’s mod_deflate documentation
Firefox plugin to view the HTTP Response headers (and lots of other useful stuff)
And here’s a sample Apache configuration section that I picked up from somewhere. I just save this in /etc/httpd/conf.d/deflate.conf and restart apache, then you are good to go (requires that mod_deflate is already compiled and installed. Actual file location may vary, depending on your OS. This works for Red-Hat derivatives)
### Enable mod_deflate to compress output
# Insert filter
SetOutputFilter DEFLATE
# Netscape 4.x has some problems...
BrowserMatch ^Mozilla/4 gzip-only-text/html
# Netscape 4.06-4.08 have some more problems
BrowserMatch ^Mozilla/4\.0[678] no-gzip
# MSIE masquerades as Netscape, but it is fine
BrowserMatch bMSIE !no-gzip !gzip-only-text/html
## Don't compress for IE5.0
BrowserMatch "MSIE 5.0" no-gzip
# Don't compress images
SetEnvIfNoCase Request_URI .(?:gif|jpe?g|png|swf)$ no-gzip dont-vary
# Make sure proxies don't deliver the wrong content
Header append Vary User-Agent env=!dont-vary
## Log some stuff for mod_deflate stats
DeflateFilterNote Input instream
DeflateFilterNote Output outstream
DeflateFilterNote Ratio ratio
LogFormat '"%r" %{outstream}n/%{instream}n (%{ratio}n%%)' deflate
## end mod_deflate stats
#### END mod_deflate configuration