goggin, edward wrote:
I'm wondering about how block device multipathing works in conjunction with the high availability features of an IP-SAN. Does the iSCSI initiator's session layer's retransmission capability and the IP-takeover capability of IP-SAN switches eliminate the need for block device multipathing in an IP-SAN? Does the iSCSI initiator present redundant paths to the SCSI mid-layer? Do these answers differ with software verses hardware iSCSI initiators?
For linux mainline and what some distros carry, software will be similar to HW iscsi becuase the linux-scsi community has not wanted MCS. For software iscsi in mainline (open-iscsi/linux-iscsi-5) and RHEL 4 (linux-iscsi-4), you can use bonding or trunking - whatever you know it as, and hide everything from the iSCSI and SCSI levels, or you can use dm-multipath. To do the latter for software iscsi, the iscsi driver creates a session to each portal, scans it, and dm-multipath/multipath assembles dm devices like it would for FC. The softare drivers drivers also support login redirect and some vendors use this for failover and load balancing. I think qlogic also support this.
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