Setting up a Local Squid Proxy

For clusters of nodes with CernVM-FS clients, we strongly recommend setting up two or more Squid forward proxy servers as well. The forward proxies will reduce the latency for the local worker nodes, which is critical for cold cache performance. They also reduce the load on the Stratum 1 servers.

From what we have seen, a Squid server on commodity hardware scales well for at least a couple of hundred worker nodes. The more RAM and hard disk you can devote for caching the better. We have good experience with memory cache and hard disk cache. We suggest setting up two identical Squid servers for reliability and load-balancing. Assuming the two servers are A and B, set


Squid is very powerful and has lots of configuration and tuning options. For CernVM-FS we require only the very basic static content caching. If you already have a Frontier Squid <> installed you can use it as well for CernVM-FS.

Otherwise, cache sizes and access control needs to be configured in order to use the Squid server with CernVM-FS. In order to do so, browse through your /etc/squid/squid.conf and make sure the following lines appear accordingly:

minimum_expiry_time 0
maximum_object_size 1024 MB

cache_mem 128 MB
maximum_object_size_in_memory 128 KB
# 50 GB disk cache
cache_dir ufs /var/spool/squid 50000 16 256

Furthermore, Squid needs to allow access to all Stratum 1 servers. This is controlled through Squid ACLs. Most sites allow all of their IP addresses to connect to any destination address. By default squid allows that for the standard private IP addresses, but if you’re not using a private network then add your public address ranges, with something like this:

acl localnet src A.B.C.D/NN

If you instead want to limit the destinations to major cvmfs Stratum 1s, it is better to use the list built in to Frontier Squid because the list is sometimes updated with new releases.

The Squid configuration can be verified by squid -k parse. Before the first service start, the cache space on the hard disk needs to be prepared by squid -z. In order to make enough file descriptors available to squid, execute ulimit -n 8192 or some higher number prior to starting the squid service.