Lenovo ThinkServer RD240 MegaRAID SAS Software User Guide - Page 36
Multipathing, The firmware determines whether enclosure modules ESMs are part
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Auto-rebuild allows a failed drive to be replaced and the data automatically rebuilt by "hot-swapping" the drive in the same drive bay. The RAID drive group continues to handle requests while the rebuild occurs. 2.4.4.1 Multipathing The firmware provides support for detecting and using multiple paths from the RAID controllers to the SAS devices that are in enclosures. Devices connected to enclosures have multiple paths to them. With redundant paths to the same port of a device, if one path fails, another path can be used to communicate between the controller and the device. Using multiple paths with load balancing, instead of a single path, can increase reliability through redundancy. Applications show the enclosures and the drives connected to the enclosures. The firmware dynamically recognizes new enclosures added to a configuration along with their contents (new drives). In addition, the firmware dynamically adds the enclosure and its contents to the management entity currently in-use. Multipathing provides the following features: • Support for failover, in the event of path failure • Auto-discovery of new or restored paths while the system is online, and reversion to system load balancing policy • Measurable bandwidth improvement to the multi-path device • Support for changing the load balancing path while the system is online The firmware determines whether enclosure modules (ESMs) are part of the same enclosure. When a new enclosure module is added (allowing multi-path) or removed (going single path), an Asynchronous Event Notification (AEN) is generated. AENs about drives contain correct information about the "enclosure", when the drives are connected by multiple paths. The enclosure module detects partner ESMs and issue events appropriately. In a system with two ESMs, you can replace one of the ESMs without affecting the virtual drive availability. For example, the controller can run heavy I/Os, and when you replace one of the ESM modules, I/Os should 2-4 Introduction to RAID
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