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DOCS-6802: Add initial inMemory documentation #2478

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53 changes: 53 additions & 0 deletions source/core/inmemory.txt
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@@ -0,0 +1,53 @@
.. _storage-inmemory:

========================
In-Memory Storage Engine
========================

.. default-domain:: mongodb

.. warning::
The ``inMemory`` storage engine is provided for beta-testing and not intended for
production use at this time.

Starting in MongoDB Enterprise 3.2, an in-memory storage engine is available in
the 64-bit builds for beta-testing purposes. By avoiding disk I/O, this storge
engine allows for more predictable latency of database operations.


Specify In-Memory Storage Engine
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
To select the in-memory storage engine, specify:
``inMemory`` for the :option:`--storageEngine` option or the
:setting:`storage.engine` setting.

Although the in-memory storage engine does not write data to the filesystem,
it maintains small metadata files in the ``--dbpath``.

.. code-block:: sh

mongod --storageEngine inMemory --dbpath <path>

See :ref:`cli-mongod-inmemory` for configuration options specific to this storage engine.

.. warning::
The ``inMemory`` storage engine is ephemeral, and all data will be lost when
the ``mongod`` instance shuts down.

Concurrency
~~~~~~~~~~~
The in-memory storage engine uses *document-level* concurrency control for write
operations. As a result, multiple clients can modify different
documents of a collection at the same time.

Durability
----------

The in-memory storage engine is ephemeral and does not write data to persistent
storage. As such, the concept of :term:`journal` or waiting for data to become
:term:`durable` does not apply to the in-memory storage engine.
Write operations that specify a write concern :writeconcern:`journaled <j>` are
acknowledged immediately. When an :program:`mongod` instance shuts down, either as
result of the :dbcommand:`shutdown` command or due to a system error, recovery is
impossible.

24 changes: 24 additions & 0 deletions source/includes/options-mongod.yaml
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Expand Up @@ -2040,4 +2040,28 @@ description: |
rather than the "mirrored mongod" write protocol.

directive: option
---
program: mongod
name: inMemorySizeGB
optional: true
args: <integer>
default: |
60% of physical RAM less 1 GB
directive: option
description: |
.. versionadded:: 3.2

Maximum amount of memory to allocate for :doc:`core/inmemory` data and indexes.
---
program: mongod
name: inMemoryStatisticsLogDelaySecs
optional: true
args: <integer>
default: |
0, do not log statistics.
directive: option
description: |
.. versionadded:: 3.2

Seconds to wait between each write to a :doc:`/core/inmemory` statistics file in the :option:`--dbpath`.
...
4 changes: 4 additions & 0 deletions source/includes/toc-storage.yaml
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Expand Up @@ -5,4 +5,8 @@ description: |
file: /core/wiredtiger
description:
An introduction to the WiredTiger storage engine.
---
file: /core/inmemory
description:
An introduction to the in-memory storage engine.
...
62 changes: 43 additions & 19 deletions source/reference/program/mongod.txt
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Expand Up @@ -47,22 +47,28 @@ Core Options

.. include:: /includes/option/option-mongod-bind_ip.rst

.. include:: /includes/option/option-mongod-ipv6.rst

.. include:: /includes/option/option-mongod-maxConns.rst

.. include:: /includes/option/option-mongod-logpath.rst

.. include:: /includes/option/option-mongod-syslog.rst

.. include:: /includes/option/option-mongod-syslogFacility.rst

.. include:: /includes/option/option-mongod-logpath.rst

.. include:: /includes/option/option-mongod-logappend.rst

.. include:: /includes/option/option-mongod-logRotate.rst

.. include:: /includes/option/option-mongod-timeStampFormat.rst

.. COMMENT diaglog is deprecated and not in mongod help output

.. include:: /includes/option/option-mongod-diaglog.rst

.. COMMENT traceExceptions is internal use only and not in mongod help output

.. include:: /includes/option/option-mongod-traceExceptions.rst

.. include:: /includes/option/option-mongod-pidfilepath.rst
Expand All @@ -73,8 +79,12 @@ Core Options

.. include:: /includes/option/option-mongod-httpinterface.rst

.. COMMENT nohttpinterface not in mongod help output

.. include:: /includes/option/option-mongod-nohttpinterface.rst

.. COMMENT clusterAuthMode moved to SSL section

.. include:: /includes/option/option-mongod-nounixsocket.rst

.. include:: /includes/option/option-mongod-unixSocketPrefix.rst
Expand All @@ -87,8 +97,6 @@ Core Options

.. include:: /includes/option/option-mongod-noauth.rst

.. include:: /includes/option/option-mongod-ipv6.rst

.. include:: /includes/option/option-mongod-jsonp.rst

.. include:: /includes/option/option-mongod-rest.rst
Expand All @@ -101,6 +109,8 @@ Core Options

.. include:: /includes/option/option-mongod-sysinfo.rst

.. COMMENT objcheck and noobjcheck missing from mongod help output

.. include:: /includes/option/option-mongod-objcheck.rst

.. include:: /includes/option/option-mongod-noobjcheck.rst
Expand All @@ -114,26 +124,14 @@ Core Options
Storage Options
~~~~~~~~~~~~~~~

.. include:: /includes/option/option-mongod-dbpath.rst

.. include:: /includes/option/option-mongod-storageEngine.rst

.. _cli-wiredtiger-options:

.. include:: /includes/option/option-mongod-wiredTigerCacheSizeGB.rst

.. include:: /includes/option/option-mongod-wiredTigerStatisticsLogDelaySecs.rst

.. include:: /includes/option/option-mongod-wiredTigerJournalCompressor.rst

.. include:: /includes/option/option-mongod-wiredTigerDirectoryForIndexes.rst

.. include:: /includes/option/option-mongod-wiredTigerCollectionBlockCompressor.rst

.. include:: /includes/option/option-mongod-wiredTigerIndexPrefixCompression.rst
.. include:: /includes/option/option-mongod-dbpath.rst

.. include:: /includes/option/option-mongod-directoryperdb.rst

.. COMMENT noIndexBuildRetry is not in mongod help output

.. include:: /includes/option/option-mongod-noIndexBuildRetry.rst

.. include:: /includes/option/option-mongod-noprealloc.rst
Expand Down Expand Up @@ -162,6 +160,23 @@ Storage Options

.. include:: /includes/option/option-mongod-journalCommitInterval.rst

.. _cli-wiredtiger-options:

WiredTiger Options
~~~~~~~~~~~~~~~~~~

.. include:: /includes/option/option-mongod-wiredTigerCacheSizeGB.rst

.. include:: /includes/option/option-mongod-wiredTigerStatisticsLogDelaySecs.rst

.. include:: /includes/option/option-mongod-wiredTigerJournalCompressor.rst

.. include:: /includes/option/option-mongod-wiredTigerDirectoryForIndexes.rst

.. include:: /includes/option/option-mongod-wiredTigerCollectionBlockCompressor.rst

.. include:: /includes/option/option-mongod-wiredTigerIndexPrefixCompression.rst

.. _cli-mongod-replica-set:

Replication Options
Expand Down Expand Up @@ -263,6 +278,15 @@ SNMP Options

.. _encryption-key-management-options:

.. _cli-mongod-inmemory:

inMemory Options
~~~~~~~~~~~~~~~~

.. include:: /includes/option/option-mongod-inMemorySizeGB.rst

.. include:: /includes/option/option-mongod-inMemoryStatisticsLogDelaySecs.rst

Encryption Key Management Options
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Expand Down