Intel AS/400 RISC Server manual Chdev -dev hdisk03 -attr pv=yes will assign a Pvid to hdisk3

14.5.1.3Specific VIOS Configuration Recommendations -- Traditional (non-blade) Machines

1.Avoid volume groups if possible. VIOS "hdisks" must have a volume identifier (PVID). Creating a volume group is an easy way to assign one and some literature will lead you to do it that way. However, the volume group itself adds overhead for no particular value in a typical IBM i operating system context where physical volumes (or, at least, RAID sets) are exported as a whole without any sort of partitioning or sub-setting. Volume groups help multiple clients share the same physical disks. In an IBM i operating system setting, this is seldom relevant and the overhead volume groups employ is therefore not needed. It is better to assign a PVID by simply changing the attribute of each individual hdisk. For instance, the VIOS command:

chdev -dev hdisk03 -attr pv=yes will assign a PVID to hdisk3.

2.For VIOS disks, use available location information to aid your RAID planning. To obtain RAID sets in IBM i operating system, you simply point DST at particular groups you want and IBM i operating system decides which disks go together. Under VIOS, for internal disks, you have to do this yourself. The names help show you what to do. For instance, suppose VIOS shows the following for a set of internal disks:

Name	Location	State	Description	Size
pdisk0	07-08-00-2,0 Active		Array Member	35.1GB
pdisk1	07-08-00-3,0 Active		Array Member	35.1GB
pdisk2	07-08-00-4,0 Active		Array Member	35.1GB
pdisk3	07-08-00-5,0 Active		Array Member	35.1GB
pdisk4	07-08-00-6,0	Active	Array Member	35.1GB
pdisk5	07-08-01-0,0	Active	Array Member	35.1GB
pdisk6	07-08-01-1,0	Active	Array Member	35.1GB

Here, it turns out that these particular physical disks are on two internal SCSI buses (00 and 01) and have device IDs of 2, 3, 4, 5, and 6 on SCSI bus 00 and device IDs of 0 and 1 on SCSI bus

01.If this was all there was, a three disk RAID set of pdisk0, pdisk1, and pdisk5 would be a good choice. Why? Because pdisk0 and 1 are on internal SCSI bus 00 and the other one is one SCSI bus 01. That provides a good balance for the available drives. This could also be repeated for pdisk2, pdisk3, pdisk4, and pdisk6. This would result in two virtual drives being created to represent the seven physical drives. The fact that these are two RAID5 disk sets (of three and four physical disks, respectively) would be unknown to IBM i operating system, but managed instead by VIOS. One or may be two virtual SCSI buses would be required to present them to IBM i operating system by VIOS. A large configuration could provide for RAID5 balance over even more SCSI buses (real and virtual).

On external storage, the discussion is slightly more complicated, because these products tend to package data into LUNs that already involve multiple physical drives. Your RAID set work would have to use whatever the external disk storage product gives you to work with in terms of naming conventions and what degree of control you have available to reflect favorable physical boundaries. Still, the principles are the same.

IBM i 6.1 Performance Capabilities Reference - January/April/October 2008
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