add doc for full text search

docs/fts.rst

@@ -0,0 +1,61 @@
+Full text search
+==========================
+
+By default, when your IMAP client searches for an email containing some
+text in its *body*, dovecot will read all your email sequentially. This
+is very slow and IO intensive. To speed body searches up, it is possible to
+*index* emails with a plugin to dovecot, ``fts_xapian``.
+
+Enabling full text search
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+To enable indexing for full text search here is an example configuration.
+
+.. code:: nix
+
+  {
+    mailserver = {
+      # ...
+      fullTextSearch = {
+        enable = true;
+        # index new email as they arrive
+        autoIndex = true;
+        # this only applies to plain text attachments, binary attachments are never indexed
+        indexAttachments = true;
+        enforced = "body";
+      };
+    };
+  }
+
+
+The ``enforced`` parameter tells dovecot to fail any body search query that cannot
+use an index. This prevents dovecot to fall back to the IO-intensive brute
+force search.
+
+If you set ``autoIndex`` to ``false``, indices will be created when the IMAP client
+issues a search query, so latency will be high.
+
+Resource requirements
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+Indices can take more disk space than the emails themselves. By default, they
+are kept in a different location (``/var/lib/dovecot/fts_xapian``) than emails
+so that you can backup emails without indices.
+
+Indexation itself is rather resouces intensive, in CPU, and for emails with
+large headers, in memory as well. Initial indexation of existing emails can take
+hours. If the indexer worker is killed or segfaults during indexation, it can
+be that it tried to allocate more memory than allowed. You can increase the memory
+limit by eg ``mailserver.fullTextSearch.memoryLimit = 2000`` (in MiB).
+
+Mitigating resources requirements
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+You can:
+
+* disable indexation of attachements ``mailserver.fullTextSearch.indexAttachments = false``
+* reduce the size of ngrams to be indexed ``mailserver.fullTextSearch.minSize`` and ``maxSize``
+* disable automatic indexation for some folders with
+  ``mailserver.fullTextSearch.autoIndexExclude``.  Folders can be specified by
+  name (``"Trash"``), by special use (``"\\Junk"``) or with a wildcard.
+

docs/index.rst

@@ -26,7 +26,7 @@ Welcome to NixOS Mailserver's documentation!
    backup-guide
    add-radicale
    rspamd-tuning
-
+   fts
 
 Indices and tables
 ==================