4.4.2 Logical volumes Fixed-block LUNs Chapter 4. Virtualization concepts  115 Figure 4-7 Creation of a fixed-block LUN The allocation process for FB volumes is illustrated in Figure 4-7. LUNs can be allocated in binary GiB (2$^{30}$ bytes), decimal GB (10$^{9}$ bytes), or 512 or 520-byte  blocks. However, the physical capacity that is allocated is a multiple of 1 GiB. For small  extents, it is a multiple of 16 MiB. Therefore, it is a good idea to use LUN sizes that are a  multiple of a gibibyte or a multiple of 16 MiB. If you define a LUN with a size that is not a  multiple of 1 GiB (for example, 25.5 GiB), the LUN size is 25.5 GiB. However, 26 GiB are  physically allocated, of which 0.5 GiB of the physical storage will be unusable. When you want  to specify a LUN size that is not a multiple of 1 GiB, specify the number of blocks. A 16 MiB  extent has 32768 blocks. Important:  DS8880 Copy Services do not support FB logical volumes larger than 2 TiB  (2 x 2$^{40}$ bytes). Do not create a LUN that is larger than 2 TiB if you want to use Copy  Services for the LUN, unless the LUN is integrated as Managed Disks in an IBM SAN  Volume Controller. Use IBM Spectrum Virtualize™ Copy Services instead. A logical volume that is composed of fixed-block extents is called a LUN. A fixed-block LUN is  composed of one or more 1 GiB (2$^{30}$ bytes) large extents or one or more 16 MiB small  extents from one FB extent pool. A LUN cannot span multiple extent pools, but a LUN can  have extents from multiple ranks within the same extent pool. You can construct LUNs up to a  size of 16 TiB (16 x 2$^{40}$ bytes, or 2$^{44}$ bytes) when using large extents. A  logical volume  consists of a set of extents from one extent pool. The DS8880 supports up  to 65,280 logical volumes (64 K CKD, or 64 K FB volumes, or a mixture of both, up to a  maximum of 64 K total volumes). The abbreviation 64 K is used in this section, even though it  is 65,536 minus 256, which is not quite 64 K in binary. 1 GB 1 GB 1 GB 1 GB free Extent Pool FBprod Ra n k-a Ra n k-b G B  LU N 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 Ra n k-a Ra n k-b G B  LU N 1 GB used 1 GB used used used Logical  3 GB LUN 2.9 GB LUN created 100 MB unused