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State of the Art: How to Build an iSCSI SAN

Talk the Talk

In iSCSI parlance, an initiator is a device or software that maps SCSI into IP: It wraps SCSI commands in an IP packet and ships them to an iSCSI target. The target machine unwraps iSCSI packets from IP and acts upon the iSCSI commands. It returns an iSCSI response or multiple responses, which are usually blocks of data.

The server is your application server, and the storage box is the machine serving up iSCSI drives. (We're using storage box to represent anything from a Linux software iSCSI target to a full-blown SAN with iSCSI support.)

You need a gigabit copper network for an iSCSI SAN. If you try running iSCSI over a 100-Mbps network, you'll be disappointed. Assuming your network connection maintains 100 percent utilization, 100 Mbps is roughly equivalent to 5 MB per second of disk transfer. Because iSCSI has a request/response for every packet transferred and network performance degrades before 100 percent saturation, the best performance you'll get is 6.25 MBps of throughput. That's a rough estimate that includes time to wrap and unwrap data packets and responses to each packet.

Bottom line: 6.25 MBps of data transfer is not good, considering that most drives run in the 40- to 320-MBps transfer range. Besides, Gigabit Ethernet is affordable: Gigabit adapters start at $60; switches, $120. And Gigabit Ethernet has the throughput for treating iSCSI as a local drive.

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