IBM DS8000 manual Protected versus unprotected volumes, Changing LUN protection

Model type		OS/400	Number	Extents	Unusable	Usable
		Device	of LBAs		space	space%
Unprotected	Protected
		size (GB)			(GiB)
2107-A05	2107-A85	35.1	68,681,728	33	0.25	99.24
2107-A04	2107-A84	70.5	137,822,208	66	0.28	99.57
2107-A06	2107-A86	141.1	275,644,416	132	0.56	99.57
2107-A07	2107-A87	282.2	551,288,832	263	0.13	99.95

Note: In Table B-1,GiB represents “Binary Gigabytes” (230 bytes) and GB represents “Decimal Gigabytes” (109 bytes)

When creating the logical volumes for use with OS/400, you will see that in almost every case, the OS/400 device size doesn’t match a whole number of extents, and so some space will be wasted. You should use the figures in Table B-1 on page 374 in conjunction with Figure 9-8 on page 176 to see how much space will be wasted for your specific configuration. You should also note that the #2766 and #2787 Fibre Channel Disk Adapters used by iSeries can only address 32 LUNs, so creating more, smaller LUNs will require more IOAs and their associated IOPs. For more sizing guidelines for OS/400, refer to “Sizing guidelines” on page 396.

Protected versus unprotected volumes

When defining OS/400 logical volumes, you must decide whether these should be protected or unprotected. This is simply a notification to OS/400 – it does not mean that the volume is protected or unprotected. In reality, all DS8000 LUNs are protected, either RAID-5 or RAID-10. Defining a volume as unprotected means that it is available for OS/400 to mirror that volume to another of equal capacity — either internal or external. If you do not intend to use OS/400 (host based) mirroring, you should define your logical volumes as protected.

Under some circumstances, you may wish to mirror the OS/400 Load Source Unit (LSU) to a LUN in the DS8000. In this case, only one LUN should be defined as unprotected; otherwise, when mirroring is started to mirror the LSU to the DS8000 LUN, OS/400 will attempt to mirror all unprotected volumes.

Changing LUN protection

It is not possible to simply change a volume from protected to unprotected, or vice versa. If you wish to do so, you must delete the logical volume. This will return the extents used for that volume to the extent pool. You will then be able to create a new logical volume with the correct protection. This is unlike ESS E20, F20, and 800, where the entire array containing the logical volume had to be reformatted.

However, before deleting the logical volume on the DS8000, you must first remove it from the OS/400 configuration (assuming it was still configured). This is an OS/400 task which is disruptive if the disk is in the System ASP or User ASPs 2-32 because it requires an IPL of OS/400 to completely remove the volume from the OS/400 configuration. This is no different from removing an internal disk from an OS/400 configuration. Indeed, deleting a logical volume on the DS8000 is similar to physically removing a disk drive from an iSeries. Disks can be removed from an Independent ASP with the IASP varied off without IPLing the system.

Appendix B. Using DS8000 with iSeries 375