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Retention

Retention in Grafana Loki is achieved either through the Table Manager or the Compactor.

By default, when table_manager.retention_deletes_enabled or compactor.retention_enabled flags are not set, then logs sent to Loki live forever.

Retention through the Table Manager is achieved by relying on the object store TTL feature, and will work for both boltdb-shipper store and chunk/index store. However retention through the Compactor is supported only with the boltdb-shipper and tsdb store.

The Compactor retention will become the default and have long term support. It supports more granular retention policies on per tenant and per stream use cases.

Compactor

The Compactor can deduplicate index entries. It can also apply granular retention. When applying retention with the Compactor, the Table Manager is unnecessary.

Run the Compactor as a singleton (a single instance).

Compaction and retention are idempotent. If the Compactor restarts, it will continue from where it left off.

The Compactor loops to apply compaction and retention at every compaction_interval, or as soon as possible if running behind.

The Compactor’s algorithm to update the index:

  • For each table within each day:
    • Compact the table into a single index file.
    • Traverse the entire index. Use the tenant configuration to identify and mark chunks that need to be removed.
    • Remove marked chunks from the index and save their reference in a file on disk.
    • Upload the new modified index files.

The retention algorithm is applied to the index. Chunks are not deleted while applying the retention algorithm. The chunks will be deleted by the Compactor asynchronously when swept.

Marked chunks will only be deleted after retention_delete_delay configured is expired because:

  • boltdb-shipper indexes are refreshed from the shared store on components using it (querier and ruler) at a specific interval. This means deleting chunks instantly could lead to components still having reference to old chunks and so they could fails to execute queries. Having a delay allows for components to refresh their store and so remove gracefully their reference of those chunks.

  • It provides a short window of time in which to cancel chunk deletion in the case of a configuration mistake.

Marker files (containing chunks to delete) should be stored on a persistent disk, since the disk will be the sole reference to them.

Retention Configuration

This Compactor configuration example activates retention.

yaml 
compactor:
  working_directory: /data/retention
  shared_store: gcs
  compaction_interval: 10m
  retention_enabled: true
  retention_delete_delay: 2h
  retention_delete_worker_count: 150
schema_config:
    configs:
      - from: "2020-07-31"
        index:
            period: 24h
            prefix: loki_index_
        object_store: gcs
        schema: v11
        store: boltdb-shipper
storage_config:
    boltdb_shipper:
        active_index_directory: /data/index
        cache_location: /data/boltdb-cache
        shared_store: gcs
    gcs:
        bucket_name: loki

Note that retention is only available if the index period is 24h.

Set retention_enabled to true. Without this, the Compactor will only compact tables.

Define schema_config and storage_config to access the storage.

The index period must be 24h.

working_directory is the directory where marked chunks and temporary tables will be saved.

compaction_interval dictates how often compaction and/or retention is applied. If the Compactor falls behind, compaction and/or retention occur as soon as possible.

retention_delete_delay is the delay after which the Compactor will delete marked chunks.

retention_delete_worker_count specifies the maximum quantity of goroutine workers instantiated to delete chunks.

Configuring the retention period

Retention period is configured within the limits_config configuration section.

There are two ways of setting retention policies:

  • retention_period which is applied globally.
  • retention_stream which is only applied to chunks matching the selector

The minimum retention period is 24h.

This example configures global retention:

yaml 
...
limits_config:
  retention_period: 744h
  retention_stream:
  - selector: '{namespace="dev"}'
    priority: 1
    period: 24h
  per_tenant_override_config: /etc/overrides.yaml
...

NOTE: You can only use label matchers in the selector field of a retention_stream definition. Arbitrary LogQL expressions are not supported.

Per tenant retention can be defined using the /etc/overrides.yaml files. For example:

yaml 
overrides:
    "29":
        retention_period: 168h
        retention_stream:
        - selector: '{namespace="prod", container=~"(nginx|loki)"}'
          priority: 2
          period: 336h
        - selector: '{container="loki"}'
          priority: 1
          period: 72h
    "30":
        retention_stream:
        - selector: '{container="nginx", level="debug"}'
          priority: 1
          period: 24h

A rule to apply is selected by choosing the first in this list that matches:

  1. If a per-tenant retention_stream matches the current stream, the highest priority is picked.
  2. If a global retention_stream matches the current stream, the highest priority is picked.
  3. If a per-tenant retention_period is specified, it will be applied.
  4. The global retention_period will be selected if nothing else matched.
  5. If no global retention_period is specified, the default value of 744h (30days) retention is used.

Stream matching uses the same syntax as Prometheus label matching:

  • =: Select labels that are exactly equal to the provided string.
  • !=: Select labels that are not equal to the provided string.
  • =~: Select labels that regex-match the provided string.
  • !~: Select labels that do not regex-match the provided string.

The example configurations will set these rules:

  • All tenants except 29 and 30 in the dev namespace will have a retention period of 24h hours.
  • All tenants except 29 and 30 that are not in the dev namespace will have the retention period of 744h.
  • For tenant 29:
    • All streams except those in the container loki or in the namespace prod will have retention period of 168h (1 week).
    • All streams in the prod namespace will have a retention period of 336h (2 weeks), even if the container label is loki, since the priority of the prod rule is higher.
    • Streams that have the container label loki but are not in the namespace prod will have a 72h retention period.
  • For tenant 30:
    • All streams except those having the container label nginx will have the global retention period of 744h, since there is no override specified.
    • Streams that have the label nginx will have a retention period of 24h.

Table Manager

In order to enable the retention support, the Table Manager needs to be configured to enable deletions and a retention period. Please refer to the table_manager section of the Loki configuration reference for all available options. Alternatively, the table-manager.retention-period and table-manager.retention-deletes-enabled command line flags can be used. The provided retention period needs to be a duration represented as a string that can be parsed using the Prometheus common model ParseDuration. Examples: 7d, 1w, 168h.

WARNING: The retention period must be a multiple of the index and chunks table period, configured in the period_config block. See the Table Manager documentation for more information.

NOTE: To avoid querying of data beyond the retention period, max_look_back_period config in chunk_store_config must be set to a value less than or equal to what is set in table_manager.retention_period.

When using S3 or GCS, the bucket storing the chunks needs to have the expiry policy set correctly. For more details check S3’s documentation or GCS’s documentation.

Currently, the retention policy can only be set globally. A per-tenant retention policy with an API to delete ingested logs is still under development.

Since a design goal of Loki is to make storing logs cheap, a volume-based deletion API is deprioritized. Until this feature is released, if you suddenly must delete ingested logs, you can delete old chunks in your object store. Note, however, that this only deletes the log content and keeps the label index intact; you will still be able to see related labels but will be unable to retrieve the deleted log content.

For further details on the Table Manager internals, refer to the Table Manager documentation.

Example Configuration

Example configuration with GCS with a 28 day retention:

yaml 
schema_config:
  configs:
  - from: 2018-04-15
    store: bigtable
    object_store: gcs
    schema: v11
    index:
      prefix: loki_index_
      period: 168h

storage_config:
  bigtable:
    instance: BIGTABLE_INSTANCE
    project: BIGTABLE_PROJECT
  gcs:
    bucket_name: GCS_BUCKET_NAME

chunk_store_config:
  max_look_back_period: 672h

table_manager:
  retention_deletes_enabled: true
  retention_period: 672h