DOCS-14598: Address eMRC=true PSA cache pressure

npentrel · jeff-allen-mongo · commit 78068d9c7ddc · 2021-07-22T11:59:49.000-04:00
diff --git a/config/redirects b/config/redirects
@@ -1821,6 +1821,7 @@ raw: /manual/core/wildcard -> ${base}/manual/core/index-wildcard/
 [v3.6-v4.4]: /${version}/core/sharding-change-shard-key-value.txt -> ${base}/${version}/core/sharding-shard-key.txt
 [v3.6-v4.4]: /${version}/core/sharding-set-missing-shard-key-fields.txt -> ${base}/${version}/core/sharding-shard-key.txt
 [v3.6-v4.4]: /${version}/core/sharding-find-shard-key.txt -> ${base}/${version}/core/sharding-shard-key.txt
+[v5.0-*]: /${version}/reference/read-concern-majority/#disable-read-concern-majority -> ${base}/${version}/reference/read-concern-majority/#primary-secondary-arbiter-replica-sets
 
 #
 # Upgrade / Downgrade redirects
diff --git a/source/administration/replica-set-maintenance.txt b/source/administration/replica-set-maintenance.txt
@@ -57,6 +57,10 @@ replica sets.
   primary-secondary-arbiter (PSA) replica set or on a replica set that
   is changing to a PSA architecture.
 
+:doc:`/tutorial/mitigate-psa-performance-issues`
+  Reduce cache pressure and increased write traffic for a deployment
+  that has a three-member primary-secondary-arbiter (PSA) architecture.
+
 .. toctree::
    :titlesonly:
    :hidden:
@@ -72,3 +76,4 @@ replica sets.
    /tutorial/configure-replica-set-secondary-sync-target
    /tutorial/rename-unsharded-replica-set
    /tutorial/modify-psa-replica-set-safely
+   /tutorial/mitigate-psa-performance-issues
diff --git a/source/core/replica-set-arbiter.txt b/source/core/replica-set-arbiter.txt
@@ -48,6 +48,11 @@ number of votes to break a tie:
 
 .. end-content-even-votes-example
 
+Performance Issues with PSA replica sets
+----------------------------------------
+
+.. include:: /includes/fact-psa-performance-issues.rst
+
 Replica Set Protocol Version and Arbiter
 -----------------------------------------
 
diff --git a/source/includes/fact-psa-performance-issues.rst b/source/includes/fact-psa-performance-issues.rst
@@ -0,0 +1,5 @@
+If you are using a three-member primary-secondary-arbiter (PSA)
+architecture, the write concern :writeconcern:`"majority"` can cause
+performance issues if a secondary is unavailable or lagging. See
+:ref:`performance-issues-psa` for advice on how to mitigate these
+issues.
diff --git a/source/reference/configuration-options.txt b/source/reference/configuration-options.txt
@@ -4165,6 +4165,10 @@ LDAP Parameters
    and is always set to ``true``. In earlier versions of MongoDB, 
    :setting:`~replication.enableMajorityReadConcern` was configurable.
 
+   .. warning::
+
+      .. include:: /includes/fact-psa-performance-issues.rst
+
 
 ``sharding`` Options
 ~~~~~~~~~~~~~~~~~~~~
diff --git a/source/reference/program/mongod.txt b/source/reference/program/mongod.txt
@@ -1837,12 +1837,17 @@ Replication Options
    *Default*: true
 
    Configures support for :readconcern:`"majority"` read concern.
- 
-   Starting in MongoDB 5.0, 
-   :option:`--enableMajorityReadConcern` cannot be changed 
-   and is always set to ``true``. In earlier versions of MongoDB, 
+
+   Starting in MongoDB 5.0,
+   :option:`--enableMajorityReadConcern` cannot be changed
+   and is always set to ``true``. In earlier versions of MongoDB,
    :option:`--enableMajorityReadConcern` was configurable.
 
+   .. warning::
+
+      .. include:: /includes/fact-psa-performance-issues.rst
+
+
 Sharded Cluster Options
 ~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/source/reference/read-concern-majority.txt b/source/reference/read-concern-majority.txt
@@ -42,6 +42,10 @@ Availability
 Read concern :readconcern:`"majority"` is available for use with or
 without causally consistent sessions and transactions.
 
+.. warning::
+
+   .. include:: /includes/fact-psa-performance-issues.rst
+
 Example
 -------
 
@@ -130,11 +134,15 @@ Read Your Own Writes
 
 .. include:: /includes/fact-read-own-writes.rst
 
-.. _disable-read-concern-majority:                                      
-                                                                        
-Disable Read Concern Majority                                           
------------------------------                                           
-                                                                        
-*For 3-Member Primary-Secondary-Arbiter Architecture*                   
+.. _disable-read-concern-majority:
+
+
+.. If you change this headline please also change the redirect that
+.. links from #disable-read-concern-majority to this headline.
+
+Primary-Secondary-Arbiter Replica Sets
+--------------------------------------
 
 .. include:: /includes/fact-eMRC-always-true-in-5.0.rst
+
+.. include:: /includes/fact-psa-performance-issues.rst
diff --git a/source/release-notes/5.0-compatibility.txt b/source/release-notes/5.0-compatibility.txt
@@ -145,6 +145,8 @@ Replica Sets
 
 .. include:: /includes/fact-eMRC-always-true-in-5.0.rst
 
+.. include:: /includes/fact-psa-performance-issues.rst
+
 ``secondaryDelaySecs`` Configuration Setting
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/source/release-notes/5.0.txt b/source/release-notes/5.0.txt
@@ -263,6 +263,8 @@ limited to an integer between ``0`` and ``255`` inclusive.
 
 .. include:: /includes/fact-eMRC-always-true-in-5.0.rst
 
+.. include:: /includes/fact-psa-performance-issues.rst
+
 Enhanced Thread Pool Timeout Control
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/source/tutorial/add-replica-set-arbiter.txt b/source/tutorial/add-replica-set-arbiter.txt
@@ -34,6 +34,11 @@ server or a monitoring host.
 Considerations
 --------------
 
+Primary-Secondary-Arbiter Replica Sets
+--------------------------------------
+
+.. include:: /includes/fact-psa-performance-issues.rst
+
 Replica Set Protocol Version
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
diff --git a/source/tutorial/mitigate-psa-performance-issues.txt b/source/tutorial/mitigate-psa-performance-issues.txt
@@ -0,0 +1,79 @@
+.. _performance-issues-psa:
+
+================================================
+Mitigate Performance Issues with PSA Replica Set
+================================================
+
+.. default-domain:: mongodb
+
+.. contents:: On this page
+   :local:
+   :backlinks: none
+   :depth: 1
+   :class: singlecol
+
+Overview
+--------
+
+In a three-member replica set with a primary-secondary-arbiter (PSA)
+architecture or a sharded cluster with three-member PSA shards, a
+data-bearing node that is down or lagged can lead to performance issues.
+
+If one data-bearing node goes down, the other node becomes the primary.
+Writes with :writeconcern:`w:1 <\<number\>>` continue to succeed in this
+state but writes with write concern :writeconcern:`"majority"` cannot
+succeed and the commit point starts to lag. If your PSA replica set
+contains a lagged secondary and your replica set requires two nodes to
+majority commit a change, your commit point also lags.
+
+With a lagged commit point, two things can affect your cluster
+performance:
+
+- The storage engine keeps **all** changes that happen after the commit
+  point on disk to retain a :term:`durable` history. The extra I/O from
+  these writes tends to increase over time. This can greatly impact
+  write performance and increase cache pressure.
+- MongoDB allows the :ref:`oplog <replica-set-oplog>` to grow past its
+  configured size limit to avoid deleting the :data:`majority commit
+  point <replSetGetStatus.optimes.lastCommittedOpTime>`.
+
+To reduce the cache pressure and increased write traffic, set
+:rsconf:`votes: 0 <members[n].votes>` and :rsconf:`priority: 0
+<members[n].priority>` for the node that is unavailable or lagging. For
+write operations issued with "majority", only voting members are
+considered to determine the number of nodes needed to perform a majority
+commit. Setting the configuration of the node to :rsconf:`votes: 0
+<members[n].votes>` reduces the number of nodes required to commit a
+write with write concern :writeconcern:`"majority"` from two to one and
+allows these writes to succeed.
+
+Once the secondary is caught up, you can use the
+:method:`rs.reconfigForPSASet()` method to set :rsconf:`votes
+<members[n].votes>` back to ``1``.
+
+.. note::
+
+   In earlier versions of MongoDB,
+   :setting:`~replication.enableMajorityReadConcern` and
+   :option:`--enableMajorityReadConcern` were configurable allowing you
+   to disable the default read concern :readconcern:`"majority"` which
+   had a similar effect.
+
+Procedure
+---------
+
+To reduce the cache pressure and increased write traffic for a
+deployment with a three-member primary-secondary-arbiter (PSA)
+architecture, set ``{ votes: 0, priority: 0 }`` for the secondary that
+is unavailable or lagging:
+
+.. code-block:: javascript
+
+   cfg = rs.conf();
+   cfg["members"][<array_index>]["votes"] = 0;
+   cfg["members"][<array_index>]["priority"] = 0;
+   rs.reconfig(cfg);
+
+
+If you want to change the configuration of the secondary later, use the
+:method:`rs.reconfigForPSASet()` method.

Original file line number	Diff line number	Diff line change
`@@ -1821,6 +1821,7 @@ raw: /manual/core/wildcard -> ${base}/manual/core/index-wildcard/`
`1821`	`1821`	`[v3.6-v4.4]: /${version}/core/sharding-change-shard-key-value.txt -> ${base}/${version}/core/sharding-shard-key.txt`
`1822`	`1822`	`[v3.6-v4.4]: /${version}/core/sharding-set-missing-shard-key-fields.txt -> ${base}/${version}/core/sharding-shard-key.txt`
`1823`	`1823`	`[v3.6-v4.4]: /${version}/core/sharding-find-shard-key.txt -> ${base}/${version}/core/sharding-shard-key.txt`
	`1824`	`+[v5.0-*]: /${version}/reference/read-concern-majority/#disable-read-concern-majority -> ${base}/${version}/reference/read-concern-majority/#primary-secondary-arbiter-replica-sets`
`1824`	`1825`
`1825`	`1826`	`#`
`1826`	`1827`	`# Upgrade / Downgrade redirects`