store: Add LRU caching for posting offset / adaptive postingOffsetInMemSampling based on traffic

[bwplotka]

After we fixed postingOffset sampling which was broken (we were keeping in memory all of them, instead of 1/32 as we intended), some of the users see extreme improvement:

This means that the about of postings is extreme in some setups, so what we keep in memory in terms of posting offsets really matters. This means we might need some caching for posting offsets as well. While we store them on disk, still disk lookup might be slower than in memory. I doubt there would be a benefit to storing this in remote cache though.

cc @d-ulyanov @mkabischev

[stale]

Hello 👋 Looks like there was no activity on this issue for last 30 days.
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[bwplotka]

This needs like some very advanced logic, which is not needed, yet. Closing for now.