Conventional vs ZNS vs FDP: Who Cleans Up?
The one picture that makes all three click — a spectrum from ‘the device does everything’ to ‘the host does everything’.
Three ways to run a flash SSD sound complicated, but one question separates them: who is responsible for garbage collection, and how much does the software have to help? Line them up on that single axis and it all clicks.
The device does everything on the left; the host takes over on the right. FDP sits in the middle.
| Conventional | FDP | ZNS | |
|---|---|---|---|
| Who does garbage collection? | the device | the device | the host |
| What the host must do | nothing | add a hint per write | manage zones & run GC |
| Write rule | write anywhere | write anywhere (+ hint) | strictly sequential |
| Software change | none | small | large |
| Typical write amplification | highest | low | lowest |
One line each
- Conventional — full service: you write wherever you like and the drive figures out cleanup by itself. Easiest, but it mixes hot and cold data, so it wastes the most writes.
- FDP — you add a small label to each write; the drive still cleans up, but now keeps similar data together, so it wastes far less. Your software barely changes.
- ZNS — you write in strict order into zones and do the cleanup yourself. Most efficient of all, but your software has to be zone-aware.
Analogy. Hotel housekeeping. Conventional = full service: you leave and they clean however they like. FDP = you leave your recycling pre-sorted so cleaning is faster. ZNS = you clean your own room and just hand back the key.
When would you pick each?
- Conventional: everyday drives, where simplicity matters more than squeezing out every last write.
- FDP: data centres that want lower write amplification and longer flash life without rewriting their software — today's industry sweet spot.
- ZNS: systems already built around logs and zones (databases, log-structured file systems) that can invest in host-managed placement for the best possible efficiency.
In the EyanaSSDSim paper & simulator. Run all three in the Live Simulator on the same trace and compare WAF directly. EyanaSSDSim implements a conventional FTL, a host-managed ZNS engine, and an FDP engine aligned to the FEMU/WARP model.
Deep dives: FDP · ZNS · Garbage Collection · Write Amplification.