How snapshots retain old data?
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Description
I am learning leveldb and have some questions about snapshots. The main problem is how to retain the old data that snapshots depend on.
1. I found that the relevant code of the snapshot has nothing to do with the version. It seems that the reference count of a version will not be increased after the snapshot is generated.
2. So I think function DoCompactionWork performed this operation.
Suppose I perform the following operations
```
Put(key1) sequence == 10
Put(key1) sequence == 20
GetSnapshot() sequence == 25
Put(key1) sequence == 30
```
When executing DoCompactionwork, according to the code, is the first key
then `if (last_sequence_for_key <= compact->smallest_snapshot)` looks like to drop ，。
This is wrong with my understanding，I think should be reserved in new Version，Can someone help me answer them?
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