Slide 8 : Log Structured Store implementation
The log is divided up into relatively large 512k pieces (called segments). Reading and writing at this coarse granularity minimises disk head seeking overheads.
There are four background threads that take on responsibility for writing segments, flushing the log, check pointing the store (setting the point from which a recovery scan is required) and cleaning partially fill segments to avoid fragmentation.
The Recoverable Packet Map (RPM) is an 8 level hierarchical map keyed by 64 bit OID used to record the locations of blobs in the store. The Segment Utilisation Table (SUT) records the utilisation of every segment in the LSS. Both of these data structures are only updated on disk during a check point. Check points are performed after writing 64MB to the store.
