In previous parts, we assumed that newer keys are in the upper level of the LSM tree, and older keys are in the lower level of the LSM tree. During compaction, we only keep the latest version of a key if multiple versions are found in multiple levels, and the compaction process will ensure that newer keys will be kept on the upper level by only merging adjacent levels/tiers. In the MVCC implementation, the key with a larger timestamp is the newest key. During compaction, we can only remove the key if no user is accessing an older version of the database. Though not keeping the latest version of key in the upper level may still yield a correct result for the MVCC LSM implementation, in our tutorial, we choose to keep the invariant, and if there are multiple versions of a key, a later version will always appear in a upper level.
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