Affects Version/s: 2.0.0
Fix Version/s: None
A memstore serves as the in-memory component of a store unit, absorbing all updates to the store. From time to time these updates are flushed to a file on disk, where they are compacted (by eliminating redundancies) and compressed (i.e., written in a compressed format to reduce their storage size).
We aim to speed up data access, and therefore suggest to apply in-memory memstore flush. That is to flush the active in-memory segment into an intermediate buffer where it can be accessed by the application. Data in the buffer is subject to compaction and can be stored in any format that allows it to take up smaller space in RAM. The less space the buffer consumes the longer it can reside in memory before data is flushed to disk, resulting in better performance.
Specifically, the optimization is beneficial for workloads with medium-to-high key churn which incur many redundant cells, like persistent messaging.
We suggest to structure the solution as 4 subtasks (respectively, patches).
(1) Infrastructure - refactoring of the MemStore hierarchy, introducing segment (StoreSegment) as first-class citizen, and decoupling memstore scanner from the memstore implementation;
(2) Adding StoreServices facility at the region level to allow memstores update region counters and access region level synchronization mechanism;
(3) Implementation of a new memstore (CompactingMemstore) with non-optimized immutable segment representation, and
(4) Memory optimization including compressed format representation and off heap allocations.