8456ch_RAS.fm Draft Document for Review January 13, 2020 5:02 pm the array rather than being rejected as under a standard RAID rebuild. A spare is brought into  the array, as an additional member, at the same time.  Note:  Smart Rebuild (SMRB) for flash drives is newly available for DS8900F. It is available  for RAID 6 arrays only.  The suspect drive and the new member-spare are set up in a temporary RAID 1 association,  allowing the troubled drive to be duplicated onto the spare rather than running a full RAID  reconstruction (rebuild) from data and parity. The new member-spare is then made a regular  member of the array and the suspect drive is rejected from the RAID array. The array never  goes through an n-1 stage in which it might suffer a complete failure if another drive in this  array encounters errors. The result saves substantial time and provides a new level of  availability that is not available in other RAID products. Smart Rebuild is not applicable in all situations, so it is not always used. Smart Rebuild runs  only for healthy RAID 6 arrays. If two drives with errors are in a RAID 6 configuration, or if the  drive mechanism failed to the point that it cannot accept any I/O, the standard RAID rebuild  procedure is used for the RAID array. If communications across a drive fabric are  compromised, such as an SAS path link error that causes the drive to be bypassed, standard  RAID rebuild procedures are used because the suspect drive is not available for a one-to-one  copy with a spare. If Smart Rebuild is not possible or does not provide the designed benefits,  a standard RAID rebuild occurs. Smart Rebuild drive error patterns are continuously analyzed as part of one of the normal  tasks that are run by the DS8900F LIC. Drive firmware is optimized to report predictive errors  to the DA. At any time, when certain drive errors (following specific criteria) reach a  determined threshold, the RAS LIC component starts Smart Rebuild within the hour. This  enhanced technique, when it is combined with a more frequent schedule, leads to  considerably faster identification if drives show signs of imminent failure. A fast response in fixing drive errors is vital to avoid a second drive failure in the same RAID 6  array, and to avoid potential data loss. The possibility of having an array that has no  redundancy, such as when a RAID rebuild occurs, is reduced by shortening the time when a  specific error threshold is reached until Smart Rebuild is triggered, as described in the  following scenarios: There is another usage for Smart Rebuild. The Smart Rebuild is also used to fix migration  conditions after a repair or rebuild in an array occurred. If too many members or spares are  unevenly distributed over the dual paths of the two enclosures in a pair after a repair then the  array members will be balanced in the enclosure pair by using the Smart Rebuild copy  function. Also if different capacity flash drives are used in the arrays of an flash enclosure pair  and a large spare jumps in for a smaller capacity drive array, the Smart Rebuild will correct  this situation later if the failing drive is replaced with the origin capacity. Once Smart Rebuild  completes, removed member by this process will be resumed as spare.  GLYPH<SM590000> Smart Rebuild might avoid the circumstance in which a suspected drive is rejected  because the Smart Rebuild process is started before rejection. Therefore, Smart Rebuild  prevents the array from going to a standard RAID rebuild, during which the array has no  redundancy and is susceptible to data loss if second drive failure occurs. GLYPH<SM590000> Crossing the drive error threshold is detected by DS8900F LIC immediately because LIC  continuously analyzes drive errors.  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. 90  IBM DS8900F Architecture and Implementation