Implement and document cache-local-dir-write-max-size

doc/relstorage-options.rst

@@ -441,6 +441,21 @@ cache-local-dir-read-count
 
         .. versionadded:: 2.0b5
 
+cache-local-dir-write-max-size
+        The *approximate* maximum size of each individual cache file
+        on disk. When not specified (the default), the maximum file
+        size will be the same as ``cache-local-mb``.
+
+        This is an approximate number because there is some overhead
+        associated with the storage format that varies based on the
+        number of entries in the cache.
+
+        RelStorage will write to disk, from most important to least
+        important, the entries in the cache until all the entries are
+        written or this limit is reached. If you use a size smaller
+        than ``cache-local-mb``, however, you may miss important
+        entries that are only used at application startup.
+
 Remote Caching
 --------------
 

relstorage/component.xml

@@ -86,6 +86,9 @@
     <key name="cache-local-dir-compress" datatype="boolean" required="no" default="false">
       <description>See the RelStorage README.txt file.</description>
     </key>
+    <key name="cache-local-dir-write-max-size" datatype="byte-size" required="no" default="0">
+      <description>See the RelStorage README.txt file.</description>
+    </key>
     <key name="cache-local-object-max" datatype="byte-size" required="no">
       <description>See the RelStorage README.txt file.</description>
     </key>

relstorage/tests/testmysql.py

@@ -65,6 +65,7 @@ class ZConfigTests(object):
             replica-conf %s
             blob-chunk-size 10MB
             cache-local-dir-read-count 12
+            cache-local-dir-write-max-size 10MB
             <mysql>
                 driver auto
                 db %s