4.3.2 Purging the on-disk cache

Polipo never removes a file in its on-disk cache, except when it finds that the instance that it represents has been superseded by a newer version. In order to keep the on-disk cache from growing without bound, it is necessary to purge it once in a while. Purging the cache typically consists in removing some files, truncating large files (see Partial instances) or moving them to off-line storage.

Polipo itself can be used to purge its on-disk cache; this is done by invoking Polipo with the -x flag. This can safely be done when Polipo is running (see Modifying the on-disk cache).

For a purge to be effective, it is necessary to cause Polipo to write-out its in-memory cache to disk (see Stopping). Additionally, Polipo will not necessarily notice the changed files until it attempts to access them; thus, you will want it to discard its in-memory cache after performing the purge. The safe way to perform a purge is therefore:

$ kill -USR1 polipo-pid
$ sleep 1
$ polipo -x
$ kill -USR2 polipo-pid

The behaviour of the -x flag is controlled by three configuration variables. The variable diskCacheUnlinkTime specifies the time during which an on-disk entry should remain unused before it is eligible for removal; it defaults to 32 days.

The variable diskCacheTruncateTime specifies the time for which an on-disk entry should remain unused before it is eligible for truncation; it defaults to 4 days and a half. The variable diskCacheTruncateSize specifies the size at which files are truncated after they have not been accessed for diskCacheTruncateTime; it defaults to 1MB.

Usually, Polipo uses a file’s modification time in order to determine whether it is old enough to be expirable. This heuristic can be disabled by setting the variable preciseExpiry to true.