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76 DS8000 Series: Concepts and Architecture
Figure 4-7 also 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.