set-parameter.md

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:

=== ":octicons-file-code-24: Configuration file"

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

```yaml
setParameter:
  <parameter>: <value>
```
  
??? example "Example: diagnosticDataCollectionEnableSystemMetricsDisks set to true"
    ```yaml
    setParameter:
      diagnosticDataCollectionEnableSystemMetricsDisks: true
    ```

??? example "Example: diagnosticDataCollectionEnableSystemMetricsMounts set to false"
    ```yaml
    setParameter:
      diagnosticDataCollectionEnableSystemMetricsMounts: false
    ```

=== ":material-console: Command line"

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

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

??? example "Example: diagnosticDataCollectionEnableSystemMetricsDisks set to false"
    ```bash
    mongod \
      --setParameter diagnosticDataCollectionEnableSystemMetricsDisks=false
    ```

??? example "Example: diagnosticDataCollectionEnableSystemMetricsMounts set to true"
    ```bash
    mongod \
      --setParameter diagnosticDataCollectionEnableSystemMetricsMounts=true
    ```

=== ":simple-mongodb: setParameter command"

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

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

??? example "Example: diagnosticDataCollectionEnableSystemMetricsDisks set to true"
    ```javascript
    db.adminCommand({setParameter: 1, diagnosticDataCollectionEnableSystemMetricsDisks: true})
    ```

??? example "Example: diagnosticDataCollectionEnableSystemMetricsMounts set to false"
    ```javascript
    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.