Any subsequent failure during the reconstruction within the same array (second drive failure, second coincident medium errors, or a drive failure and a medium error) can be recovered without data loss. 

Performance of the RAID 6 array returns to normal when the data reconstruction on the spare drive is complete. The rebuild time varies, depending on the capacity of the failed drive and the workload on the array and the DA. The completion time is comparable to a RAID 5 rebuild, but slower than rebuilding a RAID 10 array in a single drive failure. 

## 3.5.7 RAID 10 overview 

RAID 10 provides high availability by combining features of RAID 0 and RAID 1. RAID 0 optimizes performance by striping volume data across multiple drives. RAID 1 provides drive mirroring, which duplicates data between two drives. By combining the features of RAID 0 and RAID 1, RAID 10 provides a second optimization for fault tolerance. Data is striped across half of the drives in the RAID 1 array. The same data is also striped across the other half of the array, which creates a mirror. Access to data is preserved if one drive in each mirrored pair remains available. RAID 10 offers faster data reads and writes than RAID 6 or RAID 5 because it does not need to manage parity. However, with half of the drives in the group used for data and the other half mirroring that data, RAID 10 arrays have less usable capacity than RAID 6 or RAID 5 arrays. 

Note:  RAID 10 is not supported for the 7.68 TB and 15.36 TB High-Capacity Flash drives 

RAID 10 is commonly used for workloads that require the highest performance from the drive subsystem. Although with RAID 6, each front-end random write I/O might theoretically lead to 6 back-end I/Os including the parity updates (RAID penalty), this number is 4 for a RAID 5 and only 2 for a RAID 10 (not counting cache optimizations). Typical areas of operation for RAID 10 are workloads with a high random-write ratio. Either member in the mirrored pair can respond to the read requests. 

## RAID 10 implementation in DS8880 

In the DS8880, the RAID 10 implementation is achieved by using six or eight drives. If spares need to be allocated from the array site, six drives are used to make a three-drive RAID 0 array, which is then mirrored to a three-drive array (3x3). If spares do not need to be allocated, eight drives are used to make a four-drive RAID 0 array, which is then mirrored to a four-drive array (4x4). 

## Drive failure with RAID 10 

When a drive fails in a RAID 10 array, the DA rejects the failing drive and takes a hot spare into the array synthesists. Data is then copied from the good drive to the hot spare drive. The spare that is used is chosen based on a smart algorithm that looks at the location of the spares and the size and location of the failed drive. Remember, a RAID 10 array is effectively a RAID 0 array that is mirrored. Therefore, when a drive fails in one of the RAID 0 arrays, you can rebuild the failed drive by reading the data from the equivalent drive in the other RAID 0 array. 

While this data copy is occurring, the DA can still service read and write requests to the array from the hosts. Performance might degrade while the copy operation is in progress because DA and switched network resources are used to rebuild the RAID. Because a good drive is available, this effect is minimal. Read requests for data on the failed drive likely are not affected because they are all directed to the good copy on the mirrored drive. Write operations are not affected. 

