IBM RAID-5 overview, RAID-10 overview, RAID-5 theory, RAID-5 implementation in the DS8000

Figure 4-7also shows the connection paths for expansion on the far left and far right. The paths from the switches travel to the switches in the next disk enclosure. Because expansion is done in this linear fashion, the addition of more enclosures is completely non-disruptive.

4.6.2 RAID-5 overview

RAID-5 is one of the most commonly used forms of RAID protection.

RAID-5 theory

The DS8000 series supports RAID-5 arrays. RAID-5 is a method of spreading volume data plus parity data across multiple disk drives. RAID-5 provides faster performance by striping data across a defined set of DDMs. Data protection is provided by the generation of parity information for every stripe of data. If an array member fails, then its contents can be regenerated by using the parity data.

RAID-5 implementation in the DS8000

In a DS8000, a RAID-5 array built on one array site will contain either seven or eight disks depending on whether the array site is supplying a spare. A seven-disk array effectively uses one disk for parity, so it is referred to as a 6+P array (where the P stands for parity). The reason only 7 disks are available to a 6+P array is that the eighth disk in the array site used to build the array was used as a spare. This we then refer to as a 6+P+S array site (where the S stands for spare). An 8-disk array also effectively uses 1 disk for parity, so it is referred to as a 7+P array.

Drive failure

When a disk drive module fails in a RAID-5 array, the device adapter starts an operation to reconstruct the data that was on the failed drive onto one of the spare drives. The spare that is used will be chosen based on a smart algorithm that looks at the location of the spares and the size and location of the failed DDM. The rebuild is performed by reading the corresponding data and parity in each stripe from the remaining drives in the array, performing an exclusive-OR operation to recreate the data, then writing this data to the spare drive.

While this data reconstruction is going on, the device adapter can still service read and write requests to the array from the hosts. There may be some degradation in performance while the sparing operation is in progress because some DA and switched network resources are being used to do the reconstruction. Due to the switch-based architecture, this effect will be minimal. Additionally, any read requests for data on the failed drive requires data to be read from the other drives in the array and then the DA performs an operation to reconstruct the data.

Performance of the RAID-5 array returns to normal when the data reconstruction onto the spare device completes. The time taken for sparing can vary, depending on the size of the failed DDM and the workload on the array, the switched network, and the DA. The use of arrays across loops (AAL) both speeds up rebuild time and decreases the impact of a rebuild.

4.6.3 RAID-10 overview

RAID-10 is not as commonly used as RAID-5, mainly because more raw disk capacity is needed for every GB of effective capacity.

76DS8000 Series: Concepts and Architecture