Why does it work this way? Why is there a single WAL per RS? What is the point of recording the edits in a WAL rather than applying the updates in place?
The original motivation from a decade ago was that spinning disks were slow and SSDs were too expensive to be practical. In mechanical spinning disks, the seek costs were high, but if the disk head was already in the right position, writes could be performed with relatively low latency. In other words, the latency of an append operation is less than a random write. The lower latencies translate to higher throughput.
The LSM structure allows a sequence of random writes to be handled as append-only operations, as random writes are buffered in memory and then ...
