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74 DS6000 Series: Concepts and Architecture
Figure 4-7 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 DS6000 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 DS6000 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,
17.5 GB, 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 (10
9
). These capacities should be converted to GB (2
30
) when
considering effective utilization of extents which are 1 GB (2
30
). For more information on this
topic see Appendix B, “Using the DS6000 with iSeries” on page 329.
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 DS6000 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
1 GB 1 GB 1 GB
1 GB
free
Extent Pool FBprod
Rank-a
Rank-b
3 GB LUN
1 GB
free
1 GB
free
used used
Allocate a 3 GB LUN
1 GB 1 GB 1 GB
1 GB
used
Extent Pool FBprod
Rank-a
Rank-b
3 GB LUN
1 GB
used
1 GB
used
used used
Logical
3 GB LUN
2.9 GB LUN
created
100 MB unused