White Papers

9 Dell Storage PS Series Architecture: Load Balancers | TR1070
5 How the Network Load Balancer (NLB) works
Communications between application servers (iSCSI initiators) and volumes (iSCSI targets) are called
connections. A PS Series Group will present all iSCSI targets through a single virtual address known as the
Group IP address. This allows administrators to establish connections easily by configuring the iSCSI
initiator with only the Group IP address instead of the IP addresses of all of the interfaces in the Group.
As the load increases or decreases on the various Ethernet ports, the NLB automatically distributes
connections among the active Ethernet ports of the members using a feature of the iSCSI specification called
redirection. Redirection defines how the iSCSI target instructs the iSCSI initiator to log out and close the
connection to the IP address that it is currently using and immediately log in to another address and establish
a new connection. Support for redirection is required for iSCSI initiators by the iSCSI specification.
Redirection is utilized by the NLB within a PS Series Group to permit the application server to establish iSCSI
connections as needed without first needing to be updated manually to know all of the possible IP addresses
that the SAN is using. Leveraging redirection, the NLB ensures that all the network interfaces within the SAN
are optimally used. The NLB and iSCSI connection redirection are also key functions used by the PS Series
architecture to enable volumes and members to migrate seamlessly from one pool to another, and permit
members to join or leave the Group as required with no interruption in service to the applications.
The NLB should not be confused with Multi-Path I/O (MPIO), which is load-balancing that occurs on the
application host. MPIO uses redundant physical interfaces to deliver high availability to shared storage. Using
MPIO, servers can send multiple I/O streams to SAN volumes. Each of these paths uses an iSCSI connection
managed by the NLB.
In addition to the standard functionality provided by MPIO, Dell EMC provides host tools to enhance the
performance of MPIO and to automatically manage the connections for Windows (including Microsoft
®
Hyper-
V
®
), VMware and Linux environments. For a further discussion on the various MPIO enhancements provided
by Dell EMC, refer to the following Dell EMC technical reports:
Configuring and Deploying the Dell PS Series Multi-path I/O Device Specific Module with Microsoft Windows
Configuring and Installing the PS Series Multipathing Extension Module for VMware vSphere and PS Series
SANs