Re: [RFC] Separating out libata out of SCSI (finally)

[Tejun Heo <tj@kernel.org>]

James Bottomley wrote:
> On Tue, 2008-06-24 at 11:30 +0300, Boaz Harrosh wrote:
>> James Bottomley wrote:
>>> On Mon, 2008-06-23 at 17:04 -0400, Greg Freemyer wrote:
>>>> I've seen a lot of end user complaints about libata only supporting
>>>> 15(14?) partitions.  Will that limit be moved back to the traditional
>>>> drivers/ide limit as part of this?
>>> Number of partitions is directly related to number of minors, so it
>>> can't be changed without a change in the allocation of major/minor space
>>> in sd ... that could only be done compatibly by permuting the space.
>>> The only other way to do it is incompatibly by changing major (again).
>>>
>> Could we do both? I mean use the legacy, up to 15, with the old major,
>> then use the new major for bigger then 15. Since user mode that knows
>> about more then 15 partitions is new, it'll know it needs to jump a major.
> 
> Not simultaneously, which is the problem; you can't have two separate
> block devices for the same physical device unless you want aliasing
> issues in the page cache.
> 
> It might be possible to add an extra device to give access to the
> missing partitions, but that would require a bit of re-engineering in
> gendisk (which is the in-kernel code to manage the partitions).
> 
> What might be far more feasible is to set up udev to use kpartx to
> provide the missing partitions if it detects a partition table that has
> them ... of course, that requires a udev setup and most of the
> complaints about the lost partitions seem to come from non-udev systems.
> 
> But .... if everyone (particularly the people with these problems) had
> udev, we could simply migrate to a new major with more partitions, get
> udev to fix it all up for us and everyone would be happy because no-one
> would even notice that we'd moved majors ...

I'm currently working on a scheme where partitions above gd->minors get
allocated dynamic MAJ:MIN.  It looks like it can be done mostly in block
layer proper.  The only problem I can foresee is not being able to
specify MAJ:MIN as root device but that shouldn't be a major problem.
I'll report back when I make more progress.

Thanks.

-- 
tejun