Check WiredTiger cache size

Description

This advisor warns if the configured WiredTiger cache size is greater than 50% of server memory.

This is important because MongoDB uses the remaining available memory (filesystem cache) beyond its own storage engine cache to maintain connections, aggregation/sort operations, cursors, etc.

Keeping a non-default high cache size for WiredTiger can cause OOM (Out of memory) issues.

To avoid performance issues, see the WiredTiger Storage Engine blog to check and set the right value for your environment.

Rule

METRICS_INSTANT
# to fetch the value
node_memory_numa_MemTotal{node_name="{{.NodeName}}"}

METRICS_INSTANT
# to fetch the value
mongodb_ss_wt_cache_maximum_bytes_configured{service_name="{{.ServiceName}}"}            

Resolution

• Let WiredTiger use the default cache size of 50% of (RAM - 1 GB), OR
• Make sure that the storage.wiredTiger.engineConfig.cacheSizeGB value in the configuration file doesn’t exceed 50% of the server memory.

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