Comment by marcan_42
4 years ago
That's not necessarily true, it just comes down to the design. Apple seem to use a log-structured FTL which works great for performance and just requires a (very fast) log replay after a hard shutdown. You can see the syslog messages from the NVMe controller (via RTKit) talking about rebuilding the table when this happens.
Micron also talks about power loss resistant FTL design in their whitepaper, and although their older SSDs had caps, I think their recent ones mostly do away entirely.
https://www.micron.com/-/media/client/global/documents/produ...
This kind of stuff has been done in filesystems for ages (journaling etc.); there's no reason why FTLs can't be designed the same way.
The micron whitepaper you cited talks about how their higher tier strategy involves keeping enough capacitance around to write out the FTL, because it's DRAM copy is allowed to get out of sync when the write-cache is enabled.
> The hold-up circuitry also preserves enough time and energy to ensure that the FTL addressing table is properly saved to the NAND. This thorough amount of data protection not only ensures data integrity in unexpected power-loss events, but it also enables the system designer to leave the SSD’s write cache enabled, giving a significant advantage in data throughput speeds.