By Ann Silverthorn
—Organizations that don't want to deal with the high cost and complexity of Fibre Channel SANs often consider the iSCSI alternative. However, they may want to consider a third protocol—ATA over Ethernet (AoE)—that some companies are using to build lower-cost, simpler SANs. AoE allows users to put disk drives on a network connection and access them as if they were locally attached disks.
"The Fibre Channel industry grew from the need for a way to store data on a network connection," says Jack Kemp, CEO of Coraid, an AoE RAID chassis vendor. "Ethernet was available at the time but only operated at 10Mbps, which was much slower than Fibre Channel. But Ethernet has gotten faster and is now well-suited as an interface to storage."
AoE can only be used within the data center because it is not routed and operates only at the Ethernet level. iSCSI, which adds an Ethernet connection through an IP network to a storage system, can be used outside of the facility.
"Ethernet is much less expensive than Fibre Channel, and organizations are using iSCSI within the facility to save money in the local connections. That saves money versus Fibre Channel, but it's misapplying the iSCSI interface," claims Kemp. "AoE is more suited for the local connection because it's a simpler protocol, it requires less processing, and it doesn't require TCP/IP offload engine [TOE] cards. AoE uses standard Ethernet NICs and doesn't burden the host machines with a lot of processing."
AoE was invented by Brantley Coile, founder of Coraid. AoE packages ATA disk interface commands and sends them over Ethernet frames, without the need for—or overhead of—TCP/IP stacks.
Coraid supports AoE in its RAID subystems. The AoE protocol is also incorporated in the Linux kernel and is part of all Linux distributions as a natively supported protocol for storage access.
"We've also written AoE drivers for FreeBSD and Solaris," says Kemp, "and there are other companies that have written drivers for Windows and Mac OS 10. The protocol is also supported by some chip manufacturers."
Coraid's EtherDrive AoE products connect to the network using standard Ethernet. Capacities range from a few terabytes to multiple petabytes, at pricing of less than $0.70 per GB. AoE is a non-routable protocol designed to transport disk commands over Ethernet. EtherDrive subsystems support RAID 0, 1, 5, and 10, hot-swappable disks, and SATA disk drives. Coraid's 15-bay chassis (without drives) is priced at $3,995.