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

[Jeff Garzik <jeff@garzik.org>]

James Bottomley wrote:
> On Fri, 2008-06-20 at 13:06 +0900, Tejun Heo wrote:
>> The biggest problem is how to keep userland happy.  hdX -> sdX
>> transition was painful enough and I have a strong feeling that
>> everyone will come after and hunt down us if we try something like sdX
>> -> bdX now.  :-)

> In theory mounting by label or ID should have fixed a lot of this.
> However, if we need to head off a revolt, the sdX allocation algorithm
> can be placed into it's own module so both sd and a ULD ata driver could
> use it ...

> Actually, surely we can mostly dump the SAT layer?


I don't see that we can do that for a long time...  And it's not just 
the sdX allocation algorithm in question -- SCSI block devices come with 
their own partition limits and set of supported ioctls.

Therefore, my recommended path has always been

* create ata_disk block device driver (ULD, in your terminology)

* make SAT an optional piece, which maintains compatibility with 
existing SCSI blkdevs, ioctls, command sets


I just don't see a valid path moving forward that breaks userland 
/again/...  we (ATA hackers) would be drummed out of a job I think :)

Another option that's been discussed is

1) Make SCSI block devices themselves an allocate-able resource (I think 
that's what you meant by "placed into it's own module so both sd and a 
ULD ata driver could use it"?)

2) Ensure that any ata_disk ULD would support the same partition limits 
and ioctl set, enough to ensure binary compatibility.

Because that's the real need -- maintaining binary compatibility with 
SCSI block devices, so major/minor, ioctl supported set, partition 
limits, and other relevant details need to remain unchanged.

The underlying software we're of course free to change...

	Jeff