[documentation] Performance settings documentation for settings.php

[Kieran Lal]

Hello, I would appreciate some help reviewing the following  
Performance settings. I have run out of time to get this complete  
have fallen a little under the weather.  Please provide some support  
to Jeremy to help get this reviewed.

We believe this represents at 6-10X performance for anonymous pages  
on Drupal sites.

Kieran

/**
* Performance settings:
*
* The following settings can be modified to try and increase the  
performance
* of your Drupal-powered website.  It is not advised that you try  
tuning these
* settings unless you fully understand what you are doing.
*
* file_cache:
* The file_cache setting allows you to specify a directory in which  
Drupal
* should write cache files.  Anything that normally would have been  
cached to
* the database will now be cached to the filesystem.  You can specify an
* absolute path (starting with a '/'), or a path relative to your top  
level
* Drupal installation directory (that is, the directory that contains  
files
* such as index.php and cron.php).  You will need to manually create the
* specified cache directory, and to give it proper permission so that  
your
* webserver has read/write access.
*
* The file_cache may be helpful to you if your Database is under  
powered.  In
* the event that Drupal is unable to connect to the database (for  
example
* if the database is down for maintenance, or if the database is already
* handling the maximum number of connections), it will attempt to  
display the
* file-cached version of the page instead of displaying an error.  If  
it is
* unable to display the file-cached version of the page, only then will
* someone accessing your site get a database error.
*
* Example:
*   $file_cache = 'files/cache';
*   $file_cache = '/tmp/cache';
*
* file_cache_fastpath:
* When file_cache_fastpath is enabled, pages that have been cached to  
the
* filesystem will be displayed to anonymous users without making any  
database
* queries.  This bypasses Drupal's session management, as well as all  
_init
* and _exit hooks.  As always, logged in users will not be displayed the
* cached pages.
*
* cache_lifetime:
* Simply enabling the page cache will offer a sufficient performance  
boost
* for most low-traffic websites.  However, on high-traffic websites  
it can
* become necessary to enforace a minimum lifetime for cached pages.  The
* cache_lifetime is the minimum amount of time that will go by before  
the
* cache is emptied and recreated.  A larger minimum cache lifetime  
offers
* better performance, but users will not see new content for a longer  
period
* of time.
*
* The cache lifetime should be specified in seconds.  By default  
there is no
* minimum cache lifetime.
*
* roll_init:
* The roll_init variable allows you to control for which user roles the
* _init hook of each module is enabled.  As the _init hook is  
executed even
* when displaying cached pages, it can offer a significant  
performance boost
* to disable the hook for all or some roles.  Specify roles by their rid
* (obtained from your Drupal database), or use either of the  
following two
* keywords:  ALL, NONE
*
* By default, the _init hook is called for all user roles.
*
* roll_exit:
* The roll_exit variable allows you to control for which user roles  
the _exit
* hook of * each module is enabled.  As the _exit hook is executed  
even when
* displaying cached pages, it can offer a significant performance  
boost to
* disable the hook for all or some roles.  For example, the data  
stored in
* the Drupal access_log is obtained by an _exit hook in the  
statistics module.
* Specify roles by their rid (obtained from your Drupal database), or  
use
* either of the following two keywords:  ALL, NONE
*
* By default, the _exit hook is called for all user roles.
*/
$file_cache = '';
$file_cache_fastpath = FALSE;
$cache_lifetime = 0;
$roll_init = ALL;
$roll_exit = ALL;