Using Command-Line Flags

1. Setting Configuration File

Flag	Description	Default Value	Example
--config.file	Path to the configuration file	`prometheus.yml`	`--config.file=/etc/prometheus/prometheus.yml`
--config.file.sd	Configuration file service discovery	`false`	`--config.file.sd=true`

Example: Setting Configuration File

prometheus --config.file=/etc/prometheus/prometheus.yml

2. Configuring Storage Path

Flag	Description	Default Value	Notes
--storage.tsdb.path	Directory to store the TSDB (Time Series Database) data	`data/`	Must be writable by the Prometheus process
--storage.tsdb.retention.time	How long to retain samples in the database	`15d`	Can be specified in days (d), hours (h), minutes (m), or seconds (s)
--storage.tsdb.retention.size	Maximum size of the data directory (in bytes or with unit suffixes like GB, TB)	`0`	Zero means unlimited. When set, Prometheus will delete oldest data to stay under the limit

Example: Setting Storage Path and Retention

prometheus --storage.tsdb.path=/var/lib/prometheus/data \
           --storage.tsdb.retention.time=30d

Note: The data/ directory is used by default if no path is specified. Ensure sufficient disk space and proper permissions for the storage directory.

3. Setting Retention Time

Flag	Description	Default Value	Notes
--storage.tsdb.retention.time	How long to retain samples in storage	`15d`	Can be specified in days (d), hours (h), minutes (m), or seconds (s)
--storage.tsdb.retention.size	Maximum number of bytes to store in storage	`0`	Disabled by default. When set, overrides time-based retention

Example: Setting 30-day retention

prometheus --storage.tsdb.retention.time=30d

Example: Setting 50GB size limit

prometheus --storage.tsdb.retention.size=50GB

Note: If both --storage.tsdb.retention.time and --storage.tsdb.retention.size are set, Prometheus will use the more restrictive of the two limits.

4. Configuring Retention Size

Flag	Description	Default Value	Notes
--storage.tsdb.retention.size	Maximum number of bytes to retain in storage	Unlimited	Can be specified with units (e.g., 1GB, 2TB)
--storage.tsdb.retention.time	Maximum time to retain samples in storage	15d	Overrides retention size if both are set

Example: Setting retention size to 50GB

prometheus --storage.tsdb.retention.size=50GB

Note: Setting --storage.tsdb.retention.size will limit the size of the storage directory, and Prometheus will remove old data to stay within this limit.

5. Setting Web Listen Address

Flag	Description	Default Value	Example
--web.listen-address	Address to listen on for web interface and telemetry	`:9090`	`--web.listen-address=:8080`

Example: Change Web Listen Address

prometheus --web.listen-address=:8080

Note: The address must be in the format <host>:<port>. If host is omitted, it defaults to all interfaces.

6. Configuring External URL

Flag	Description	Default Value	Example
--web.external-url	Sets the URL under which Prometheus is externally reachable	Empty string (not set)	`--web.external-url=https://prometheus.example.com`

Example: Setting External URL

prometheus --web.external-url=https://prometheus.example.com

Note: The --web.external-url flag is used to set the URL under which Prometheus is externally reachable, useful for reverse proxies or when Prometheus is behind a load balancer.

7. Enabling Feature Flags

Flag	Description	Default	Status
--enable-feature	Enables experimental features	None	EXPERIMENTAL
--enable-feature=exemplar-storage	Enables exemplar storage support	false	NEW
--enable-feature=remote-write-receiver	Enables remote write receiver capability	false	NEW

Example: Enabling Feature Flags

prometheus --enable-feature=exemplar-storage --enable-feature=remote-write-receiver

Note: Feature flags are intended for experimental use and may change or be removed in future versions.

8. Setting Query Timeout

Flag	Description	Default Value	Notes
--query.timeout	Maximum time a query is allowed to run	`2m`	Timeout for query execution

Example: Setting Query Timeout

prometheus --query.timeout=5m

Note: Setting a query timeout prevents long-running queries from consuming excessive resources and affecting system performance.

9. Configuring Max Samples

Flag	Description	Default Value	Notes
--storage.tsdb.max-samples-per-chunk	Maximum number of samples per chunk	`120`	Controls the size of chunks in the time series database
--storage.tsdb.max-chunks-per-raw-samples	Maximum number of chunks per raw sample	`1000`	Used to limit the number of chunks generated per sample
--storage.tsdb.max-chunk-size	Maximum size of a chunk in bytes	`1048576`	Defines the maximum size of a chunk in the database

Example: Setting Max Samples Per Chunk

prometheus --storage.tsdb.max-samples-per-chunk=240

10. Setting Log Level

Flag	Description	Default	Values
--log.level	Set the log level for the Prometheus server	`info`	`debug`, `info`, `warn`, `error`

Example: Setting Log Level to Debug

prometheus --log.level=debug

11. Configuring WAL Compression

Flag	Description	Default Value	Notes
--storage.tsdb.wal-compression	Enables or disables WAL compression	`false`	Set to `true` to enable compression using Snappy
--storage.tsdb.wal-compression-level	Compression level for WAL (1-9)	`4`	Higher levels = better compression but more CPU usage

Example: Enabling WAL Compression

prometheus --storage.tsdb.wal-compression=true --storage.tsdb.wal-compression-level=6

Note: WAL compression can reduce disk I/O and storage usage, but may increase CPU overhead during writes.

12. Setting Max Block Duration

Flag	Description	Default Value	Notes
--storage.tsdb.max-block-duration	Maximum duration a block can last before being closed and persisted to disk.	`2h`	Used to control the size and retention of time series blocks in the TSDB.

Example: Setting Max Block Duration

prometheus --storage.tsdb.max-block-duration=3h

Note: Setting this value too low may result in excessive block creation, leading to increased I/O and storage overhead. Setting it too high can lead to larger blocks that are harder to manage or query efficiently.