On Mon, May 13, 2019 at 02:03:44PM +0200, Christoph Hellwig wrote:
> On Mon, May 13, 2019 at 05:45:45PM +0800, Ming Lei wrote:
> > On Mon, May 13, 2019 at 08:37:45AM +0200, Christoph Hellwig wrote:
> > > Currently ll_merge_requests_fn, unlike all other merge functions,
> > > reduces nr_phys_segments by one if the last segment of the previous,
> > > and the first segment of the next segement are contigous. While this
> > > seems like a nice solution to avoid building smaller than possible
> >
> > Some workloads need this optimization, please see 729204ef49ec00b
> > ("block: relax check on sg gap"):
>
> And we still allow to merge the segments with this patch. The only
> difference is that these merges do get accounted as extra segments.
Trying mkfs.xfs there were no merges at all, which is expected as
it does perfectly sized direct I/O.
Trying mkfs.ext4 I see lots of bio merges. But for those this patch
nothing changes at all, as we never decrement nr_phys_segments to start
with, we only every did that for request merges, and for those also
only for those on non-gappy devices due to the way the
req_gap_back_merge check was placed.
