- io architecture
- kernel page cache, direct IO
- what is IO performance
  - CPU time
  - synchronous blocking operations
  - latency
  - IO speed
- issue: reads
  - little prefetching
  - no concurrent IO
  - unable to use CPU time for other tasks
- issue: background writer
  - lots of random io
  - no write combining
  - no clock sweep
- issue: latency jitter
  - too many outstanding "data" writes
  - kernel allowed to do readahead
  - lots of outstanding WAL
- issue: backend writes
  - too many
  - not flushed by default
- issue: bulk write speed (checkpointer)
- issue: double buffering