Dell High Availability Clustering
iSCSI Cluster Overview
iSCSI Clusters
A PowerEdgeTM iSCSI Cluster is an integrated system of components, including Dell PowerEdge servers, Gigabit Ethernet (GbE) adapters, Microsoft iSCSI Software Initiator, and Dell PowerVaultTM or Dell | EMC® iSCSI storage system(s). Microsoft Cluster Service (MSCS) in Microsoft® Windows® Server 2003 Enterprise Edition enables two-node to eight-node failover clusters, which deliver high availability for applications and services.
A PowerEdgeTM iSCSI Cluster uses readily available GbE technology to provide the storage connections between the PowerEdge servers and the shared Dell PowerVaultTM or Dell | EMC® iSCSI storage system(s). The cluster employs GbE components and Microsoft MPIO to provide multiple data paths and redundancy.
Active/Active and Active/Passive Configurations
MSCS and all PowerEdge iSCSI Clusters support both active/active and active/passive cluster configurations. The term active/active refers to a cluster with at least one virtual server running on each node. For example, in a 2-node cluster configuration, when an application is running on Node 1, Node 2 does not need to remain idle waiting for Node 1 to fail. Node 2 can run its own cluster-aware applications (or another instance of the same application) while providing failover capabilities for resources on Node 1. Node 1 in turn can provide failover capabilities for resources on Node 2. An active/active cluster node must be sized appropriately to handle the increased load that would be incurred in the event of a failover.
The term active/passive refers to failover cluster configurations in which one or more cluster nodes is actively processing requests for cluster applications while at least one cluster node simply waits for an active node to fail. An active/passive configuration can appear more costly in terms of price/performance because one or more servers remain idle most of the time, but simplifies planning. This type of deployment is appropriate for business-critical systems since the application can use the full power of a standby server in case of a failure.
Direct-Attached Configurations
A direct-attached PowerEdge iSCSI Cluster configuration includes the two cluster nodes and a single storage system. In direct-attached configurations, the storage controller(s) on the Dell PowerVault MD3000i storage system – or the storage processors on the Dell | EMC iSCSI storage system – are connected directly to iSCSI Initiator controlled GbE adapters in the cluster nodes (Figure 1).
Each clustered server has access to the shared storage system. In a direct-attached configuration with dual storage controllers or storage processors, the path failover software running on each node monitors the paths to the storage system and can reroute traffic in the event of a failure in the data path from the iSCSI Initiator controlled adapter to the storage controller or storage processor.
Figure 1
NOTE : In this figure and the other figures on this page, the term "storage processor" is used to represent either a storage processor on a Dell | EMC storage system or a storage controller on the Dell PowerVault MD3000i storage system.
NOTE : A Non-Redundant Direct-Attached configuration is supported with single-controller MD3000i. In this configuration, there is only a single data path from a host to the MD3000i controller. This configuration does not offer high-availability within the storage system, and is not recommended for applications that need critical data storage.
Switch-Attached Configurations
A PowerEdge iSCSI Cluster can be integrated into an iSCSI storage network using Ethernet/IP switches. Switch-attached clusters are superior to direct-attached clusters in terms of configuration flexibility and expandability. These clusters provide secure data communication over an existing IP infrastructure with minimized interoperability barriers. A best practice for switch-attached iSCSI cluster configurations is to employ dedicated networks for iSCSI traffic and to use redundant Ethernet switch fabrics.
If storage capacity beyond what is attainable by connecting expansion enclosures to a Dell PowerVault MD3000i or a Dell/EMC storage system is desired for the cluster, a Switch-attached cluster configuration can be connected to multiple iSCSI storage systems through an IP network (Figure 2).
Figure 2
Cluster consolidation
Cluster consolidation is an extension of a Switch-attached cluster configuration. For an iSCSI network, cluster consolidation is commonly defined as the ability to allow multiple iSCSI clusters and stand-alone iSCSI servers to access a portion of the storage capacity offered by one or more storage systems (Figure 3).
In a cluster consolidation, storage system software performs LUN mapping and masking to partition the storage system and assign each data volume to specific server(s) attached to the iSCSI network. This software grants servers (or clustered sets of servers) explicit ownership of a data volume provided by the common iSCSI storage enclosure. For example, if two clusters are sharing a storage system that has four available LUNs (or volumes of data), Cluster 1 can be assigned LUN 0 and 1, and Cluster 2 can be assigned LUN 2 and 3. The LUN masking software prevents each server or cluster from discovering or accessing storage assigned to other servers or other sets of cluster nodes.
Figure 3