Does Mongo cache the whole index or just the parts that it needs to service a Mongo Query?

[Gregory_Chavez]

@John_Cantu

If I have an index that is sized at 1 GB and have the WiredTiger cache set to 100 GB. Then, I perform that very first query that uses the 1GB index, but the query only returns back one row. Will Mongo store the whole 1 GB index in cache or just the parts that it needed to service that query?

Thank you,

John

Hello, @John_Cantu

When using the WiredTiger storage engine in MongoDB, the cache behavior is interesting. Here’s how it works:

Cache Allocation:
WiredTiger doesn’t reserve separate portions of the cache for reads and writes.
Instead, it allocates its cache to the entire mongod instance.
There’s no per-database or per-collection cache allocation.
Index Caching:
When you perform a query, WiredTiger prioritizes index caching.
So, even if your query only returns one row, it will still load the relevant index pages into the cache.
However, it won’t necessarily load the entire 1 GB index; it’ll focus on the parts needed for the query.
Heavy Write Workload:
If there’s a heavy write workload, it can affect performance.
But WiredTiger still tries to keep index pages in cache for efficient querying.

I hope this info is helpful to you.

Best Regard,
Gregory Chavez