Edit: Do you still manage disks manually?
Originally posted December 2, 2014 on AIXchange
A customer was looking to assign SAN LUNs to a pair of VIO servers using vSCSI. When the VIO servers were configured, VIO1 was assigned adapters containing odd numbers of internal disks:
VIO1
lsdev -Cc disk
hdisk0 Available 04-00-00 SAS RAID 0 SSD Array
hdisk1 Available 04-00-00 SAS RAID 0 SSD Array
hdisk2 Available 04-00-00 SAS RAID 0 SSD Array
hdisk3 Available 0C-00-00 SAS Disk Drive
hdisk4 Available 0H-00-00 SAS Disk Drive
VIO2
lsdev –Cc disk
hdisk0 Available 05-00-00 SAS Disk Drive
hdisk1 Available 06-00-00 SAS Disk Drive
Because of this, the shared LUNs would be out of sync when they were allocated to the VIO servers. On VIO1, the next hdisk number would be hdisk5, while on VIO2, the next hdisk number would be hdisk2. The customer wanted to keep the hdisk numbers consistent on the two servers to simplify the process of mapping the LUNs to client LPARs. Consistent numbering would also make it a bit easier to conduct any future troubleshooting.
There was no shortage of ways to resolve this issue. The SAN team could assign some small LUNs to VIO1 only. That way both servers would have the same number of hdisks once mapping of the “real” shared LUNs began.
I found more options online (see here, here and here). Unfortunately, we couldn’t get any of them to work.
Finally, I was shown this blog post on dummy devices, which includes the following command:
mkdev -l hdisk100 -c disk -s sas -t scsd -p sas0 -w sas –d
Sure enough, this did the job:
hdisk100 Defined
lsdev –Cc disk
hdisk100 Defined 00-00-00-00 MPIO Other SAS Disk Drive
Basically, this method allows you, the admin, to add in all the dummy hdisks you need without involving SAN personnel.
When you’re using vSCSI, do you take the time to keep your hdisks in sync? In what other instances do you find it necessary to create a dummy device? How do you go about it?