Modern SSDs are believed to be random IO friendly devices, unlike HDDs. ScyllaDB's low-level library called Seastar is believed to exploit share-nothing design to achieve peak performance on any hardware. IO scheduling in ScyllaDB is where both beliefs meet together ... and are both refuted.

1. Brought to you by
A New IO Scheduler
Algorithm for Mixed Workloads
Pavel Emelyanov
Principal Engineer at ScyllaDB

2. Pavel Emelyanov
Principal engineer
■ Linux kernel in the past
■ Seastar in the present

3. The list
■ Mixed mode handling
■ More metrics
■ Full-duplex scheduling
■ Bandwidth limiting
■ Improved classes preemption

4. Disk throughput

5. Disk profile (i3)

6. Disk profile (i3en)

7. Disk profile (im4gn)

8. Disk profile (i4i)

9. Disk profile (HDD)

10. Scheduler “safety area”

11. Scheduling equation

12. Scheduling algo

13. Scheduler “safety area” (cont)

14. Current status
■ In 5.0
■ Accepts legacy io-properties.yaml
● rate_factor to fine-tune the safety area
■ New metrics
● scylla_io_queue_consumption
● scylla_io_queue_adjusted_consumption

15. Advanced “advanced” dashboard
■ scylla_io_queue_{disk_}?queue_length
■ scylla_io_queue_total_{delay|exec}_sec
■ scylla_io_queue_total_{read|write}_{ops|bytes}

16. Full duplex disks

17. Full duplex disk stacks
SSD
EBS
RAID / DRBD
R/W
Backup (write-only)
Disclaimer: don’t try this at home

18. Full duplex scheduling
io_queue
fair_queue
AIO
io_queue
fair_queue
AIO
fair_queue
R W

19. Bandwidth limiting
io_queue
AIO
compaction
IO class
query
IO class
commitlog
IO class

20. Bandwidth limiting
io_queue
AIO
Token
bucket
compaction
IO class
query
IO class
commitlog
IO class
io_properties.yaml

21. Bandwidth limiting
io_queue
AIO
Token
bucket
compaction
IO class
query
IO class
commitlog
IO class
Token
bucket
--compaction-throughput-mb-per-sec

22. Requests preemption
■ IO class with low concurrency
■ Two contradicting options
● Keep history for longer
● Monopolize the queue

23. Improved interactivity

24. Brought to you by
Pavel Emelyanov
xemul@scylladb.com
/xemul