I wish SSDs gave you CPU performance style metrics about their activity

Summary

Modern CPUs have an impressive collection of performance counters for detailed, low level information on things like cache misses, branch mispredictions, various sorts of stalls, and so on; on Linux you can use 'perf list' to see them all. Modern SSDs (NVMe, SATA, and SAS) are all internally quite complex, and their behavior under load depends on a lot of internal state. It would be nice to have CPU performance counter style metrics to expose some of those details. For a relevant example that's on my mind (cf), it certainly would be interesting to know how often flash writes had to stall while blocks were hastily erased, or the current erase rate. Having written this, I checked some of our SSDs (the ones I'm most interested in at the moment) and I see that our SATA SSDs do expose some of this information as (vendor specific) SMART attributes, with things like 'block erase count' and 'NAND GB written' to TLC or SLC (as well as the host write volume and so on stuff you'd expect). NVMe does this in a different way that doesn't have the sort of easy flexibility that SMART attributes do, so a random one of ours that I checked doesn't seem to provide this sort of lower level information. It's understandable that SSD vendors don't necessarily want to expose this sort of information, but it's quite relevant if you're trying to understand unusual drive performance. For example, for your workload do you need to TRIM your drives more often, or do they have enough pre-erased space available when you need it? Since TRIM has an overhead, you may not want to blindly do it on a frequent basis (and its full effects aren't entirely predictable since they depend on how much the drive decides to actually erase in advance). (Having looked at SMART 'block erase count' information on one of our servers, it's definitely doing something when the server is under heavy fsync() load, but I need to cross-compare the numbers from it to other systems in order to get a better sense of what's excep...