End Node Load-Balancing Techniques

End nodes can load-balance SCSI commands across multiple iSCSI or FCP sessions using DMP software. Two basic approaches exist: LUN-to-path mapping and SCSI command distribution. When a host uses the LUN-to-path mapping approach, all I/O operations initiated to a given LUN are mapped to a single session (the active session). Recall from chapter 8, "The OSI Session, Presentation and Application Layers," that an iSCSI or FCP session is identified by the concatenation of the initiator port identifier and the target port identifier. So, the active session is inherently associated with only one of the available paths (the active path) between the host and storage array. A different path is designated within the DMP configuration as the backup path in case the active path fails. Upon failure of the active path, the DMP software establishes a new session via the backup path and resumes I/O to the LUN via the new session. Load balancing is accomplished by distributing the active sessions across the available paths. For example, all I/O for LUN 13 traverses path A, and path B is the backup path for LUN 13. Simultaneously, all I/O for LUN 17 traverses path B, and path A is the backup path for LUN 17. Obviously, the LUN-to-path mapping approach enables load balancing only when multiple LUNs are being accessed simultaneously. When a host uses the SCSI command distribution approach, all I/O operations initiated to all LUNs are distributed across all available paths. The DMP software establishes a session via each available path and then determines which LUNs are accessible via each session. For any given SCSI command, the session is selected by the configured DMP algorithm. Some algorithms simply perform round-robin distribution. Other algorithms attempt to distribute commands based on real-time utilization statistics for each session. Many other algorithms exist. Note that optimal use of all available paths requires LUN access via each path, which is controlled by the storage array configuration. The SCSI command distribution approach enables load balancing even if the host is accessing only one LUN.