Multipath I/O provides failover and might improve performance. You can configure multiple physical I/O paths between server nodes and storage arrays into a single multipath device.
Multipathing thus aggregates the physical I/O paths, creating a new device that consists of the aggregated paths.
Linux multipathing provides I/O failover and path load sharing for multipathed block devices. In Linux, multipathing is implemented with multi-path tools that provide a user-space daemon for monitoring and an interface to the device mapper. The device-mapper, which provides a container for configurations, maps block devices to each other.
A single SCSI device (or a single zfcp unit) constitutes one physical path to the storage. The multipath user-space configuration tool scans sysfs for SCSI devices and then groups the paths into multipath devices. This mechanism that automatically puts each detected SCSI device underneath the correct multipath device is called coalescing.
Use a multipath setup to access SCSI storage in an FC SAN. The multipath device automatically switches to an alternate path in case of an interruption on the storage system controllers or due to maintenance on one path.
The multipath daemon has default configuration entries for most storage systems, and thus you only need do basic configuration for these systems. This chapter describes how to access, configure, and use FCP multipathing with Linux kernel 2.6 with minimal setup. This minimal setup uses the default configuration entries.