Disable write-ahead log

Since it's a cache, we do not need to retain the data from one
MaxScale invocation to the next. Consequently, we can turn off
write ahead logging completely if we simply wipe the RocksDB
database at each startup. In addition, the location of the
cache directory can now be specified explicitly, so it can be
placed, for instance, on a RAM disk.

Documentation/Filters/Cache.md

@@ -381,4 +381,26 @@ regardless of what host the `admin` user comes from.
 
 # Storage
 
-## Storage RocksDB
+## `storage_rocksdb`
+
+This storage module uses RocksDB database for storing the cached data. The
+directory where the RocksDB database will be created is by default created
+into the _MaxScale cache_ directory, which usually is not on a RAM disk. For
+maximum performance, you may want to explicitly place the RocksDB database
+on a RAM disk.
+
+### Parameters
+
+#### `cache_directory`
+
+Specifies the directory under which the filter instance specific RocksDB
+databases will be placed. Note that at startup, each RocksDB database will
+be deleted and recreated. That is, cache content will not be retained across
+MaxScale restarts.
+
+```
+storage_options=cache_directory=/mnt/maxscale-cache
+```
+
+With the above setting a directory `/mnt/macscale-cache/storage_rocksdb` will
+created, under which the actual instance specific cache directories are created.