Re: [PATCH][RFC] fast file mapping for loop

[Chris Mason <chris.mason@oracle.com>]

On Fri, 11 Jan 2008 10:01:18 +1100
Neil Brown <neilb@suse.de> wrote:

> On Thursday January 10, jens.axboe@oracle.com wrote:
> > On Thu, Jan 10 2008, Chris Mason wrote:
> > > On Thu, 10 Jan 2008 09:31:31 +0100
> > > Jens Axboe <jens.axboe@oracle.com> wrote:
> > > 
> > > > On Wed, Jan 09 2008, Alasdair G Kergon wrote:
> > > > > Here's the latest version of dm-loop, for comparison.
> > > > > 
> > > > > To try it out, 
> > > > >   ln -s dmsetup dmlosetup
> > > > > and supply similar basic parameters to losetup.
> > > > > (using dmsetup version 1.02.11 or higher)
> > > > 
> > > > Why oh why does dm always insist to reinvent everything? That's
> > > > bad enough in itself, but on top of that most of the extra
> > > > stuff ends up being essentially unmaintained.
> > > 
> > > I don't quite get how the dm version is reinventing things.  They
> > > use
> > 
> > Things like raid, now file mapping functionality. I'm sure there are
> > more examples, it's how dm was always developed probably originating
> > back to when they developed mostly out of tree. And I think it's a
> > bad idea, we don't want duplicate functionality. If something is
> > wrong with loop, fix it, don't write dm-loop.
> 
> I'm with Jens here.
> 
> We currently have two interfaces that interesting block devices can be
> written for: 'dm' and 'block'.
> We really should aim to have just one.  I would call it 'block' and
> move anything really useful from dm into block.
> 
> As far as I can tell, the important things that 'dm' has that 'block'
> doesn't have are:
> 
>   - a standard ioctl interface for assembling and creating interesting
>      devices.
>      For 'block', everybody just rolls there own. e.g. md, loop, and
>      nbd all use totally different approaches for setup and tear down
>      etc. 
> 
>   - suspend/reconfigure/resume.
>     This is something that I would really like to see in 'block'.  If
>     I had a filesystem mounted on /dev/sda1 and I wanted to make it a
>     raid1, it would be cool if I could
> 	suspend /dev/sda1
> 	build a raid1 from sda1 and something else
> 	plug tha raid1 in as 'sda1'.
> 	resume sda1
> 
>   - Integrated 'linear' mapping.
>     This is the bit of 'dm' that I think of as yucky.  If I read the
>     code correctly, every dm device is a linear array of a bunch of
>     targets.  Each target can be a stripe-set(raid0) or a multipath or
>     a raid1 or a plain block device or whatever.
>     Having 'linear' at a different level to everything else seems a
>     bit ugly, but it isn't really a big deal.
>

DM is also a framework where you can introduce completely new types of
block devices without having to go through the associated pain of
finding major numbers.  In terms of developing new things with greater
flexibility, I think it is easier.
 
> I would really like to see every 'dm' target being just a regular
> 'block' device.  Then a 'linear' block device could be used to
> assemble dm targets into a dm device.  Or the targets could be used
> directly if the 'linear' function wasn't needed.
> 
> Each target/device could respond to both dm ioctls and 'adhoc'
> ioctls.  That is a bit ugly, but backwards compatibility always is,
> but it isn't a big cost.
> 
> I think the way forward here is to put the important
> suspend/reconfig/resume functionality into the block layer, then
> work on making code work with multiple ioctl interfaces.
> 
> I *don't* think the way forward is to duplicate current block devices
> as dm targets.  This is duplication of effort (which I admit isn't
> always a bad thing) and a maintenance headache (which is).
> 

raid in dm aside (that's an entirely different debate ;), loop is a
pile of things which dm can nicely layer out into pieces (dm-crypt vs
loopback crypt).  Also, dm doesn't have to jump through hoops to get a
variable number of minors.

Yes, the loop side was recently improved for # of minors, and it does
have enough in there for userland to do variable number of minors, but
this is one specific case where dm is just easier.

At any rate, I'm all for ideas that make dm less of the evil stepchild
of the block layer ;)  I'm not saying everything should be dm, but I
did want to point out that dm-loop isn't entirely silly.

I have a version of Jens' patch in testing here that makes a new API
with the FS for mapping extents and hope to post it later today.

-chris