GLYPH<SM590000> RAS LIC component starts Smart Rebuild after the Smart Rebuild threshold criteria are met. The Smart Rebuild process runs every hour and does not wait for 24 hours as in the past. 

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. 

Important:  Even with additional optimizations like Smart Rebuild, given the relative probabilities of a dual-drive failure in a RAID 5 in combination with potentially long rebuild times of increasingly large-capacity drives, RAID 6 or RAID 10 is preferred to achieve the highest availability levels possible with such a storage system. Consult your IBM representative if you still need RAID 5. 

## 3.5.9 Spare creation 

This section describes methods of spare creation. 

## Standard drive enclosures 

When the arrays are created on a DS8880 standard drive enclosure, the LIC determines the array sites that contain spares. The first array sites on each 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 four spares, with two spares placed on each loop. 

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 four spares per DA pair exist. 

GLYPH<SM590000> A minimum of four spares for the largest capacity array site on the DA pair exist. 

GLYPH<SM590000> A minimum of two spares of capacity and revolutions per minute (RPM) greater than or equal to the fastest array site of any capacity on the DA pair exist. 

## Spare rebalancing 

The DS8880 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, DS8880 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 FC-AL loops 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 disk drive module (DDM) is migrated in the background onto an existing spare by using the Smart Rebuild technique. For more information, see 3.5.4, “Disk scrubbing” on page 92. This process does not fail the disk that is being migrated. However, the process reduces the number of available spares in the DS8880 until the migration process is complete. 

In a drive intermix on a standard DA pair, it is possible to rebuild the contents of a 300 GB drive onto a 600 GB spare drive. However, approximately one-half of the 600 GB drive is wasted because that space cannot be used. When the failed 300 GB drive is replaced with a new 300 GB drive, the DS8880 LIC migrates the data back onto the recently replaced 300 GB drive. 

