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In NVMe, what is the difference between a reserved log and an unsupported log?

mx flag

I'm reading through the NVMe specification. On page 237 it says the following for get-log return value 0x09:

Invalid Log Page: The log page indicated is invalid or not supported. This error condition is also returned if a reserved log page is requested. Controllers compliant with NVM Express Base Specification revision 1.3 and earlier may return Invalid Field in Command for this condition.

What I'm not clear on is what is a reserved log page? I understand reserve values in the rest of the spec but this log page has been defined so it's not as if it is reserved for future use. There is a distinction made here between reserved and not supported. Does reserved mean you need a vendor key or something to look at it?

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mx flag

Asked a coworker: here, reserved means what it typically means in that the log page is reserved for future use. If you receive this error and the log is defined that means, assuming that the vendor is compliant with the NVMe spec, that the log is an optional log and that the vendor has chosen not to implement it.

Going through the list, most of the logs in question appear to be related to endurance groups / NVM groups which many drive vendors don't support.

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