Novell LINUX ENTERPRISE SERVER 10 - STORAGE ADMINISTRATION GUIDE 7-2007 Administration Manual

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Novell LINUX ENTERPRISE SERVER 10 - STORAGE ADMINISTRATION GUIDE 7-2007 Administration Manual Manual pdf 45 page image

Managing Multipath I/O for Devices 45novdocx (en) 6 April 20075.2.1 Device Mapper Multipath I/O ModuleThe Device Mapper Multipath I/O (DM-MPIO) module provides the multipathing capability forLinux. Multipath protects against failures in the paths to the device, and not failures in the deviceitself. If one of the paths is lost (for example, a network adapter breaks or a fiber-optic cable isremoved), I/O will be redirected to the remaining paths. If an active path fails, the DM continues tobalance traffic across the healthy paths. If all active paths fail, inactive secondary paths must bewaked up, so failover occurs with a delay of approximately 30 seconds.Table 5-1 Multipath I/O FeaturesDevice Mapper detects every path for a multipathed device as a separate SCSI device. The SCSIdevice names take the form /dev/sdN, where N is an autogenerated letter for the device, beginningwith a and issued sequentially as the devices are created, such as /dev/sda, /dev/sdb, and soon. If the number of devices exceeds 26, the letters are duplicated such that the next device after /dev/sdz will be named /dev/sdaa, /dev/sdab, and so on.If multiple paths are not automatically detected, you can configure them manually in the /etc/multipath.conf file.5.2.2 Multipath I/O Management ToolsThe multipath-tools user-space package takes care of automatic path discovery and grouping.It automatically tests the path periodically, so that a previously failed path is automatically reinstatedwhen it becomes healthy again. This minimizes the need for administrator attention in a productionenvironment.Features DescriptionActive/passive If the storage array has multiple controllers, and only one controller isactive at a time, then only the paths from the host to the active storagecontroller are active. Connections to the second and subsequentcontrollers are passive.Active/active If the storage array has multiple controllers that are concurrently active, allconnections from the host to the controllers are active and treated equallyin a load-balanced setup.Load balancing The Device Mapper driver automatically load balances traffic across allactive paths.Controller failover When the active controller fails over to the passive, or standby, controller,the Device Mapper driver automatically activates the paths between thehost and the standby, making them the primary paths. When the failedprimary controller is reactivated as primary, the Device Mapper driver alsoactivates the previously-downed paths.Boot/Root device support Multipathing is supported for the root (/) device in SUSE Linux EnterpriseServer 10 and later.