Summary:
## Problem Description
Our process was abort when it call `CheckConsistency`. And the information in `stderr` show that "`L0 files seqno 3001491972 3004797440 vs. 3002875611 3004524421` ". Here are the causes of the accident I investigated.
* RocksDB will call `CheckConsistency` whenever `MANIFEST` file is update. It will check sequence number interval of every file, except files which were ingested.
* When one file is ingested into RocksDB, it will be assigned the value of global sequence number, and the minimum and maximum seqno of this file are equal, which are both equal to global sequence number.
* `CheckConsistency` determines whether the file is ingested by whether the smallest and largest seqno of an sstable file are equal.
* If IntraL0Compaction picks one sst which was ingested just now and compacted it into another sst, the `smallest_seqno` of this new file will be smaller than his `largest_seqno`.
* If more than one ingested file was ingested before memtable schedule flush, and they all compact into one new sstable file by `IntraL0Compaction`. The sequence interval of this new file will be included in the interval of the memtable. So `CheckConsistency` will return a `Corruption`.
* If a sstable was ingested after the memtable was schedule to flush, which would assign a larger seqno to it than memtable. Then the file was compacted with other files (these files were all flushed before the memtable) in L0 into one file. This compaction start before the flush job of memtable start, but completed after the flush job finish. So this new file produced by the compaction (we call it s1) would have a larger interval of sequence number than the file produced by flush (we call it s2). **But there was still some data in s1 written into RocksDB before the s2, so it's possible that some data in s2 was cover by old data in s1.** Of course, it would also make a `Corruption` because of overlap of seqno. There is the relationship of the files:
> s1.smallest_seqno < s2.smallest_seqno < s2.largest_seqno < s1.largest_seqno
So I skip pick sst file which was ingested in function `FindIntraL0Compaction `
## Reason
Here is my bug report: https://github.com/facebook/rocksdb/issues/5913
There are two situations that can cause the check to fail.
### First situation：
- First we ingest five external sst into Rocksdb, and they happened to be ingested in L0. and there had been some data in memtable, which make the smallest sequence number of memtable is less than which of sst that we ingest.
- If there had been one compaction job which compacted sst from L0 to L1, `LevelCompactionPicker` would trigger a `IntraL0Compaction` which would compact this five sst from L0 to L0. We call this sst A, which was merged from five ingested sst.
- Then some data was put into memtable, and memtable was flushed to L0. We called this sst B.
- RocksDB check consistency , and find the `smallest_seqno` of B is less than that of A and crash. Because A was merged from five sst, the smallest sequence number of it was less than the biggest sequece number of itself, so RocksDB could not tell if A was produce by ingested.
### Secondary situaion
- First we have flushed many sst in L0, we call them [s1, s2, s3].
- There is an immutable memtable request to be flushed, but because flush thread is busy, so it has not been picked. we call it m1. And at the moment, one sst is ingested into L0. We call it s4. Because s4 is ingested after m1 became immutable memtable, so it has a larger log sequence number than m1.
- m1 is flushed in L0. because it is small, this flush job finish quickly. we call it s5.
- [s1, s2, s3, s4] are compacted into one sst to L0, by IntraL0Compaction. We call it s6.
- compacted 4@0 files to L0
- When s6 is added into manifest, the corruption happened. because the largest sequence number of s6 is equal to s4, and they are both larger than that of s5. But because s1 is older than m1, so the smallest sequence number of s6 is smaller than that of s5.
- s6.smallest_seqno < s5.smallest_seqno < s5.largest_seqno < s6.largest_seqno
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5958
Differential Revision: D18601316
fbshipit-source-id: 5fe54b3c9af52a2e1400728f565e895cde1c7267
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