Ordering guarantee inside a single bio?

[Lukas Straub]

On Mon, 27 Jan 2020 12:27:58 -0500
"Valdis Klētnieks" <valdis.kletnieks at vt.edu> wrote:

> On Sun, 26 Jan 2020 13:07:38 +0100, Lukas Straub said:
> 
> > I am planing to write a new device-mapper target and I'm wondering if there
> > is a ordering guarantee for the operation inside a single bio? For example if I
> > issue a write bio to sector 0 of length 4, is it guaranteed that sector 0 is
> > written first and sector 3 is written last?  
> 
> I'll bite.  What are you doing where the order of writing out a single bio matters?

I plan to improve the performance of dm-integrity on HDDs by removing the requirement for bitmap or journal (which causes head seeks even for sequential writes). I also want to avoid cache flushes and FUA. The problem with dm-integrity is that the data and checksum update needs to be atomic.
So I came up with the following Idea:

The on-disk layout will look like this:
|csum_next-01|data-chunk-01|csum_prev-01|csum_next-02|data-chunk-02|csum_prev-02|...

Under normal conditions, csum_next-01 (a single sector) contains the checksums for data-chunk-01 and csum_prev-01 is a duplicate of csum_next-01.

Updating data will first update csum_next (with FUA), then update the data (FUA) and finally update csum_prev (FUA).
But if there is a ordering guarantee we have a fast path: If a full chunk of data is written, we simply issue a single big write with csum_next, data and csum_prev, all without FUA (except if the incoming request asks for that).
So that's why I'm asking.