[FEATURE] Add a buffer pool cache clear API for performance testing #148
Description
Is your feature request related to a problem?
Currently there isn't a quick way to flush the bufferpool cache to test cold path.
What solution would you like?
Have a debug API to clear the cache, similar to https://docs.opensearch.org/latest/api-reference/index-apis/clear-index-cache/. This doesn't need to be a cluster level request, primary performance testing will be done single a single node. If this command is run on coordinator, it will have no effect.
What alternatives have you considered?
Brute force way would be close and re-open an index, or worse restart the process. This will siginficantely increase the testing time.
Do you have any additional context?
Part of performance testing is to test cold path latency tests, i.e. search queries that request data that are not in bufferpool cache and thus issue IO requests. There are a number of related features in development that would help: IOUring, prefetch, async io, concurrent segment search, all of which to aim to increase the IO parallelism.