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Automatic stream sharding

Automatic stream sharding will attempt to keep streams under a desired_rate by adding new labels and values to existing streams. When properly tuned, this should eliminate issues where log producers are rate limited due to the per-stream rate limit.

To enable automatic stream sharding:

  1. Edit the global limits_config of the Loki configuration file:
          enabled: true
  2. Optionally lower the desired_rate in bytes if you find that the system is still hitting the per_stream_rate_limit:
        enabled: true
        desired_rate: 2097152 #2MiB
  3. Optionally enable logging_enabled for debugging stream sharding. Note: this may affect the ingestion performance of Loki.
        enabled: true
        logging_enabled: true

When to use automatic stream sharding

Large log streams present several problems for Loki, namely increased and uneven resource usage on Ingesters and Distributors. The general recommendation is to explore existing log streams for additional label values that are both useful for querying and sufficiently low cardinality. There are many cases, however, where no more labels can be extracted, or cardinality for a label is dangerously large. To protect itself from such volume leading to operational failure, Loki implements per-stream rate limits; but the result is that some data is lost. The per-stream limit also needs human intervention to change, which is not ideal when log volumes increase and decrease.

Loki uses automatic stream sharding to avoid rate limiting and large streams for any log stream by ensuring it is close to a configured desired_rate.

How automatic stream sharding works

Automatic stream sharding works by adding a new label, __stream_shard__, to streams and incrementing its value to try and keep all streams below a configured desired_rate.

The feature adds a new API to Ingesters that reports the size of all existing log streams. Once per second, Distributors query the API to get a picture of all stream rates in the system. Distributors use the existing stream-rate data and a configured desired_rate to determine how many shards a given stream should have. The desired number of new log streams are created with the label __stream_shard__ and logs are divided evenly among the streams.

Because automatic stream sharding is reactive and relies on successive calls to Ingesters, the view of current rates is always somewhat behind. As a result, the actual size of sharded streams will always be higher than the desired_rate. In practice, this is still sufficient to keep log producers from being rate limited by per-stream rate limits.

Automatic stream sharding metrics

Use these metrics to help tune Loki so that it is sharding streams aggressively enough to avoid the per-stream rate limit:

  • loki_rate_store_refresh_failures_total: The total number of failed attempts to refresh the distributor’s view of stream rates.
  • loki_rate_store_streams: The number of unique streams reported by all Ingesters. Sharded streams are reported as if they were unsharded.
  • loki_rate_store_max_stream_shards: The maximum number of shards for any tenant of the system.
  • loki_rate_store_stream_shards: A histogram of the distribution of shard counts across all streams.
  • loki_rate_store_max_stream_rate_bytes: The maximum stream size in bytes/second for any tenant of the system. Sharded streams are reported as if they are unsharded.
  • loki_rate_store_max_unique_stream_rate_bytes: The maximum size of any stream across all tenants. Stream shards are individually reported.
  • loki_rate_store_stream_rate_bytes: A histogram of the distribution of stream sizes across all tenants in bytes/second.
  • loki_stream_sharding_count: The total number of times that streams have been sharded. Useful for calculating the sharding rate.