On Thu, 30 Jan 2020 15:16:17 +0100, Lukas Straub said:

> No, csum_next (and csum_prev) is a whole sector (i.e. physical block)
> containing all the checksums for the following chunk of data (which
> spans multiple sectors)

Oh, OK.  That works too.. :)

> Regarding the ordering guarantee, I have now gathered that the kernel
> will happily split the bio if the size is not optimal for the hardware
> which means it's not guaranteed - right?

And if the kernel doesn't split it and reorder the chunks, the hardware will
happily do so - and lie to you the whole way.  Ever since the hardware people
realized they could get away with lying about turning off the writeback cache,
it's been more and more of a challenge to guarantee correct performance from
storage subsystems.