docs: add recovery mode for leveled compaction (#86)

Signed-off-by: Alex Chi <iskyzh@gmail.com>

mini-lsm-book/src/week2-05-manifest.md

@@ -75,6 +75,10 @@ src/lsm_storage.rs
 
 Now, you may modify the `open` function to recover the engine state from the manifest file. To recover it, you will need to first generate the list of SSTs you will need to load. You can do this by calling `apply_compaction_result` and recover SST ids in the LSM state. After that, you may iterate the state and load all SSTs (update the sstables hash map). During the process, you will need to compute the maximum SST id and update the `next_sst_id` field. After that, you may create a new memtable using that id and increment the id by one.
 
+If you have implemented leveled compaction, you might have sorted the SSTs every time you apply the compaction result. However, with manifest recover, your sorting logic will be broken, because during the recovery process, you cannot know the start key and the end key of each of the SST. To resolve this, you will need to read the `in_recovery` flag of the `apply_compaction_result` function. During the recovery process, you should not attempt to retrieve the first key of the SST. After the LSM state is recovered and all SSTs are opened, you can do a sorting at the end of the recovery process.
+
+Optionally, you may include the start key and the end key of each of the SSTs in the manifest. This strategy is used in RocksDB/BadgerDB, so that you do not need to distinguish the recovery mode and the normal mode during the compaction apply process.
+
 You may use the mini-lsm-cli to test your implementation.
 
 ```