IBM DS8000 manual ISeries LUNs, Allocation and deletion of LUNs/CKD volumes

Extent Pool FBprod

Rank-a	1 GB	1 GB	1 GB	1 GB
	3	GB LUN		free
Rank-b
	used	1 GB	used	1 GB
		free		free

Logical

3 GB LUN

Allocate a 3 GB LUN

Rank-a

Rank-b

Extent Pool FBprod
1 GB	1 GB	1 GB 1 GB
			used
3 GB LUN			2.9 GB LUN
			created
used	1 GB	used	1 GB
	used		used
			100 MB unused

Figure 5-8 Creation of an FB LUN

iSeries LUNs

iSeries LUNs are also composed of fixed block 1 GB extents. There are, however, some special aspects with iSeries LUNs. LUNs created on a DS8000 are always RAID protected. LUNs are based on RAID-5 or RAID-10 arrays. However, you might want to deceive OS/400® and tell it that the LUN is not RAID protected. This causes OS/400 to do its own mirroring. iSeries LUNs can have the attribute unprotected, in which case the DS8000 will lie to an iSeries host and tell it that the LUN is not RAID protected.

OS/400 only supports certain fixed volume sizes, for example model sizes of 8.5 GB,

17.5GB, and 35.1 GB. These sizes are not multiples of 1 GB and hence, depending on the model chosen, some space is wasted. iSeries LUNs expose a 520 byte block to the host. The operating system uses 8 of these bytes so the usable space is still 512 bytes like other SCSI

LUNs. The capacities quoted for the iSeries LUNs are in terms of the 512 byte block capacity and are expressed in GB (109). These capacities should be converted to GB (230) when considering effective utilization of extents that are 1 GB (230). For more information on this topic see Appendix B, “Using DS8000 with iSeries” on page 373.

Allocation and deletion of LUNs/CKD volumes

All extents of the ranks assigned to an extent pool are independently available for allocation to logical volumes. The extents for a LUN/volume are logically ordered, but they do not have to come from one rank and the extents do not have to be contiguous on a rank. The current extent allocation algorithm of the DS8000 will not distribute the extents across ranks. The algorithm will use available extents within one rank, unless there are not enough free extents available in that rank, but free extents in another rank of the same extent pool. While this algorithm exists, the user may want to consider putting one rank per extent pool to control the

Chapter 5. Virtualization concepts 93