Originally had disabled the ability to lookup individual values when running in head_only mode. This is a saving of about 11% at PUT time (about 3 microseconds per PUT) on a macbook.
Not sure this saving is sufficient enought to justify the extra work if this is used as an AAE Keystore with Bitcask and LWW (when we need to lookup the current value before adjusting).
So reverted to re-adding support for HEAD requests with these keys.
Initial commit to add head_only mode to leveled. This allows leveled to receive batches of object changes, but where those objects exist only in the Penciller's Ledger (once they have been persisted within the Ledger).
The aim is to reduce significantly the cost of compaction. Also, the objects ar enot directly accessible (they can only be accessed through folds). Again this makes life easier during merging in the LSM trees (as no bloom filters have to be created).
Previously done at Slot Level - but Blocks were still read from disk after the Slot CRC had been checked.
This seems safer. It requires an extra CRC check for every fetch. However, CRC chekcing smaller binaries during the buld process appears to be beneficial to performance.
Hoped this will be an enabler to turning off compression at Levels 0 and 1 to improve performance (wihtout having a compensating issues with reduced CRC performance)
Previouslythe tinybloom was used within the SST file as an extra check to remove false fetches.
However the SST already has a low FPR check in the slot_index. If the newebloom was used (which is no longer per slot, but per sst), this can be shared with the penciller and then the penciller could use it and avoid the message pass.
the message pass may be blocked by a 2i query or a slot fetch request for a merge. So this should make performance within the Penciller snappier.
This is as a result of taking sst_timings within a volume test - where there was an average of + 100microsecs for each level that was dropped down. Given the bloom/slot checks were < 20 microsecs - there seems to be some further delay.
The bloom is a binary of > 64 bytes - so passing it around should not require a copy.
Bloom filter that can take largere keys but is still efficient to build. Allows bloom filter to be checked without first detemrining the slot. Also, as represents the whole SST - it could be sent to the penciller to remove the need for a message pass.
The bloom is smaller and has a worse fpr than leveled_tinybloom. Failing the bloom check isn't so bad - due to the slot index check being relatively fast and having a very low fpr.
