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So what can you do?

You can use the data in access.log to get a list of URIs that might be in the cache. Then, feed them to squidclient or another utility to generate PURGE requests. For example:

% awk '{print $7}' /usr/local/squid/var/logs/access.log \

| grep www.example.com \

| xargs -n 1 squidclient -m PURGE

Removing All Objects

In extreme circumstances you may need to wipe out the entire cache, or at least one of the cache directories. First, you must make sure that Squid isn't running.

One of the easiest ways to make Squid forget about all cached objects is to overwrite the swap.state files. Note that you can't simply remove the swap.state files because Squid then scans the cache directories and opens all the object files. You also can't simply truncate swap.state to a zero-sized file. Instead, you should put a single byte there, like this:

# echo '' > /usr/local/squid/var/cache/swap.state

When Squid reads the swap.state file, it gets an error because the record that should be there is too short. The next read results in an end-of-file condition, and Squid completes the rebuild procedure without loading any object metadata.

Note that this technique doesn't remove the cache files from your disk. You've only tricked Squid into thinking that the cache is empty. As Squid runs, it adds new files to the cache and may overwrite the old files. In some cases, this might cause your disk to run out of free space. If that happens to you, you need to remove the old files before restarting Squid again.

One way to remove cache files is with rm. However, it often takes a very long time to remove all the files that Squid has created. To get Squid running faster, you can rename the cache directory, create a new one, start Squid, and remove the old one at the same time. For example:

# squid -k shutdown

# cd /usr/local/squid/var

# mv cache oldcache

# mkdir cache

# chown nobody:nobody cache

# squid -z

# squid -s

# rm -rf oldcache &

Another technique is to simply run newfs (or mkfs) on the cache filesystem. This works only if you have the cache_dir on its own disk partition.

refresh_pattern

The refresh_pattern directive controls the disk cache only indirectly. It helps Squid decide whether or not a given request can be a cache hit or must be treated as a miss. Liberal settings increase your cache hit ratio but also increase the chance that users receive a stale response. Conservative settings, on the other hand, decrease hit ratios and stale responses.

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Tip

The refresh_pattern rules apply only to responses without an explicit expiration time. Origin servers can specify an expiration time with either the Expires header, or the Cache-Control: max-age directive.

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You can put any number of refresh_pattern lines in the configuration file. Squid searches them in order for a regular expression match. When Squid finds a match, it uses the corresponding values to determine whether a cached response is fresh or stale. The refresh_pattern syntax is as follows:

refresh_pattern [-i] regexp

min

percent

max [options]

For example:

refresh_pattern -i \.jpg$ 30 50% 4320 reload-into-ims

refresh_pattern -i \.png$ 30 50% 4320 reload-into-ims

refresh_pattern -i \.htm$ 0 20% 1440

refresh_pattern -i \.html$ 0 20% 1440

refresh_pattern -i . 5 25% 2880

The regexp parameter is a regular expression that is normally case-sensitive. You can make them case-insensitive with the -i option. Squid checks the refresh_pattern lines in order; it stops searching when one of the regular expression patterns matches the URI.

The min parameter is some number of minutes. It is, essentially, a lower bound on stale responses. A response can't be stale unless its time in the cache exceeds the minimum value. Similarly, max is an upper limit on fresh responses. A response can't be fresh unless its time in the cache is less than the maximum time.

Responses that fall between the minimum and maximum are subject