[dm-devel] [PATCH 0/2] dm: add new loop and ram targets

[Mike Snitzer]

On Thu, Jan 18 2018 at  6:42am -0500,
Bryn M. Reeves <bmr at redhat.com> wrote:

> On Wed, Jan 17, 2018 at 04:29:36PM -0500, Mike Snitzer wrote:
> > On Wed, Jan 17 2018 at  2:33pm -0500,
> > As for dm-loop, doubling the performance of the loopback driver is quite
> > nice (especially with only 1/7 the number of lines of code as
> > drives/block/loop.c).
> 
> Isn't this going to raise the same objection that akpm had years ago,
> with the original dm-loop (block mapping) target?
> 
> We had an even bigger performance boost with that but it was rejected
> on the grounds that a second loop back block device implementation was
> not welcome unless the two could share code.

Could.  But I wasn't around for that particular spat.  It seems quite
misplaced to swoop in with an aire of design purity to defeat a DM
target that shows such clear wins.

This idea that our poor Linux users will lose their heads because they
have multiple options is also idiotic.

But we'll cross that bridge as needed (before burning it down?) ;)

> We had out of tree users for years from folks looking for better
> performance.
>
> I've been through a few revisions of a patch set that refactors the
> loop.c driver into an interface and back end, which both /dev/loopN and
> dm-loop could use. This was at the request of various groups who were
> interested in a DM loop target but ultimately interest waned each time
> and I ended up stopping maintenance of it a few years back.

It is make work.  Yes there is duplicated logic but loop is now
encumbered by blk-mq complexity now.  To think that we have to factor
out a front and back end to stand up a DM bio-based loop device is
_insane_.  Especially when you consider the front-end would need to
differentiate between bio-based vs blk-mq.

It could also easily be that we don't need or want feature compatibility
(which I assume is the intent of a common front-end?).

This isn't rocket science.  If we're willing to maintain ~400 line
dm-loop then nobody else should worry their pretty little heads about
it.  Problems arise when code isn't maintained.

Mike