Squid vs httpd mod_cache

[Neil Gunton <neil@xxxxxxxxxxxx>]

Hi all,

I'm running a LAMP community website (Debian Lenny, Apache 2.2.9, MySQL, 
mod_perl) which gets around 100,000 page requests per day. I currently 
use two builds of apache - one lightweight front end caching reverse 
proxy, and a heavy back-end mod_perl. This worked well for years while I 
was using Apache 1.3, since I was using Igor Sysoev's mod_accel and 
mod_deflate modules to do the reverse proxy and caching. Now I have 
upgraded to Apache 2.2, I can't use his modules any more, so I've been 
trying to use the stock mod_cache. The server is a dual Opteron 265 
(i.e. 4 cores), 4GB RAM, 4x10k SCSI drives in RAID0 (I know it's risky, 
buy I need the space and performance, and backup is instantaneous with 
MySQL replication).

Everything's working fine, mostly, but I'm having some issues with the 
cache management. In a nutshell, htcacheclean just doesn't seem to be 
able to keep up with managing the cache pruning (i.e. keeping it down to 
a reasonable size). If I run htcacheclean in cron mode, then it takes 
hours to complete its run, and while running it hogs the disks and 
produces big iowait times. If I run it in daemon mode, then it just sits 
there and produces about half the iowait (if I run with the -n "nice" 
option), in which case it just isn't keeping up with the cache growth.

I'm concerned about the cache structure - it's a 3-level directory, and 
it seems to take a long time just to traverse it. Even doing a simple du 
on it seems to take forever, currently about 3 hours or more, and that's 
for about 10GB of cache. I'd prefer to keep the cache down to more like 
1GB at the most. In fact, that's what I have htcacheclean set to - 
1000MB. But it doesn't seem to be doing the job.

I've been asking around the Apache and mod_perl lists about ways to 
improve this. Someone suggested using Squid instead. So here I am - I've 
never used Squid, mostly because I always used Apache and really need 
the mod_rewrite capabilities for doing things like blocking image 
hotlinking from other sites. I really need a front-end reverse proxy 
that has capability to do access control stuff like this, as well as 
redirects for old content etc - you know, all the things you can do with 
mod_rewrite. I really don't want to have to pass all that back to the 
mod_perl processes.

I would like to know how good Squid's cache management (i.e. pruning) 
is. I get the impression that mod_cache in Apache 2.2 is not very mature 
- some of the cache management features don't even seem to be 
implemented yet. I assume that Squid is a much more mature product, and 
thus I'd hope that it has cache management pretty much down pat.

How does Squid manage its disk cache? Does it consume a lot of disk io 
when doing it?

Has anybody else here migrated from using Apache's mod_cache to Squid, 
and if so do you have any insights?

Lastly, if I do decide to use Squid, is the O'Reilly book from 2004 
still relevant, or is it out of date now? I know there's a lot of stuff 
online, but I like to have a handy book reference, plus a well-written 
book often has a good intro to the tool. This book seems to get only 
5-star reviews on Amazon. Is it still up to date?

Thanks in advance,

Neil