Ephemeral files and temporary filesystems

[Дилян Палаузов]

Hello,

the file docsrc/imap/concepts/deployment/performance_recommendations.rst says:

In-memory filesystems are faster then disk filesystems, but are limited in space and volatile as well. The following
list includes configuration settings that could make the corresponding databases, files and directory trees be created
and maintained on a temporary filesystem.

* ``proc_path``: /dev/shm/cyrus-imapd/proc
* ``mboxname_lockpath``: /dev/shm/cyrus-imapd/mboxname_lock
* ``duplicate_db_path``: /dev/shm/cyrus-imapd/duplicate_db
* ``statuscache_db_path``: /dev/shm/cyrus-imapd/statuscache_db

----
lib/imapoptions says:
{ "prometheus_stats_dir", NULL, STRING }
/* Directory to use for gathering prometheus statistics.  If specified,
   must be an absolute path.  If not specified, the default path
   $configdirectory/stats/ will be used.  It may be advantageous to locate this
   directory on ephemeral storage. */

----
doc/examples/imapd_conf/murder-backend.conf, doc/examples/imapd_conf/murder-frontend.conf,
doc/examples/imapd_conf/murder-mupdate.conf, doc/examples/imapd_conf/normal-master.conf, doc/examples/imapd_conf/normal-
replica1.conf and doc/examples/imapd_conf/normal.conf say:

# Directories for proc and lock files
proc_path: /run/cyrus/proc
mboxname_lockpath: /run/cyrus/lock

# Locations for DB files
# The following DB are recreated upon initialization, so should live in
# ephemeral storage for best performance.
duplicate_db_path: /run/cyrus/deliver.db
ptscache_db_path:  /run/cyrus/ptscache.db
statuscache_db_path: /run/cyrus/statuscache.db
tls_sessions_db_path: /run/cyrus/tls_sessions.db

----
docsrc/imap/download/packagers.rst says:
Ephemeral files and temporary filesystems
#########################################

In addition to Unix domain sockets and lock files, several databases
used by Cyrus programs may be located in temporary filesystems, such as
those backed by RAM (i.e. tmpfs, md, etc.).  Here's a list of such
files.  In this example, the filesystem ``/run`` is on tmpfs::

    proc_path: /run/cyrus/proc
    mboxname_lockpath: /run/cyrus/lock
    duplicate_db_path: /run/cyrus/deliver.db
    statuscache_db_path: /run/cyrus/statuscache.db
    ptscache_db_path: /run/cyrus/ptscache.db
    tls_sessions_db_path: /run/cyrus/tls_sessions.db
    lmtpsocket: /run/cyrus/socket/lmtp
    idlesocket: /run/cyrus/socket/idle
    notifysocket: /run/cyrus/socket/notify

----
docsrc/imap/download/upgrade.rst says:
You don't need to copy the following databases as Cyrus 3.0 will
recreate these for you automatically:

* duplicate delivery (deliver.db),
* TLS cache (tls_sessions.db),
* PTS cache (ptscache.db),
* STATUS cache (statuscache.db).

.. note::
    You may wish to consider relocating these four databases to ephemeral
    storage, such as ``/run/cyrus`` (Debian/Ubuntu) or ``/var/run/cyrus``
    or whatever suitable tmpfs is provided on your distro.  It will place
    less IO load on your disks and run faster.

----
docsrc/imap/reference/admin/locations/configuration-state.rst says:

Ephemeral Databases
^^^^^^^^^^^^^^^^^^^

Several of the state databases may be located in RAM-backed locations,
such as ``/run`` or ``/var/run`` or where ever your OS roots such
filesystems (i.e. ``tmpfs`` on Linux or ``mfs`` on FreeBSD). Cyrus 3.0 will
recreate the following databases for you automatically:

* duplicate delivery (deliver.db),
* TLS cache (tls_sessions.db),
* PTS cache (ptscache.db),
* STATUS cache (statuscache.db).

Relocating these DBs to ephemeral storage will place less IO load on
your disks and tend to run faster.

----
Where is the truth?  What is the complete list of things, that will benefit on being on a tmpfs?  On one place or
another, STATUS cache, TLS cache, PTS cache, duplicate delivery, proc_path, mboxname_path, prometheus_stats_dir are
mentioned as candidates.

Do sockets (lmtpsocket) benefit from being on tmpfs, as only download/packagers.rst suggests ?

Putting on a single place all candidates for tmpfs will be good, and using consistently /dev/shm or /run or /var/run to
mean tmpfs in all documentation will also be good.

Greetings
  Дилян