7. SSTable SSTable SSTable SSTable 1 2 3 4 SSTableSSTables are merged to maintain read performance COMPACT
8. X X X X SSTable SSTable SSTable SSTableSSTable New SSTable is streamed to disk and old SSTables are erased
9. TAKEAWAYS• All disk writes are sequential, append- only operations• On-disk tables (SSTables) are written in sorted order, so compaction is linear complexity O(N)• SSTables are completely immutable
10. TAKEAWAYS• All disk writes are sequential, append- only operations• On-disk tables (SSTables) are written in sorted order, so compaction is linear IMPORTANT complexity O(N)• SSTables are completely immutable
11. COMPARED• Most popular data storage engines rewrite modified data in-place: MySQL (InnoDB), PostgreSQL, Oracle, MongoDB, Membase, BerkeleyDB, etc• Most perform similar buffering of writes before flushing to disk• ... but flushes are RANDOM writes
12. SPINNING DISKS• Dirt cheap: $0.08/GB• Seek time limited by time it takes for drive to rotate: IOPS = RPM/60• 7,200 RPM = ~120 IOPS• 15,000 RPM has been the max for decades• Sequential operations are best: 125MB/ sec for modern SATA drives
13. THAT WAS THE WORLDIN WHICH CASSANDRA WAS BUILT
14. 2012: MLC NAND FLASH* • Affordable: ~$1.75/GB street • Massive IOPS: 39,500/sec read, 23,000/ sec write • Latency of less than 100µs • Good sequential throughput: 270MB/ sec read, 205MB/sec write • Way cheaper per IOPS: $0.02 vs $1.25* based on specifications provided by Intel for 300GB Intel 320 drive
15. WITH RANDOM ACCESSSTORAGE, ARE CASSANDRA’S LSM-TREES OBSOLETE?
16. SOLID STATE HASSOME MAJOR BUTS...
17. ... BUT• Cannot overwrite directly: must erase first, then write• Can write in small increments (4KB), but only erase in ~512KB blocks• Latency: write is ~100µs, erase is ~2ms• Limited durability: ~5,000 cycles (MLC) for each erase block
18. WEAR LEVELING is usedto reduce the number of total erase operations
19. WEAR LEVELING
20. WEAR LEVELINGErase Block
21. WEAR LEVELING
22. WEAR LEVELING
23. WEAR LEVELING Disk Page
24. WEAR LEVELING Write 1
25. WEAR LEVELING Write 1 Write 2
26. WEAR LEVELING Write 1 Write 2 Write 3
27. Remember: the whole block must be erased Write 1 Write 2 Write 3 How is data from only Write 2 modified?
28. Mark Garbage
29. Empty BlockMark Garbage Append Modified Data
30. Wait... GARBAGE?
31. THAT MEANS...
32. ... fragmentation,WHICH MEANS...
33. Garbage Collection!
34. GARBAGE COLLECTION• Compacts fragmented disk blocks• Erase operations drag on performance• Modern SSDs do this in the background... as much as possible• If no empty blocks are available, GC must be done before ANY writes can complete
35. WRITE AMPLIFICATION• When only a few kilobytes are written, but fragmentation causes a whole block to be rewritten• The smaller & more random the writes, the worse this gets• Modern “mark and sweep” GC reduces it, but cannot eliminate it
36. Torture test shows massive write performance drop-off for heavily fragmented driveSource: http://www.anandtech.com/show/4712/the-crucial-m4-ssd-update-faster-with-fw0009/6
37. Some poorly designed drives COMPLETELY fall apartSource: http://www.anandtech.com/show/5272/ocz-octane-128gb-ssd-review/6
38. Even a well-behaved drivesuffers significantly from the torture testSource: http://www.anandtech.com/show/4244/intel-ssd-320-review/11
39. Post-torture, all disk blockswere marked empty, and the “fast” comes back...Source: http://www.anandtech.com/show/4244/intel-ssd-320-review/11
40. “TRIM”• Filesystems don’t typically immediately erase data when files are deleted, they just mark them as deleted and erase later• TRIM allows the OS to actively tell the drive when a region of disk is no longer used• If an entire erase block is marked as unused, GC is avoided, otherwise TRIM just hastens the collection process
41. TRIM only reduces thewrite amplification effect, it can’t eliminate it.
42. THEN THERE’S LIFETIME...
43. AnandTech estimates that modern MLC SSDsonly last about 1.5 years under heavy MySQL load, which causes around 10x write amplification
44. REMEMBER THIS?
45. TAKEAWAYS• All disk writes are sequential, append- only operations• On-disk tables (SSTables) are written in sorted order, so compaction is linear complexity O(N)• SSTables are completely immutable
46. CASSANDRA ONLY WRITESSEQUENTIALLY
47. “For a sequential write workload, write ampliﬁcation is equal to 1, i.e., there is no write ampliﬁcation.”Source: Hu, X.-Y., and R. Haas, “The Fundamental Limitations of Flash Random Write Performance: Understanding, Analysis, and Performance Modeling”