Draft Document for Review January 13, 2020 5:02 pm 8456ch_RAS.fm IBM remote support representatives can manually start a Smart Rebuild if needed, such as  when two drives in the same array logged temporary media errors. In this case, it is  considered appropriate to manually start the rebuild.  3.5.11 Spare creation This section describes methods of spare creation. Flash drive enclosures Spare rebalancing Hot pluggable drives Enhanced sparing When the arrays are created on a DS8900F flash drive enclosure, the LIC determines the  array sites that contain spares. The first array sites on each flash RAID controller (DA) pair  that are assigned to arrays contribute one or two spares (depending on the RAID option) until  the DA pair has access to at least two spares, with spares placed on each enclosure in the  enclosure pair.  A minimum of one spare is created for each array site that is assigned to an array until the  following conditions are met: GLYPH<SM590000> A minimum of two spares per DA pair exist. GLYPH<SM590000> A minimum of two spares for the largest capacity array site on the DA pair exist. The DS8900F implements a spare rebalancing technique for spare drives. When a drive fails  and a hot spare is taken, it becomes a member of that array. When the failed drive is repaired,  DS8900F LIC might choose to allow the hot spare to remain where it was moved. However, it  can instead choose to migrate the spare to a more optimum position. This migration is  performed to better balance the spares across the two dual flash enclosure paths to provide  the optimum spare location based on drive capacity and spare availability.  It might be preferable that the drive that is in use as an array member is converted to a spare.  In this case, the data on that flash drive module is migrated in the background onto an existing  spare by using the Smart Rebuild technique. For more information, see 3.5.5, “Disk  scrubbing” on page 85 and 3.5.10, “Smart Rebuild” on page 89. This process does not fail the  disk that is being migrated. However, the process reduces the number of available spares in  the DS8900F until the migration process is complete.  In a flash drive intermix on a DA pair, it is possible to rebuild the contents of a smaller flash  drive onto a larger spare drive. When the failed origin flash drive is replaced with a new drive,  the DS8900F LIC migrates the data back onto the recently replaced drive.  When this process completes, the smaller flash drive rejoins the array and the larger drive  becomes the spare again. This same algorithm applies when the hot spare that is taken at the  time of the initial drive failure has a tier mismatch. Replacement of a failed flash drive does not affect the operation of the DS8900F because the  drives are fully hot pluggable. Each drive plugs into a SAS expander switch, so no path break  is associated with the removal or replacement of a drive. In addition, no potentially disruptive  loop initialization process occurs. The drive sparing policies support having spares for all capacity and speed drives on the DA  pair. When any DA pair has only a single spare for any drive type, a call home to IBM is  generated. Because of spare over-allocation, several drives can be in a Failed/Deferred  Chapter 3. DS8900 reliability, availability, and serviceabilty  91