vsphere-esxi-vcenter-server-55-storage-guide

Understanding Multipathing and23FailoverTo maintain a constant connection between a host and its storage, ESXi supports multipathing. Multipathingis a technique that lets you use more than one physical path that transfers data between the host and anexternal storage device.In case of a failure of any element in the SAN network, such as an adapter, switch, or cable, ESXi can switchto another physical path, which does not use the failed component. This process of path switching to avoidfailed components is known as path failover.In addition to path failover, multipathing provides load balancing. Load balancing is the process ofdistributing I/O loads across multiple physical paths. Load balancing reduces or removes potentialbottlenecks.NOTE Virtual machine I/O might be delayed for up to sixty seconds while path failover takes place. Thesedelays allow the SAN to stabilize its configuration after topology changes. In general, the I/O delays mightbe longer on active-passive arrays and shorter on active-active arrays.This chapter includes the following topics:• “Failover with Fibre Channel,” on page 215• “Host-Based Failover with iSCSI,” on page 216• “Array-Based Failover with iSCSI,” on page 218• “Path Failover and Virtual Machines,” on page 219• “Managing Multiple Paths,” on page 220• “VMware Multipathing Module,” on page 221• “Path Scanning and Claiming,” on page 223• “Managing Storage Paths and Multipathing Plug-Ins,” on page 226Failover with Fibre ChannelTo support multipathing, your host typically has two or more HBAs available. This configurationsupplements the SAN multipathing configuration that generally provides one or more switches in the SANfabric and one or more storage processors on the storage array device itself.In the following illustration, multiple physical paths connect each server with the storage device. Forexample, if HBA1 or the link between HBA1 and the FC switch fails, HBA2 takes over and provides theconnection between the server and the switch. The process of one HBA taking over for another is calledHBA failover.VMware, Inc. 215