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The Compatibility of VIOS and IBM i

July 10, 2012

"What do you mean you can't see the disk?"

I was surprised. I'd just run mkvdev –vdev hdisk2 –vadapter vhost0 on my VIO server and mapped hdisk2 to an IBM i client, and the guy doing the install couldn't see the disk I just mapped to his LPAR.

I wondered why I was having VIOS manage these disks anyway. Why not give him the physical adapters and let him go nuts? There were two good reasons to go VIOS. First, multiple IBM i clients would be using the internal disk. Second, the company wanted to be positioned for the future. They planned to attach the IBM i partitions to a SAN.

I was working with a large RAID5 array made out of 6x600G internal SAS disks. It was nearly 3TB in size.

How do you create a big RAID array out of internal disks? In the VIOS I was logged in as padmin, and I'd assigned the storage adapters to the VIOS LPAR in the HMC GUI. In this instance we only had one VIOS, so it was fairly straightforward.

I ran diagmenu from the $ prompt and got into my normal diag screens that I'm familiar with in AIX.  From there I hit Enter, then I ran Task Selection>RAID array manager>IBM SAS Disk Array Manager.

The next step was to "Create an Array Candidate pdisk and Format to 528 Byte Sectors." I selected all of the relevant hdisks I was planning to use in the array and let diag format and then delete them as hdisks. They would then be recreated as pdisks. (This took a while, but I needed a break. I stepped outside to confirm that the sun still occupies the sky.)

Once the formatting/deleting/recreating was complete, I was able to "Create a SAS Disk Array" from within the IBM SAS Disk Array Manager menu. I chose RAID5 and my stripe size, and the new hdisk was created. Now we're back at the point where I mapped the hdisk that my IBM i admin couldn't see. So I unmapped the single large disk and carved it up into smaller logical volumes. I added hdisk2 into the datavg volume group using this command:

mkvg –vg datavg hdisk2

Then I made some smaller logical volumes to present to IBM i using this command:

mklv –lv disk1 datavg 500G

Once I'd carved up the logical volumes, I mapped them with this command:

mkvdev –vdev disk1 –vadapter vhost0

Lowering my logical disk sizes was the key. Once I did that and the remapping, all was well, and the IBM i admin was able to use the disk as presented.

While this is an older IBM Redbook, it has good information about IBM i and VIOS and disk limitations:

http://www.redbooks.ibm.com/redbooks/pdfs/sg247668.pdf

From Section 4.5.4: "The maximum available size of a logical drive for System i is 2 TB - 512 bytes. But for performance reasons we recommend much smaller sizes, as is described further in this section."

I seem to do more and more with IBM i. I guess these shops are recognizing how well IBM i and VIOS work together. That's a good thing for them, and it's good for us AIX users, too. Anytime we can work together, we can learn from one another.


Posted July 10, 2012 | Permalink

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