Forced Unit Access, plp

[edge]

This is one of those questions that I find answered with a remarkable degree of uncertainty.

FUA (forced unit access) is a flag for a write to stable media on a disk drive. One would think persistent cache (battery backed or capacitor backed) would qualify as stable, but over the decades of dealing with storage I have known a number of engineers who don't consider cache "media". This has caused me to observe major write performance degradation when there are large numbers of writes with the FUA flag set. This experience has spanned battery backed raid controllers, SAN storage arrays, and flash cached disk drives.

Given that history, I asked the simple question: do plp SSDs consider the FLT stable or does the FUA flag force write through to NAND?

 

[ajs]

This is one of those questions that I find answered with a remarkable degree of uncertainty.

FUA (forced unit access) is a flag for a write to stable media on a disk drive. One would think persistent cache (battery backed or capacitor backed) would qualify as stable, but over the decades of dealing with storage I have known a number of engineers who don't consider cache "media". This has caused me to observe major write performance degradation when there are large numbers of writes with the FUA flag set. This experience has spanned battery backed raid controllers, SAN storage arrays, and flash cached disk drives.

Given that history, I asked the simple question: do plp SSDs consider the FLT stable or does the FUA flag force write through to NAND?

In my experience, yes. The enterprise SSDs I've worked on all consider the write complete and safe once it hits the cache. A write with FUA set will return complete at that time.

Rotational media is a different story... not enough time/power to flush the write cache on power loss, so FUA in that case will wait for the sync to media.