Managing and scaling IP SAN

Computer Technology Review,  Nov, 2004  by Peter Wang

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All network interconnects among these iSCSI devices are implemented with point-to-point, switched gigabit Ethernet/IP networks. The use of switched Ethernet breaks the limitation of internal busses and loops of the conventional storage arrays. One can extend and expand the switched network easily and quickly while riding the price/performance improvement trends of Ethernet. The switched IP/Ethernet network can be configured with redundant paths. In the event of a switch failure, the network automatically re-routes the data paths. This is transparent to the application hosts and the storage controllers. Thus, the need for additional, multi-pathing software and source-route configurations are obviated.

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As shown in Figure 1, separating the storage controllers and the storage elements enables both to be scaled and/or repurposed with the network, either independently or together, to meet various application storage requirements.

Clustered Storage Services Controllers

The storage services controller (SSC) is a specialized IP server executing policy-based volume management and intelligent resource management. The SSCs of a Native IP SAN operate as a loosely coupled, active-active cluster, which is presented to the application hosts as a single storage system with a single virtual IP address. At any point in time, one member of the cluster serves as the primary SSC and hosts the virtual IP address that represents the cluster.

When an application host initiates an iSCSI session to the virtualized storage system (via the virtual IP address of the cluster), the primary SSC designates a member of the cluster to service the data-path to the storage elements that comprise the volume to be accessed by the requesting application host. The primary SSC then issues an iSCSI re-direct command to the application host, instructing it to connect to the physical IP address of the designated SSC servicing the data-path (Figure 2).

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As any of the cluster members can assume the responsibility of another failed SSC, the same iSCSI re-direct mechanism for the initial iSCSI session establishment is used to reestablish the data-path to the storage elements through one of the remaining functional SSCs. Similarly, as additional SSCs are added to the cluster, or as a member of the cluster is consistently overloaded, the storage traffic can be re-directed through other cluster members. Thus, linear I/O performance scaling is achieved by expanding the cluster size.

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In short, high-end FC SAN storage subsystem capabilities such as N-1 levels of fault tolerance and high availability, load balancing and I/O bandwidth scaling become available in even an entry level native IP SAN.

True Single Storage Pool and Global Sparing

The back-end iSCSI disks/JBODs/arrays form a single pool of storage behind the SSCs. The SSCs offer up virtualized volumes to the application hosts. DHCP is employed to manage the IP addresses of the iSCSI storage elements and the SSCs.