[Drbd-dev] [PATCH v4 01/11] block: make generic_make_request handle arbitrarily sized bios

[Mike Snitzer]

On Tue, May 26 2015 at 11:02am -0400,
Ming Lin <mlin at kernel.org> wrote:

> On Tue, May 26, 2015 at 7:36 AM, Mike Snitzer <snitzer at redhat.com> wrote:
> > On Fri, May 22 2015 at  2:18pm -0400,
> > Ming Lin <mlin at kernel.org> wrote:
> >
> >> From: Kent Overstreet <kent.overstreet at gmail.com>
> >>
> >> The way the block layer is currently written, it goes to great lengths
> >> to avoid having to split bios; upper layer code (such as bio_add_page())
> >> checks what the underlying device can handle and tries to always create
> >> bios that don't need to be split.
> >>
> >> But this approach becomes unwieldy and eventually breaks down with
> >> stacked devices and devices with dynamic limits, and it adds a lot of
> >> complexity. If the block layer could split bios as needed, we could
> >> eliminate a lot of complexity elsewhere - particularly in stacked
> >> drivers. Code that creates bios can then create whatever size bios are
> >> convenient, and more importantly stacked drivers don't have to deal with
> >> both their own bio size limitations and the limitations of the
> >> (potentially multiple) devices underneath them.  In the future this will
> >> let us delete merge_bvec_fn and a bunch of other code.
> >
> > This series doesn't take any steps to train upper layers
> > (e.g. filesystems) to size their bios larger (which is defined as
> > "whatever size bios are convenient" above).
> >
> > bio_add_page(), and merge_bvec_fn, served as the means for upper layers
> > (and direct IO) to build up optimally sized bios.  Without a replacement
> > (that I can see anyway) how is this patchset making forward progress
> > (getting Acks, etc)!?
> >
> > I like the idea of reduced complexity associated with these late bio
> > splitting changes I'm just not seeing how this is ready given there are
> > no upper layer changes that speak to building larger bios..
> >
> > What am I missing?
> 
> See: [PATCH v4 02/11] block: simplify bio_add_page()
> https://lkml.org/lkml/2015/5/22/754
> 
> Now bio_add_page() can build lager bios.
> And blk_queue_split() can split the bios in ->make_request() if needed.

That'll result in quite large bios and always needing splitting.

As Alasdair asked: please provide some performance data that justifies
these changes.  E.g use a setup like: XFS on a DM striped target.  We
can iterate on more complex setups once we have established some basic
tests.

If you're just punting to reviewers to do the testing for you that isn't
going to instill _any_ confidence in me for this patchset as a suitabe
replacement relative to performance.