splitting iovecs to bios

[Max Gurtovoy]

Hi Konstantin,

>
>> There might be indeed a reason, it could be that some drivers expect
>> blocking to be done by the userspace.  The drivers could have some
>> restrictions on transfer sizes and atomicity of transfer, which would
>> be broken by the unconditional merge.  I cannot give you an example
>> of such driver, known block-aware drivers like sa(4) only require the
>> bio size to be multiple of the basic block size.
>
> I'm surprised to learn that the generic access layer splits IO requests
> just because some block drivers cannot handle it. I'd expect that this
> sort of limitation would be communicated by the drivers in the form of
> device flag SI_NOMERGE.
>
>> OTOH, I see no issue with adding a SI_PHYSIOMERGE flag and doing the
>> merges for the driver in physio(), when unmapped request has consequtive
>> iov elements ending and starting at the page boundary.
>
> I'd say it should be the other way around, physio would always strive
> to append/merge iov elements but wouldn't in case the device does not
> support it. Moreover, some modern devices does not even require the page
> boundary alignment you mentioned. These devices can execute IO to/from
> any arbitrary scatter list of buffers.

Do you know if this issue is on someone's plate ?
If it doesn't, maybe we can try to advance it and start implementing 
some solutions.
As I said earlier and as Sagi mentioned, this feature can improve the 
performance of modern devices.

Thanks,
Max.