Feature #8674
Updated by Sage Weil almost 10 years ago
Earlier we proposed adding a new 'read-forward' mode that never promotes on reads (which may make sense for flash). Currently we promote into the cache on first read.
I suspect what will work best for most workloads is something in between, where we promote either on, say, the second read. That is easy: promote only if the object is in the current HitSet.
If we want to promote on 3rd or later read, then it's harder; we'd need to look at multiple HitSets, or make the HitSet a counting bloom filter (potentially useful), or something.
Either way, though, this should either be a tunable that controls the promotion behavior in 'writeback' or 'readonly' cache modes. Another pg_pool_t property?