Multi-initiator and high availability
You can increase availability using multi-initiator and high availability to connect multiple controllers to a common set of disk expansion drawers.
The terms multi-initiator and high availability (HA) in the SAS arena refer to connecting multiple controllers (typically two controllers) to a common set of disk expansion drawers for the purpose of increasing availability. IBM® SAS RAID controllers support high availability with up to two controllers in one or two systems.
IBM SAS RAID controllers support the following configurations:
- HA two-system configuration
An HA two-system configuration provides a high-availability environment for system storage by enabling two systems or partitions to have access to the same set of disks and disk arrays. This feature is typically used with high availability cluster applications such as the IBM PowerHA® SystemMirror®. The IBM HACMP™ software provides a commercial computing environment that ensures that mission-critical applications can recover quickly from hardware and software failures.
The HA two-system configuration is intended for using disk arrays. The disks must be formatted to RAID format. Any RAID level, or combination of RAID levels, can be used.
Use of disks without RAID (referred to as JBOD) is also possible. The disks must be formatted to JBOD format. This JBOD alternative is supported only on particular controllers and requires unique setup and cabling.
- HA single-system configuration
An HA single-system configuration provides for redundant controllers from a single system to the same set of disks and disk arrays. This feature is typically used with Multi-Path I/O (MPIO). MPIO support in Linux is provided by Device-Mapper Multipath and can be used to provide a redundant IBM SAS RAID controller configuration with RAID-protected disks.
When using an HA single-system configuration, the disks must be formatted to RAID format and used in one or more disk array. Any RAID level, or combination of RAID levels, may be used. Disks formatted to JBOD format are not supported in an HA single-system configuration.
Disks formatted to 512 bytes per sector are allowed in an HA single-system configuration. These disks are detected through both SAS controllers with two different paths and assigned with two different /dev/sdX names, for example, /dev/sde and /dev/sdf. The disks can be accessed using either path. However, there is no additional coordination between the two SAS controllers to automatically switch between the two paths. Multipath IO software such as Device-Mapper Multipath needs to be configured to monitor and switch paths. Using Device-Mapper Multipath to provide HA function is a software-only solution that is independent of IBM SAS RAID controllers. Its use is beyond the scope of this document.
Not all controllers support all configurations. Refer to the comparison tables and look for controllers that have the entry Yes for HA two-system RAID, HA two-system JBOD, or HA single-system RAID.
