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Add memtable section
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rfds/001_xks_storage_engine.md
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rfds/001_xks_storage_engine.md
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The `pair_count` records the number of pairs in the subtable,
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which is needed to binary search the slot array.
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## Memtables
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A memtable is an in-memory table used to buffer writes before they are merged with the on-disk tables.
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There is one active memtable, which accepts writes and maintains sorted order.
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Once the active memtable grows large enough (64 MiB), it is frozen and a new active memtable is created.
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Once enough memtables are accumulated (four), they are merged with the first level of the LSM on disk.
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A memtable is implemented as an `:ets` table with the following layout:
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```
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Key: {key, version}
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Value: value
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```
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The `key` and `value` are binaries, and the `version` is the integer database version.