splitting iovecs to bios

[Konstantin Belousov]

On Tue, Dec 22, 2015 at 11:37:23AM +0200, Max Gurtovoy wrote:
> 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.

Sure, feel free to implement it.  If you need a help, just ask.