[dm-devel] About WWID

[Brian Bunker]

Nicolas,

The details of this are spelled out in the SPC-4 SCSI document available from t10.org. The ‘3’ in the value you are asking about tells what format that the value in question is using. ‘3’ being ‘NAA’. The ‘6’ too is also again a format designation as well. The rest of the number contains a company id and then the part that is unique to a particular volume on your array:

So it is:

‘3’ - NAA
‘6’ - NAA
‘000144’ - ‘EMC corporation’
‘0000000102058055965f660d8’ - Some unique id (some part will likely be a serial number you can see on the array side)

Multipathing software uses this number to determine if a device is a new device or a path to an existing one. The answer to your question is that a wwid is not defined in the SPC-4 spec so what is right is not really relevant, it is more just how they choose to display uniqueness. multipath is displaying the entire designator while EMC is stripping the ‘3’ from the front. Either way it is in the VPD page 0x83 of an inquiry to your device. You can do ‘sg_inq -p 0x83 device’ where device is /dev/*** to see what I mean.

Thanks,
Brian

On Jan 17, 2014, at 4:19 AM, Nicolas Michel <be.nicolas.michel at gmail.com> wrote:

> Hello guys,
> 
> I noticed a difference with the WWID of a multipath device as reported
> by the device mapper native multipath, and by the EMC propriatary tool
> powermt. In the latter case it misses a leading 3 (which seems to be
> an invariant prefix with the device mapper).
> 
> Exemple :
> 
> Reported by multipath -ll : 360001440000000102058055965f660d8
> Reported by powermt :       60001440000000102058055965F660D8
> 
> My questions are : what means the leading 3 given by the device-mapper
> and which one is the right WWID?
> 
> Thank you a lot ;)
> 
> -- 
> Nicolas MICHEL
> 
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Brian Bunker
brian at purestorage.com