What are the Q1 and Q2 parameters?

In the source code, these are called magic1 and magic2. These numbers refer to the number of oustanding requests on a message queue. They are specified on the cache_dir option line, after the L1 and L2 directories:

cache_dir diskd /cache1 1024 16 256 Q1=72 Q2=64

If there are more than Q1 messages outstanding, then Squid will intentionally fail to open disk files for reading and writing. This is a load-shedding mechanism. If your cache gets really really busy and the disks can not keep up, Squid bypasses the disks until the load goes down again.

If there are more than Q2 messages outstanding, then the main Squid process ``blocks'' for a little bit until the diskd process services some of the messages and sends back some replies.

Reasonable Q1 and Q2 values are 64 and 72. If you would rather have good hit ratio and bad response time, set Q1 > Q2. Otherwise, if you would rather have good response time and bad hit ratio, set Q1 < Q2.