Caching

Loki supports caching of index writes and lookups, chunks and query results to speed up query performance. This sections describes the recommended Memcached configuration to enable caching for chunks and query results. The index lookup cache is configured to be in-memory by default.

Before you begin

It is recommended to deploy three, or in the case of the Helm chart two, dedicated Memcached clusters.
As of 2023-02-01, the memcached:1.6.17-alpine version of the library is recommended.
Consult the Loki ksonnet memcached deployment and the ksonnet memcached library.

Steps

To enable and configure Memcached:

Deploy each Memcached service with at least three replicas and configure each as follows:
1. Chunk cache
```
--memory-limit=4096 --max-item-size=2m --conn-limit=1024
```
2. Query result and index queries cache
```
--memory-limit=1024 --max-item-size=5m --conn-limit=1024
```

Configure Loki to use the cache.

If the Helm chart is used
Set memcached.chunk_cache.host to the Memcached address for the chunk cache, memcached.results_cache.host to the Memcached address for the query result cache, memcached.chunk_cache.enabled=true and memcached.results_cache.enabled=true.
Ensure that the connection limit of Memcached is at least number_of_clients * max_idle_conns.
The options host and service depend on the type of installation. For example, using the bitnami/memcached Helm Charts with the following commands, the service values are always memcached.
```
helm upgrade --install chunk-cache -n loki bitnami/memcached -f memcached-overrides-chunk.yaml
helm upgrade --install results-cache -n loki bitnami/memcached -f memcached-overrides-results.yaml
```
The current Helm Chart only supports the chunk and results cache.
In this case, the Loki configuration would be
yaml
```
loki:
  memcached:
    chunk_cache:
      enabled: true
      host: chunk-cache-memcached.loki.svc
      service: memcached-client
      batch_size: 256
      parallelism: 10
    results_cache:
      enabled: true
      host: results-cache-memcached.loki.svc
      service: memcached-client
      default_validity: 12h
```

If the Loki configuration is used, modify the following three sections in the Loki configuration file.

Configure the chunk and index write cache

chunk_store_config:
  chunk_cache_config:
    memcached:
      batch_size: 256
      parallelism: 10
    memcached_client:
      host: <chunk cache memcached host>
      service: <port name of memcached service>

Configure the query result cache

query_range:
  cache_results: true
  results_cache:
    cache:
      memcached_client:
        consistent_hash: true
        host: <memcached host>
        service: <port name of memcached service>
        max_idle_conns: 16
        timeout: 500ms
        update_interval: 1m

Configure the index queries cache

storage_config:
  index_queries_cache_config:
    memcached:
      batch_size: 100
      parallelism: 100
    memcached_client:
      host: <memcached host>
      service: <port name of memcached service>
      consistent_hash: true