It seems that new BIOs do not have BIO_SEG_VALID set. So when you do sequential IO, the IO being back-merged should always have not
had valid segments.
I ran bonnie++ and it shows the same thing.
> Well you'd want to optimize for the busy case, right, no
> point in optimizing for a more idle system.
>
> I'm not at all uninterested in this, I'd just like to see a
> more intelligent/controlled work load that actually stresses
> the io subsystem being profiled. If you have a not-so-busy
> system, you like don't do enough IO to trigger a lot of
> merges. Or maybe you do, and we just have a bug somewhere so
> that we unfortunately repeatedly recount segments.
>
> Care to run a simple io benchmark and profile that?
>
> --
> Jens Axboe
>
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