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<h4 class="subsection">4.3.2 Purging the on-disk cache</h4>
<p><a name="index-purging-214"></a><a name="index-diskCacheUnlinkTime-215"></a><a name="index-diskCacheTruncateTime-216"></a><a name="index-diskCacheTruncateSize-217"></a><a name="index-preciseExpiry-218"></a>
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that the instance that it represents has been superseded by a newer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 <dfn>purge</dfn> it once in a while. Purging the
cache typically consists in removing some files, truncating large
files (see <a href="Partial-instances.html#Partial-instances">Partial instances</a>) or moving them to off-line storage.
<p>Polipo itself can be used to purge its on-disk cache; this is done by
invoking Polipo with the <samp><span class="option">-x</span></samp> flag. This can safely be done
when Polipo is running (see <a href="Modifying-the-on_002ddisk-cache.html#Modifying-the-on_002ddisk-cache">Modifying the on-disk cache</a>).
<p>For a purge to be effective, it is necessary to cause Polipo to
write-out its in-memory cache to disk (see <a href="Stopping.html#Stopping">Stopping</a>).
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:
<pre class="example"> $ kill -USR1 <var>polipo-pid</var>
$ sleep 1
$ polipo -x
$ kill -USR2 <var>polipo-pid</var>
</pre>
<p>The behaviour of the <samp><span class="option">-x</span></samp> flag is controlled by three
configuration variables. The variable <code>diskCacheUnlinkTime</code>
specifies the time during which an on-disk entry should remain unused
before it is eligible for removal; it defaults to 32 days.
<p>The variable <code>diskCacheTruncateTime</code> 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
<code>diskCacheTruncateSize</code> specifies the size at which files are
truncated after they have not been accessed for
<code>diskCacheTruncateTime</code>; it defaults to 1<span class="dmn">MB</span>.
<p>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 <code>preciseExpiry</code> to true.
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