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minor nits for week 2 overview

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Signed-off-by: Alex Chi <iskyzh@gmail.com>
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Alex Chi
2024-01-22 10:38:05 +08:00
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mini-lsm-book/src/week2-overview.md
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@@ -57,9 +57,9 @@ If the workload is like a time-series database, it is possible that the user alw
Compaction strategies usually aim to control the number of sorted runs, so as to keep read amplification in a reasonable amount of number. There are generally two categories of compaction strategies: leveled and tiered.
In leveled compaction, the user can specify a maximum number of levels, which is the number of sorted runs in the system (except L0). For example, RocksDB usually keeps 6 levels (sorted runs) in leveled compaction mode. During the compaction process, SSTs from two adjacent levels will be merged and then the produced SSTs will be put to the lower level of the two levels. The sorted runs (levels) grow exponentially in size -- the lower level will be < some number x > of the upper level in size.
In leveled compaction, the user can specify a maximum number of levels, which is the number of sorted runs in the system (except L0). For example, RocksDB usually keeps 6 levels (sorted runs) in leveled compaction mode. During the compaction process, SSTs from two adjacent levels will be merged and then the produced SSTs will be put to the lower level of the two levels. Therefore, you will usually see a small sorted run merged with a large sorted run in leveled compaction. The sorted runs (levels) grow exponentially in size -- the lower level will be < some number x > of the upper level in size.
In tiered compaction, the engine will dynamically adjust the number of sorted runs by merging them or letting new SSTs flushed as new sorted run (a tier) to minimize write amplification. The number of tiers can be high if the compaction strategy does not choose to merge tiers, therefore making read amplification high. In this tutorial, we will implement RocksDB's universal compaction, which is a kind of tiered compaction strategy.
In tiered compaction, the engine will dynamically adjust the number of sorted runs by merging them or letting new SSTs flushed as new sorted run (a tier) to minimize write amplification. In this strategy, you will usually see the engine merge two equally-sized sorted runs. The number of tiers can be high if the compaction strategy does not choose to merge tiers, therefore making read amplification high. In this tutorial, we will implement RocksDB's universal compaction, which is a kind of tiered compaction strategy.
## Space Amplification