Resolve "cache size = "max""

Closes #524 (closed)

daemon/bindings/cache.rst

@@ -37,6 +37,16 @@ Now you can configure cache size to be 90% of the free memory 14 928 MB, i.e. 13
    -- 90 % of free memory after machine restart
    cache.size = 13453 * MB
 
+Or you can use the entire cache partition with small free disk space, i.e. 10 MB:
+
+.. code-block:: lua
+
+   cache.size = fssize(cache.current_storage) - 10*MB
+
+.. tip:: We recommended that you use ``fssize`` on a cache-only partition (like RAM disk).
+   So you can better estimate the available free space. It is also a good idea to
+   leave 1-10% free disk space (for small files, locks etc.).
+
 .. note:: The :ref:`garbage-collector` can be used to periodically trim the
    cache. It is enabled and configured by default when running kresd with
    systemd integration.
@@ -167,6 +177,12 @@ Configuration reference
 
    .. note:: This may or may not clear the cache, depending on the cache backend.
 
+.. function:: cache.fssize(storage)
+
+   :param string storage: Cache storage.
+
+   :return: Partition size of cache storage.
+
 .. function:: cache.stats()
 
    Return table with low-level statistics for each internal cache operation.