An LSM tree first puts writes in an in-memory blank, flushes it to disk as immutable sorted files called blank, and merges those files in the background, which is called blank.

Answer

An LSM tree first puts writes in an in-memory memtable, flushes it to disk as immutable sorted files called SSTables, and merges those files in the background, which is called compaction.

Also accepted: SSTable, sorted string tables for SSTables.

Why

A write-ahead log alongside the memtable keeps writes safe until the flush.

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