Re: [Scst-devel] Algorithm for the exposed WWN

[Bart V. A. <bva...@ac...>]

On 01/17/13 15:22, Bigby wrote:
> at initiator side (open-iscsi) I need to guarantee a fixed WWN for a
> certain scst device (which is currently coming from /dev/vdb at scst
> side but in the future might be /dev/vdc,  or /dev/sdb or whatever... ).
> That WWN must never change even if I change the LU-Number, add
> additional devices to the target, change the backing "filename" or
> something like that . In other words, I want to decide which exported LU
> gets a certain WWN.
>
> The WWN I am seeing from client side seems a hash of something, and the
> scst manual is not 100% clear on which factors contribute to such hash .
> I have found this doc of 2010 https://lkml.org/lkml/2010/9/14/197 which
> says that it depends on a module parameter (ok I will not change that
> value) and from a seemingly random value which is generated at device
> creation time and is probably made persistent by writing it somewhere
> (where?) ... ok I would like to prevent random things made persistent
> out of my control so I would like to override that...
>
> My scst.conf is like this:
>
> --------------
> HANDLER vdisk_blockio {
>      DEVICE mydisk1 {
>           filename /dev/vdb
>           t10_dev_id "myid1"
>      }
> }
> .....
> TARGET_DRIVER iscsi {
>           enabled 1
>           TARGET {......
>           .....}
> }
> --------------
>
> by specifying the t10_dev_id manually like above inside the device
> "mydisk1", is it guaranteed that the WWN will always stay the same for
> "mydisk1" even after reconfigurations of other parts of scst.conf ?

If you set the t10_dev_id explicitly in scst.conf then you are safe - 
the first eight characters of that ID are what is communicated to the 
initiator as the SCSI ID of that LUN. On the initiator side, 
/lib/udev/scsi_id will use these first eight characters, encode these in 
hex, prepend a "2" and will use that as the SCSI ID.

The "setup_id" parameter only influences the t10_dev_id if it has not 
been specified explicitly in scst.conf.

Bart.