The Next Step in SSD Evolution: NVMe Zoned Namespaces Explainedby Billy Tallis on August 6, 2020 12:00 PM EST
How to Enable NVMe Zoned Namespaces
Hardware changes for ZNS
At a high level, in order to enable ZNS, most drives on the market only require a firmware update. ZNS doesn't put any new requirements on SSD controllers or other hardware components; this feature can be implemented for existing drives with firmware changes alone.
The critical element in hardware comes down to when an SSD is designed to support only zoned namespaces. First and foremost, a ZNS-only SSD doesn't need anywhere near as much overprovisioning as a traditional enterprise SSD. ZNS SSDs are still responsible for performing wear leveling, but this no longer requires a large spare area for the garbage collection process. Used properly, ZNS allows the host software to avoid almost all of the circumstances that would lead to write amplification inside the SSD. Enterprise SSDs commonly use overprovisioning ratios up to 28% (800GB usable per 1024GB of flash on typical 3 DWPD models) and ZNS SSDs can expose almost all of that capacity to the host system without compromising the ability to deliver high sustained write performance. ZNS SSDs still need some reserve capacity (for example, to cope with failures that crop up in flash memory as it wears out), but Western Digital says we can expect ZNS to allow roughly a factor of 10 reduction in overprovisioning ratios.
Better control over write amplification also means QLC NAND is a more viable option for use cases that would otherwise require TLC NAND. Enterprise storage workloads often lead to write amplification factors of 2-5x. With ZNS, the SSD itself causes virtually no write amplification and clever host software can avoid causing much write amplification, so the overall effect is a boost to drive lifespan that offsets the lower endurance of QLC compared to TLC (or beyond QLC). Even in a ZNS SSD, QLC NAND is still fundamentally slower than TLC, but that same near-elimination of background data management within the SSD means a QLC-based ZNS SSD can probably compete with TLC-based traditional SSDs on QoS metrics even if the total throughput is lower.
The other major hardware change enabled by ZNS is a drastic reduction in DRAM requirements. The Flash Translation Layer (FTL) in traditional block-based SSDs requires about 1GB of DRAM for every 1TB of NAND flash. This is used to store the address mapping or indirection tables that record the physical NAND flash memory address that is currently storing each Logical Block Address (LBA). The 1GB per 1TB ratio is a consequence of the FTL managing the flash with a granularity of 4kB. Right off the bat, ZNS gets rid of that requirement by letting the SSD manage whole zones that are hundreds of MB each. Tracking which physical NAND erase blocks comprise each zone now requires so little memory that it could be done with on-controller SRAM even for SSDs with tens of TB of flash. ZNS doesn't completely eliminate the need for SSDs to include DRAM, because the metadata that the drive needs to store about each zone is larger than what a traditional FTL needs to store for each LBA, and drives are likely to also use some DRAM for caching writes - more on this later.