Re: [PATCH] O_DIRECT with hardware blocksize alignment

[Badari Pulavarty]

Wouldn't this cause, single IO to split into (possibly) 3 I/Os for 
RAW VARY on all unaligned IOs ? In that case, wouldn't it be better
of doing entire I/O with 512byte buffer heads, so that all of them 
can be merged ?

IMHO, preventing the merge due to b_size mismatch just to handle few
drivers/hardware may be overkill. This will penalize all the drivers
which can handle variable size IOs also. Isn't it ? Leaving it to
the driver and doing variable size I/O on only the drivers that can 
handle them, may be a better option ?

In my initial version of RAW VARY patch, I split the I/Os into possibly
3 I/O in raw.c. Do brw_kiovec(.., 512) for the first page and last pages &
4K for the middle pages. It showed reduction in CPU utilization for
large IOs. But it since it could split single I/O into 3 I/Os, i did
not see significant I/O throughput improvement.

Regards,
Badari

> 
> On Tue, Jan 15 2002, Jens Axboe wrote:
> > On Tue, Jan 15 2002, Andrea Arcangeli wrote:
> > > actually we could also forbid merging at the ll_rw_block layer if b_size
> > > is not equal, maybe that's the simpler solution to that problem after
> > > all, merging between kiovec I/O and buffered I/O probably doesn't
> > > matter.
> > 
> > Agreed, this is also what I suggested.
> 
> Here's the right version, sorry. This still potentially decrements
> elevator sequence wrongly for a missed front merge, but that's an issue
> I can definitely live with :-)
> ....