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Donald K. Burleson

Oracle RAC Tips

Cluster Interconnect Products

Most cluster vendors utilize very competitive technology. Many of the interconnect products approach the latency levels of the SMP bus. Technologies like Memory Channel, SCI, and Myrinet support virtual shared memory space by means of internodal memory address space mapping. Connections between nodes are established by mapping part of the nodes virtual address space interconnect interface. Because of the memory-mapped nature of the interface, transmitting or receiving overhead is similar to an access of local main memory. This mechanism is the fundamental reason for the low latency levels seen in Memory Channel, SCI, and Myrinet. Table 3.2 summarized the various interconnect capabilities.

Table 3.2- Interconnect capabilities

Let’s examine some of the popular interconnect products and technologies.

  • Memory Channel - The memory channel interconnect is a high-speed network interconnect and provides applications with a cluster-wide address space. Applications map portions of this address space into their own virtual address space as 8 Kbyte pages, and then read and write from this address space just like normal memory. This is available for Alpha-based HP (now Compaq) clusters.
  • Myrinet - Myrinet is a cost effective, high performance packet communication and switching technology. It is widely used in Linux Clusters. Myrinet software supports most common hosts and operating systems. The software is supplied ‘open source’. Myrinet implements host interfaces that execute a control program to interact directly with host processes (‘OS bypass’) for low latency communication, and directly with networks to send, receive, and buffer packets.
  • Scalable Coherent Interconnect - SCI is Sun's best performing cluster interconnect because of its high data rate and low latency. Applications that stress the interconnect will scale better using SCI, compared to using lower performing alternatives. Sun SCI implements Remote Shared Memory (RSM), a feature that bypasses the TCP/IP communication overhead of Solaris. This improves cluster performance. SCI is available for the Sun Fire 4800 and 6800 servers.
  • Veritas - Database Edition / Advanced Cluster communications consists of LLT (low level transport) and GAB (Group Membership Services). LLT provides kernel-to-kernel communications and functions as a high performance replacement for the IP stack. The use of LLT rather than IP reduces the latency and overhead associated with the IP stack.
  • HP Hyper Fabric HMP - HP Hyper-Fabric supports standard TCP/UDP over IP and HP’s proprietary Hyper Message Protocol. Hyper Fabric extends the scalability and reliability of TCP/UDP by providing transparent load balancing of connection traffic across multiple network interface cards. HMP, coupled with OS bypass capability and the hardware support for protocol offload, provides low latency and extremely low CPU utilization.

For more information, see the book Oracle 11g Grid and Real Application Clusters 30% off if you buy it directly from Rampant TechPress . 

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