IBM DS6000 Series manual Disk subsystem RAS, RAID-5 overview, RAID-5 theory

A physical FICON path is established when the DS6800 port sees light on the FICON fiber (for example, a cable is plugged in to a DS6800 host adapter, or a processor, or the DS6800 is powered on, or a path is configured online by OS/390). At this time, logical paths are established through the FICON port between the host and some or all of the LCUs in the DS6800, controlled by the HCD definition for that host. This happens for each physical path between a zSeries CPU and the DS6800. There may be multiple system images in a CPU. Logical paths are established for each system image. The DS6800 then knows which FICON paths can be used to communicate between each LCU and each host.

Provided you have the correct maintenance level, all major zSeries operating systems should support preferred path (z/OS, z/VM, VSE/ESA™, TPF).

CUIR

Control Unit Initiated Reconfiguration (CUIR) prevents loss of access to volumes in zSeries environments due to wrong path handling. This function automates channel path management in zSeries environments in support of selected DS6800 service actions.

Control Unit Initiated Reconfiguration is available for the DS6800 when operating in the z/OS and z/VM environments. The CUIR function automates channel path vary on and vary off actions to minimize manual operator intervention during selected DS6800 service actions.

CUIR allows the DS6800 to request that all attached system images set all paths required for a particular service action, to the offline state. System images with the appropriate level of software support will respond to such requests by varying off the affected paths, and either notifying the DS6800 subsystem that the paths are offline, or that it cannot take the paths offline. CUIR reduces manual operator intervention and the possibility of human error during maintenance actions, at the same time reducing the time required for the maintenance. This is particularly useful in environments where there are many systems attached to a DS6800.

Note: CUIR support will be included in a future release of microcode.

3.3 Disk subsystem RAS

The DS6000 currently supports only RAID-5 and RAID-10. It does not support non-RAID configurations of disks (JBOD - just a bunch of disks).

3.3.1 RAID-5 overview

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

RAID-5 theory

The DS6000 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 DS6000

In a DS6000, a RAID-5 array built on one array site will contain either three disks or four disks, depending on whether the array site chosen had a pre-allocated spare. A three disk array effectively uses 1 disk for parity, so it is referred to as a 2+P array (where the P stands for parity). The reason only three disks are available to a 2+P array is that the fourth disk in the array site used to build the array, was used as a spare. This can be referred to as a 2+P+S

52DS6000 Series: Concepts and Architecture