2021-03-05 02:06:05 +05:00
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Full text search
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==========================
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By default, when your IMAP client searches for an email containing some
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text in its *body*, dovecot will read all your email sequentially. This
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is very slow and IO intensive. To speed body searches up, it is possible to
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*index* emails with a plugin to dovecot, ``fts_xapian``.
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Enabling full text search
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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To enable indexing for full text search here is an example configuration.
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.. code:: nix
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{
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mailserver = {
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# ...
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fullTextSearch = {
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enable = true;
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# index new email as they arrive
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autoIndex = true;
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# this only applies to plain text attachments, binary attachments are never indexed
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indexAttachments = true;
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enforced = "body";
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};
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};
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}
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The ``enforced`` parameter tells dovecot to fail any body search query that cannot
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use an index. This prevents dovecot to fall back to the IO-intensive brute
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force search.
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If you set ``autoIndex`` to ``false``, indices will be created when the IMAP client
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issues a search query, so latency will be high.
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Resource requirements
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~~~~~~~~~~~~~~~~~~~~~~~~
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2021-03-12 00:53:38 +05:00
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Indices created by the full text search feature can take more disk
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space than the emails themselves. By default, they are kept in the
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emails location. When enabling the full text search feature, it is
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recommended to move indices in a different location, such as
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(``/var/lib/docecot/indices/%d/%n``) by using the option
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``mailserver.indexDir``.
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.. warning::
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When the value of the ``indexDir`` option is changed, all dovecot
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indices needs to be recreated: clients would need to resynchronize.
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2021-03-05 02:06:05 +05:00
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Indexation itself is rather resouces intensive, in CPU, and for emails with
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large headers, in memory as well. Initial indexation of existing emails can take
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hours. If the indexer worker is killed or segfaults during indexation, it can
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be that it tried to allocate more memory than allowed. You can increase the memory
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limit by eg ``mailserver.fullTextSearch.memoryLimit = 2000`` (in MiB).
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Mitigating resources requirements
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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You can:
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* disable indexation of attachements ``mailserver.fullTextSearch.indexAttachments = false``
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* reduce the size of ngrams to be indexed ``mailserver.fullTextSearch.minSize`` and ``maxSize``
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* disable automatic indexation for some folders with
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``mailserver.fullTextSearch.autoIndexExclude``. Folders can be specified by
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name (``"Trash"``), by special use (``"\\Junk"``) or with a wildcard.
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