As is the case for HDDs, the SSD RAID array format helps to protect against individual SSD failures. Depending on your requirements, you can achieve more high availability (HA) protection above the RAID level by using volume mirroring. 

The internal storage configuration of flash arrays can differ depending on an array vendor. Regardless of the methods used to configure flash-based storage, the flash system maps a volume to a host, in this case to the IBM Spectrum Virtualize or Storwize system. From the IBM Spectrum Virtualize or Storwize system perspective, a volume presented from a flash storage is also seen as a normal managed disk. 

Starting with SVC 2145-DH8 nodes and software version 7.3, up to two expansion drawers can be connected to the one IBM Spectrum Virtualize I/O Group. Each drawer can have up to 24 SDDs, and only SDD drives are supported. The SDD drives are then gathered together to form RAID arrays in the same way that RAID arrays are formed in the IBM Storwize systems. 

After creation of an SDD RAID array, it appears as a usual MDisk but with a tier of tier0_flash or tier1_flash , which differs from MDisks presented from external storage systems or RAID arrays made of HDDs. Because IBM Spectrum Virtualize/Storwize does not know what kind of physical disks that external MDisks are formed from, the default MDisk tier that the system adds to each external MDisk is tier2_hdd . It is up to the user or administrator to change the tier of MDisks to tier0_flash , tier1_flash , tier2_hdd , or tier3_nearline . 

To change a tier of an MDisk in the CLI, use the chmdisk  command as in Example 7-1. 

IBM_2145:SVC_ESC:superuser>lsmdisk -delim " " id name status mode mdisk_grp_id mdisk_grp_name capacity ctrl_LUN_# controller_name UID tier encrypt site_id site_name distributed dedupe 1 mdisk1 online managed 1 POOL_V7K_SITEB 250.0GB 0000000000000001 V7K_SITEB_C2 6005076802880102c00000000000002000000000000000000000000000000000 tier2_hdd  no 2 SITE_B no no 2 mdisk2 online managed 1 POOL_V7K_SITEB 250.0GB 0000000000000002 V7K_SITEB_C2 6005076802880102c00000000000002100000000000000000000000000000000 tier2_hdd no 2 SITE_B no no IBM_2145:SVC_ESC:superuser> chmdisk -tier tier3_nearline 1 IBM_2145:SVC_ESC:superuser>lsmdisk -delim " " id name status mode mdisk_grp_id mdisk_grp_name capacity ctrl_LUN_# controller_name UID tier encrypt site_id site_name distributed dedupe 1 mdisk1 online managed 1 POOL_V7K_SITEB 250.0GB 0000000000000001 V7K_SITEB_C2 6005076802880102c00000000000002000000000000000000000000000000000 tier3_nearline  no 2 SITE_B no no 2 mdisk2 online managed 1 POOL_V7K_SITEB 250.0GB 0000000000000002 V7K_SITEB_C2 6005076802880102c00000000000002100000000000000000000000000000000 tier2_hdd no 2 SITE_B no no 

It is also possible to change the MDisk tier from the graphical user interface (GUI), but this only applies to external MDisks. To change the tier, complete the following steps: 

1. Click Pools →  External Storage and click the Plus  sign ( + ) next to the controller that owns the MDisks for which you want to change the tier. 

2. Right-click the wanted MDisk and select Modify Tier (Figure 7-4 on page 313). 

