Percona Server for MongoDB parameter tuning guide

Disable FTDC metric groups

Overview

FTDC collects diagnostic samples such as serverStatus, replSetGetStatus, and Operating System level systemMetrics. By default, systemMetrics are collected in all environments. This can create significant noise and overhead, as disk and mount statistics are gathered from every available mount point.

When using FUSE, autofs, or NFS, reading disk stats from an unresponsive mount may cause the FTDC thread to enter an uninterruptible sleep (D-state), halting all FTDC sampling until the node is restarted. To prevent this issue, you can disable specific subsections of systemMetrics while still collecting all other essential metrics.

Parameters

Two new server parameters control the collection of disks and mounts subsections within systemMetrics in FTDC:

• diagnosticDataCollectionEnableSystemMetricsDisks

  • Enables or disables collection of disk-level statistics.
  • Type: Boolean (true/false)
  • Default: true (enabled)
  • Scope: Startup; runtime configurable via setParameter

• diagnosticDataCollectionEnableSystemMetricsMounts

  • Enables or disables collection of mount-level statistics.
  • Type: Boolean (true/false)
  • Default: true (enabled)
  • Scope: Startup; runtime configurable via setParameter

Configuration methods

You can modify parameters in Percona Server for MongoDB using the following methods:

Configuration file Command line setParameter command

Use the setParameter admonitions in the configuration file for persistent changes in production:

setParameter:
  <parameter>: <value>

Example: diagnosticDataCollectionEnableSystemMetricsDisks set to true

setParameter:
  diagnosticDataCollectionEnableSystemMetricsDisks: true

Example: diagnosticDataCollectionEnableSystemMetricsMounts set to false

setParameter:
  diagnosticDataCollectionEnableSystemMetricsMounts: false

Use the --setParameter command line option arguments when running the mongod process for development or testing purposes:

mongod \
  --setParameter <parameter>=<value>\

Example: diagnosticDataCollectionEnableSystemMetricsDisks set to false

mongod \
  --setParameter diagnosticDataCollectionEnableSystemMetricsDisks=false

Example: diagnosticDataCollectionEnableSystemMetricsMounts set to true

mongod \
  --setParameter diagnosticDataCollectionEnableSystemMetricsMounts=true

Use the setParameter command on the admin database to make changes at runtime:

db.adminCommand( { setParameter: 1, <parameter>: <value> } )

Example: diagnosticDataCollectionEnableSystemMetricsDisks set to true

db.adminCommand({setParameter: 1, diagnosticDataCollectionEnableSystemMetricsDisks: true})

Example: diagnosticDataCollectionEnableSystemMetricsMounts set to false

db.adminCommand({setParameter: 1, diagnosticDataCollectionEnableSystemMetricsMounts: false})

See what parameters you can define in the parameters list.

Comparison: Default vs. tuned FTDC behavior

Mode	Metrics Collected	Risks / Overhead
Default (no tuning)	systemMetrics (disks and mounts), serverStatus.connections, replSetGetStatus, plus all other FTDC groups	Provides full visibility, but may lead to increased noise and overhead. FTDC can become unresponsive in FUSE, autofs, or NFS environments.
Disable disks only	All FTDC groups, with systemMetrics excluding disk-level stats	Reduces overhead while retaining mount-level visibility.
Disable mounts only	All FTDC groups, with systemMetrics excluding mount-level stats	Avoids interruptions from unresponsive mounts while continuing retaining disk-level visibility.