Network design

Create a reliable and fast SAN design with iSCSI

ISCSI is a versatile and affordable SAN access technology, but deployment involves more than just installing an iSCSI filer and connecting network nodes to it.

It helps to understand the Internet Protocol Small Computer System Interface (iSCSI) before designing a iSCSI network. In iSCSI, the blocks to be stored are sent over the network in an IP packet, rather than over dedicated SCSI cables. This iSCSI traffic is addressed by default on TCP port 3260. The iSCSI block devices are configured on a node called the iSCSI target, which is usually the Network of storage areas (SAN) in your environment. The servers connected to the SAN are ISCSI initiators.

An efficient SAN requires sufficient bandwidth and speed, which means a dedicated iSCSI network. While iSCSI packets could theoretically travel over the user’s network, the bursty nature of iSCSI requires a dedicated path so that packets are not lost. You should only use mixed networks in test environments.

Figure 1. This overview of the iSCSI network shows a redundant topology.

Take into account the size of the packets that will pass through the iSCSI network for your SAN. By default, an Ethernet packet is 1500 bytes, which actually fragments SAN traffic. To optimize iSCSI traffic, configure all devices on the iSCSI network to use a maximum transmission unit of 9000 bytes, the largest allowed at the Ethernet level. On some devices, this means enabling jumbo frames. Since the network requires 9000 byte packets throughout the block path, be sure to set this packet size larger at both ends of the communication channel. If a node is set to 1500 byte packets, you will keep this size smaller throughout.

For the iSCSI network topology, use redundant networks and connect to redundant controllers if possible in your deployment. A redundant topology requires two network interfaces for each server that connects to the iSCSI network, each interface being connected to a separate switch. Two controllers must be available on the iSCSI SAN and connect each switch to each controller (see Figure 1). To prevent the same device from appearing multiple times through its redundant paths, install a multipath driver on servers using the iSCSI network. With this redundancy, nodes should survive a failure of the network topology.

After you have properly designed the iSCSI network for your SAN, consider the optimization settings available for your iSCSI target and the initiator. These will vary. For example, a Linux server connected to the iSCSI SAN has a configuration file for each active iSCSI connection in the / var / lib / iscsi directory, where you will find several optimization parameters. An iSCSI host bus adapter with an iSCSI offload engine will effectively smooth network traffic, thereby avoiding server latency while handling high volume of traffic.

A well-designed iSCSI network will not need much optimization from the iSCSI initiator.

Sander van Vugt is an independent trainer and consultant based in the Netherlands. He is an expert in high availability, virtualization and Linux performance. He is the author of numerous books on Linux topics, including Start the Linux command line, Start the administration of the Ubuntu LTS server and Ubuntu pro server administration.

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