Re: [PATCH 2/2] hw/block/nvme: add write uncorrectable command

[Keith Busch]

On Thu, Feb 11, 2021 at 09:43:05AM +0100, Klaus Jensen wrote:
> On Feb 11 12:37, Keith Busch wrote:
> 
> > Is there a use case with a real qemu guest wanting this?
> 
> Like for the extended metadata case (which also does not have a lot of
> "public" exposure, but definitely have enterprise use), our main
> motivation here was to ease testing for compliance suites and frameworks
> like SPDK. 

I'm okay with the metadata patches.

> I'm honestly have no clue what so ever what a real world use
> of Write Uncorrectable would be. It's been in the spec since 1.0, so
> there must have been some reason, Is it just to align with SCSI WRITE
> LONG? I'm not SCSI expert at all, but from what I can read it looks like
> that was also intended as a feature for testing read error conditions.

I don't think it's for testing purposes.

If you need to send a burst of non-atomic writes (ex: writing a RAID
stripe), a power failure can result in an inconsistent state where you
don't know at a block level which ones have old data or new data. If you
Write Uncorrectable first, you can never read old data, and thus have no
"write hole".

Journalling solves this better, and I'm not aware of any real
implementation relying on uncorrectable.