Tweaking Cyrus IMAP
*******************


Storage Tiering
===============

Various opportunities exist to tier storage. With storage tiering, we
mean to distribute payload over different levels of storage, where
levels mean to refer to storage cost, performance and capacity, in a
way that makes optimal use of the storage solution(s) – with an eye on
excellent performance yet remain cost-effective.

Imagine that you have the following levels of storage:

* A **low-cost** solution with large capacity,

  such as many consumer-grade 4 TB HDDs,

* A **high-cost** solution with little capacity,

  such as a few business-grade 512 GB SSDs.

The following options are available to split up the payload for a
Cyrus IMAP server:

1. Partitioning the mail spool;

   where you divide a single backend server’s mail spool in to
   multiple *partitions*, presumably at least two, and make sure that
   mail folders end up on the correct partition.

   You would have one partition mounted off of the fast storage
   contain the *day-to-day* payload, such as user’s INBOX folders,
   while the partition mounted off of the slower, cheap storage holds
   less frequently used data, such as archive folders.

   See also:

     * Partitioning the Mail Spool

2. Distributing the mail spool;

   where you would have one server hold *day-to-day* payload, and
   other servers hold less frequently used data, such as archive
   folders.

   Note that distribution of the mail spool requires a Cyrus IMAP
   Murder topology, so that access to the mailboxes remains
   transparent.

   Also note that such archive folders may be hosted using not only a
   low-cost storage tier, but perhaps also using a less resourceful
   compute node.

   See also:

     * Distributing the Mail Spool

3. Caching I/O using dm-cache;

   This mechanism is a feature of Linux distributions that ship a
   kernel version of 3.9 or later, called dm-cache, and allows
   multiple disk volumes to be used as tiered caching storage.

   Using this technology, frequently accessed data can automatically
   be promoted to the fast storage, while less frequently accessed
   data can automatically be demoted to the cheaper storage.

4. Using metadata partitions;

   While the access patterns for data and metadata are different, so
   could their storage locations be.

   In this scenario, you would distinguish between a mail folder’s
   message content and metadata – the metadata consists of mail
   headers, indexes created for the purpose of searching, and such,
   while mail messages are stored in separate files.

   When a mail folder is opened, and the messages in the folder are
   listed, no mail message content is touched – the results come
   entirely from metadata.

   It is only when the individual mail message is being fetched, that
   the message file is opened.


Partitioning the Mail Spool
---------------------------


Distributing the Mail Spool
---------------------------


Using Metadata Partitions
-------------------------


Synchronous File Operations
===========================

A default Kolab Groupware installation comes with a set of settings
suitable for the vast majority of our users – mostly home users and
other small(er) deployments.

It is typical for these installations to **not** have battery-backed
I/O controllers, and/or some other form of enterprise-level storage.

To prevent data from being corrupted during a power outage, the
default for our Cyrus IMAP packages is to ensure the mail spool
("/var/spool/cyrus/") and configuration directory ("/var/lib/imap/")
and all files contained therein have the *synchronous* filesystem flag
set.

To gain performance, execute the following:

1. Remove the synchronous flag from the directories and files:

      **chattr -RV -S /var/lib/imap/ /var/spool/cyrus/**

2. In "/etc/sysconfig/cyrus-imapd" (or "/etc/default/cyrus-imapd"),
   change the following:

      CHATTRSYNC=1

   to:

      CHATTRSYNC=0


Mailbox locking
---------------

Cyrus IMAP uses fcntl(2) based file locking for mailboxes, for example
during SELECT commands. To mitigate race conditions it locks mailbox
names even for non-existing mailboxes. For example, if user foo issued
the following command

      SELECT INBOX.x

for non-existing mailbox x, it creates a lock file

      $CYRUS_CONFDIR/lock/user/foo/x.lock

which might be left on the filesystem after completion of the command.

This has in practice not shown to be an issue. If this is a concern
however, keeping the lock file directory in a tmpfs allows for both
fast locking and to purge stale locks during controlled Cyrus
downtimes.
