Re: [PATCH 21/21] nvme: Support atomic writes

John Garry
2023-11-09T19:09:22Z

On 09/11/2023 15:46, Christoph Hellwig wrote:
> On Thu, Nov 09, 2023 at 03:42:40PM +0000, Matthew Wilcox wrote:
>> That wasn't the model we had in mind. In our thinking, it was fine to
>> send a write that crossed the atomic write limit, but the drive wouldn't
>> guarantee that it was atomic except at the atomic write boundary.
>> Eg with an AWUN of 16kB, you could send five 16kB writes, combine them
>> into a single 80kB write, and if the power failed midway through, the
>> drive would guarantee that it had written 0, 16kB, 32kB, 48kB, 64kB or
>> all 80kB. Not necessarily in order; it might have written bytes 16-32kB,
>> 64-80kB and not the other three.

I didn't think that there are any atomic write guarantees at all if we
ever exceed AWUN or AWUPF or cross the atomic write boundary (if any).

> I can see some use for that, but I'm really worried that debugging
> problems in the I/O merging and splitting will be absolute hell.

Even if bios were merged for NVMe the total request length still should
not exceed AWUPF. However a check can be added to ensure this for a
submitted atomic write request.

As for splitting, it is not permitted for atomic writes and only a
single bio is permitted to be created per write. Are more integrity
checks required?

Thanks,
John