As disk storage density increases and data availability requirements become ever more demanding, data replication is increasingly an indispensable feature of enterprise-class storage systems. For highly available storage systems, every disk block is typically replicated on a local mirror server and a remote mirror server in addition to being stored on the main storage server. In a network storage environment, this N-way data replication increases the write traffic load on the storage client''s network interface by N times. Multicast is a natural solution to this problem. However, existing storage area network technologies such as Fibre Channel and Ethernet do not provide adequate support for linklayer multicast. This paper describes a novel reliable multicast mechanism for Ethernet-based storage area networks that effectively exploits the Virtual LAN technology and is able to keep the traffic load of N-way replication roughly at the same level as the no-replication case regardless of the value of N. This technology greatly increases the appeal of using Ethernet as the physical-layer technology for storage area networking, as Fibre Channel networks currently do not support such in-network replication. Performance measurements on an iSCSI-based network storage system demonstrate that the proposed reliable multicast mechanism is able to reduce the end-to-end data transport time by a factor of 2.6 for three-way replication when the disk write size is more than 4Mbytes.
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