Disk Arrays - IBM SAS RAID Series Manual

given time. Using expanders creates more nodes in the path from the controller to the I/O device. If an
I/O device supports multiple ports, more than one path to the device can exist when there are expander
devices included in the path.
A SAS fabric refers to the summation of all paths between all SAS controller ports and all I/O device
ports in the SAS subsystem including cables, enclosures, and expanders.
The following example SAS subsystem shows some of the concepts described in this SAS overview. A
controller is shown with eight SAS phys. Four of those phys are connected into two different wide ports.
One connector contains four phys grouped into two ports. The connectors have no significance in SAS
other than causing a physical wire connection. The four-phy connector can contain between one and four
ports depending on the type of cabling that is used. The uppermost port in the figure shows a
controller-wide port number 6 that consists of phy numbers 6 and 7. Port 6 connects to an expander,
which attaches to one of the dual ports of the I/O devices. The dashed red line indicates a path between
the controller and an I/O device. Another path runs from the controller's port number 4 to the other port
of the I/O device. These two paths provide two different possible connections for increased reliability by
using redundant controller ports, expanders, and I/O device ports. The SCSI Enclosure Services (SES) is a
component of each expander.
Figure 7. Example SAS Subsystem

Disk arrays

Disk arrays are groups of disks that work together with a specialized array controller to take advantage of
potentially higher data transfer rates and data redundancy.
Disk arrays use RAID technology to offer data redundancy and to provide improved data transfer rates
over single large disks. If a disk failure occurs, the disk can typically be replaced without interrupting
normal system operation.
Data redundancy
The disk array controller tracks how the data is distributed across the disks. RAID 5 and RAID 6 disk
arrays provide data redundancy, ensuring that data is not lost if a disk in the array fails. If a disk failure
occurs, the disk can typically be replaced without interrupting normal system operations. System
mirroring provides data redundancy by mirroring the same data across pairs of disks.
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Chapter 2. SAS RAID controllers for IBM i