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    How DCB Makes iSCSI Better

    Posted by Allen Ordoubadian

    A challenge with traditional iSCSI deployments is the non-deterministic nature of Ethernet networks. When Ethernet networks only carried non-storage traffic, lost data packets where not a big issue as they would get retransmitted. However; as we layered storage traffic over Ethernet, lost data packets became a “no no” as storage traffic is not as forgiving as non-storage traffic and data retransmissions introduced I/O delays which are unacceptable to storage traffic. In addition, traditional Ethernet also had no mechanism to assign priorities to classes of I/O.

    Therefore a new solution was needed. Short of creating a separate Ethernet network to handle iSCSI storage traffic, Data Center Bridging (DCB), was that solution.

    The DCB standard is a key enabler of effectively deploying iSCSI over Ethernet infrastructure. The standard provides the framework for high-performance iSCSI deployments with key capabilities that include:
    – Priority Flow Control (PFC)—enables “lossless Ethernet”, a consistent stream of data between servers and storage arrays. It basically prevents dropped frames and maximizes network efficiency. PFC also helps to optimize SCSI communication and minimizes the effects of TCP to make the iSCSI flow more reliably.
    – Quality of Service (QoS) and Enhanced Transmission Selection (ETS)—support protocol priorities and allocation of bandwidth for iSCSI and IP traffic.
    – Data Center Bridging Capabilities eXchange (DCBX) — enables automatic network-based configuration of key network and iSCSI parameters.

    With DCB, iSCSI traffic is more balanced over high-bandwidth 10GbE links. From an investment protection perspective, the ability to support iSCSI and LAN IP traffic over a common network makes it possible to consolidate iSCSI storage area networks with traditional IP LAN traffic networks. There is also another key component needed for iSCSI over DCB. This component is part of Data Center Bridging eXchange (DCBx) standard, and it’s called TCP Application Type-Length-Value, or simply “TLV”! TLV allows the DCB infrastructure to apply unique ETS and PFC settings to specific sub-segments of the TCP/IP traffic. This is done through switches which can identify the sub-segments based on their TCP socket or port identifier which are included in the TCP/IP frame. In short, TLV directs servers to place iSCSI traffic on available PFC queues, which separates storage traffic from other IP traffic. PFC also eliminates data retransmission and supports a consistent data flow with low latency. IT administrators can leverage QoS and ETS to assign bandwidth and priority for iSCSI storage traffic, which is crucial to support critical applications.

    Therefore, depending on your overall datacenter environment, running iSCSI over DCB can improve:
    – Performance by insuring a consistent stream of data, resulting in “deterministic performance” and the elimination of packet loss that can cause high latency
    – Quality of service through allocation of bandwidth per protocol for better control of service levels within a converged network
    – Network convergence

    For more information on this topic or technologies discussed in this blog, please visit some of our other blog articles:
    What Up with DCBX Blog and iSCSI over DCB: Reliability and predictable performance or check out the IEEE website on DCB

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