Add some kind of unique_id attribute to disks in scsi mid layer?

[scameron@beardog.cce.hp.com]

Would it be a reasonable idea to add some sort of unique_id attribute
to scsi devices in the mid layer?

For example, sg_inq, from sg3_utils, will print out a serial number:

[root@localhost scameron]# sg_inq /dev/sda
standard INQUIRY:
  PQual=0  Device_type=0  RMB=0  version=0x05  [SPC-3]
  [AERC=0]  [TrmTsk=0]  NormACA=0  HiSUP=0  Resp_data_format=2
  SCCS=0  ACC=0  TPGS=0  3PC=0  Protect=0  BQue=0
  EncServ=0  MultiP=0  [MChngr=0]  [ACKREQQ=0]  Addr16=0
  [RelAdr=0]  WBus16=0  Sync=0  Linked=0  [TranDis=0]  CmdQue=1
    length=56 (0x38)   Peripheral device type: disk
 Vendor identification: HP      
 Product identification: LOGICAL VOLUME  
 Product revision level: 3.66
 Unit serial number: 50014380065020C0

And there's also some stuff in page 0x83 on some devices which might
be useful as suck an identifier:

[root@localhost scameron]# sg_inq --page 0x83 /dev/sda
VPD INQUIRY: Device Identification page
  Designation descriptor number 1, descriptor length: 20
    designator_type: NAA,  code_set: Binary
    associated with the addressed logical unit
      NAA 6, IEEE Company_id: 0x508b
      Vendor Specific Identifier: 0x1001ce88b
      Vendor Specific Identifier Extension: 0x746f381eedf6577
      [0x600508b1001ce88b0746f381eedf6577]
  Designation descriptor number 2, descriptor length: 8
    designator_type: vendor specific [0x0],  code_set: Binary
    associated with the addressed logical unit
 00     00 00 00 00                                         ....            
[root@localhost scameron]#

The hpsa driver, for example, currently exports a "unique_id"
attribute itself for disks, which is the same data reported by
inquire page 0x83.

[root@localhost 0:0:0:1]# pwd 
/sys/bus/scsi/devices/0:0:0:1
[root@localhost 0:0:0:1]# cat unique_id 
600508B1001CE88B0746F381EEDF6577

It occurs to me that maybe something
like that would be useful for all disks serviced by the scsi mid layer.
For example, kdump userland stuff, as typically implemented,
has a very small ramdisk which it operates from, and has a fairly
simple minded way of recognizing which disk is which (e.g. looks at
vendor, model, rev, and type attributes, arguably not a very good way
to do it, as just flashing firmware on a drive will change the rev, but
won't mean the disk is not the same disk, for example, and likewise, all
disks on a given Smart Array will carry the same vendor/model/rev/type,
and type?  Type is always going to be zero for a disk anyhow.) 

So kdump needs some kind of a unique_id to help it out, but would prefer
not to carry around e.g. sg_inq in the kdump initrd (so you may ask
"stuffing it in the kernel is a better idea?")  Well, _if_ the LLDs are
typically already getting this info (as in the case of hpsa and cciss
at least) maybe they should have some standard way of informing the
mid layer, so the midlayer could expose this attribute in a standard way?
Or, maybe the mid layer could send down the necessary inquiries to get
the information itself with no interface changes needed?

Well, this seems like such an obvious idea, that the mere fact that it 
is not already implemented makes me think there may be some good reason
why not.

-- steve