On Thu, Nov 09, 2023 at 07:08:40PM +0000, John Garry wrote: >>> 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). You're quoting a few mails before me, but I agree. >> 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. Yes. > 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? I'm more worried about the problem where we accidentally add a split. The whole bio merge/split path is convoluted and we had plenty of bugs in the past by not looking at all the correct flags or opcodes.
