RocksDB is a new storage engine for MySQL that provides better storage efficiency than InnoDB. It achieves lower space amplification and write amplification than InnoDB through its use of compression and log-structured merge trees. While MyRocks (RocksDB integrated with MySQL) currently has some limitations like a lack of support for online DDL and spatial indexes, work is ongoing to address these limitations and integrate additional RocksDB features to fully support MySQL workloads. Testing at Facebook showed MyRocks uses less disk space and performs comparably to InnoDB for their queries.

1. MySQL + RocksDB
Better Storage Efficiency Than InnoDB
Feb 3, 2016
Siying Dong, Software Engineer
Database Engineering Team @ Facebook

2. Why Another MySQL Storage
Engine ?

3. Facebook Website Architecture
MySQL
Databases
Web servers
Data center
Caches

4. Facebook Website Architecture
MySQL
Databases
Web servers
Data center
Caches
SSD

5. Facebook Website Architecture
▪ What do the system metrics
look like?
MySQL
Databases
Web servers
Data center
Caches
SSD

6. Measurements of MySQL Hosts’ Actual
Resource Usages
▪ Read IOPS: < 5%
▪ Write IOPS: < 5%
▪ Peak Write Bandwidth: < 15%
▪ CPU: < 20%
▪ Write Endurance: last more than 3 years.
▪ Space is the bottleneck

7. RocksDB Storage Engine in
MySQL(MyRocks)
▪ https://github.com/MySQLOnRocksDB/mysql-5.6

8. RocksDB vs. InnoDB

9. DB Size Comparison
0
0.2
0.4
0.6
0.8
1
1.2
InnoDB RocksDB
DB Size (Relative)

10. Write Amplification Comparison
0
0.2
0.4
0.6
0.8
1
1.2
InnoDB RocksDB
Bytes Written (Relative)

11. Why is RocksDB better?

12. Lower Space Amplification
Row
Row
Row
Wasted
Row
Row
Row
Wasted
Using 8KB space
on storage
Waste
4KB 4KB
Compressed to 5KB
Uncompressed 16KB
page
Row
Row
Row
Wasted
Level 1
Level 2
Level 3
Level 4
Target 1GB
Target 10 GB
Target 100 GB
Target 1000 GB
stale
Level 0
stale
stale
stale
stale
stale

13. Lower Write Amplification (Worst Case)
Row
Read
Modify
Write
Row
Row
Row
Row
Row
Row
Row
Row
Row
Write Amp = Page size / row size
Level 1
Level 2
Level 3
Level 4
Target 1GB
Target 10 GB
Target 100 GB
Target 1000 GB
Level 0
Merge
Merge
Merge
Merge
flush
Write amp 1
Write Amp 10
Write Amp 10
Write Amp 10
Write Amp 10

14. How about query
performance?

15. Queries Per Second (LinkBench)

16. Reads Per Query (LinkBench)

17. New RocksDB Features to
Support MySQL

18. Transactions
▪ Optimistic and Pessimistic
▪ MyRocks Uses Pessimistic

19. “Single Delete”
▪ Secondary index is insert/delete only, never upate
▪ Updating a column in secondary key generates one delete + one insert
▪ We can drop “single delete” tombstone as soon as it meets a value

20. Delete Files in Range
▪ Drop Table/Index Needs to claim space fast

21. Delete Files in Range
▪ Drop Table/Index Needs to claim space fast
L0
L1
L2
L3
Table A Table B Table C

22. Delete Files in Range
L0
L1
L2
L3
Table A Table B Table C
▪ Drop Table/Index Needs to claim space fast

23. Delete Files in Range
L0
L1
L2
L3
Table A Table B Table C
▪ Drop Table/Index Needs to claim space fast

24. Delete Files in Range
L0
L1
L2
L3
Table A Table C
▪ Drop Table/Index Needs to claim space fast

25. What we worked around?

26. Reverse Column order
▪ RocksDB’s Prev() is much slower than Next()
▪ Reverse column order to serve common query better

27. Optimizer Stats
▪ Query plans need index statistics
▪ MyRocks stores index statistics in data dictionary (similar to InnoDB)
▪ When creating data files (SST files), statistics are also added to the
SST files, and data dictionary is also updated

28. Limitation?

29. MyRocks Limitation
Limitation Plan to Address

30. MyRocks Limitation
Limitation Plan to Address
Not yet support Online DDL,
Foreign Key, Spatial Index, and
Fulltext Index
Support them

31. MyRocks Limitation
Limitation Plan to Address
Not yet support Online DDL,
Foreign Key, Spatial Index, and
Fulltext Index
Support them
No next key locking support

32. MyRocks Limitation
Limitation Plan to Address
Not yet support Online DDL,
Foreign Key, Spatial Index, and
Fulltext Index
Support them
No next key locking support
Only support replication using
row-based binary logging.
(durability with XA not supported)
RocksDB to support Two-Phase-
Commit. MyRocks to use it to
support replication using
statement-based binary logging.

33. MyRocks Limitation
Limitation Plan to Address
Not yet support Online DDL,
Foreign Key, Spatial Index, and
Fulltext Index
Support them
No next key locking support
Only support replication using
row-based binary logging.
(durability with XA not supported)
RocksDB to support Two-Phase-
Commit. MyRocks to use it to
support replication using
statement-based binary logging.
Either ORDER BY DESC or ASC
is slower
Improve RocksDB Prev()
performance, to narrow
performance gap between
ORDER BY DESC and ASC.

34. Conclusion

35. Conclusion
▪ MyRocks vs. InnoDB
▪ Better Space and Write Amplification
▪ Comparable performance
▪ Features in RocksDB to Support MyRocks:
▪ Transactions
▪ “Single Delete”
▪ Delete Files in Range
▪ MyRocks has some limitations and they are being
addressed.

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