ATA over Ethernet: Worth considering?

Scott Lowe discusses the pros and cons of ATA over Ethernet (AoE) as a storage solution. Has AoE found a solid niche, or will it fade against Fibre Channel and iSCSI?

When it comes to shared block storage, your choices are ever-expanding. Even better, they are expanding in the downward direction when it comes to price. As prices come down, the technical nature of each solution changes drastically and, although the price might be right, the solution may not fit the bill. I've written quite a bit already on the comparative features of Fibre Channel and iSCSI, but the newcomer to the game—ATA over Ethernet (AoE)—aims to find its niche at the lower, simpler, cheaper end of the storage market. But while it's simpler and cheaper, can AoE get comfortable in its niche or is it too little, too late? I’ll discuss some pros and cons, give you a brief overview of the technology, and then give you my opinion at the end of this article.

Recapping Fibre Channel and iSCSI in the enterprise

Both Fibre Channel and iSCSI have a definitive, if blurring, spot in the enterprise. When it comes to raw performance and ability to scale, Fibre Channel holds the prize. But, with 10-Gb Ethernet just around the corner, and serious enterprise players, iSCSI is no slouch.

However, both technologies have their drawbacks. Fibre Channel is expensive and relatively complex. It takes a lot to get going. iSCSI, on the other hand, is much simpler, particularly since many iSCSI implementations stay local to the main data center. Even though iSCSI uses TCP/IP, traffic in a lot of iSCSI SAN implementations stays on a local subnet. However, iSCSI traffic still has to be wrapped up in relatively fat TCP/IP packets.

What AoE has to offer

AoE provides a simple, very low-level transport mechanism for ATA commands to travel between a host and a hard drive. In effect, AoE trades the traditional IDE or SATA cable for Ethernet cabling and inexpensive switches. Sure, it’s more expensive than that IDE cable, but it also provides cheap, central block-based storage for your network. AoE communicates with other devices on the network through the use of device MAC addresses, which limits AoE to the local broadcast domain. Storage devices are, as you might expect, ATA/IDE disks.

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Like iSCSI, AoE uses "initiator" software, which is installed on each host and enables that host to communicate over the network to remote storage devices. Some companies, such as RocketDivision, have already developed working, supported AoE initiators. Other companies, including Coraid, sell complete AoE arrays and controllers.

Pros:

  • AoE doesn’t require the use of a heavyweight transport protocol. Instead, it rides right on the wire (okay, right on top of Ethernet, so not really right on the wire, but darn close).
  • AoE is cheap! An array capable of supporting up to 11.25 TB from Coraid starts at less than $4,000 without disks. Today’s price for a 750-GB disk at NewEgg.com is $400 and the unit supports 15 disks. So, for less than 10 grand, you can get 11.25 TB of shared block storage. If you do the math, that runs at about $888/TB or $0.87/GB. Not bad!
  • AoE uses Ethernet. You already know how to deal with Ethernet and already know how to handle things like flow control.
  • AoE can spare processing cycles. iSCSI requires TCP/IP and its requisite complexity. AoE doesn’t.
  • AoE has initiator support for Windows, Linux, and Mac OS X.

Cons:

  • AoE is cheap, but the more supported iSCSI isn’t that much more expensive when you consider the outstanding resources behind it.
  • AoE isn’t "enterprise-ready" yet.
  • AoE is not as scalable as iSCSI or Fibre Channel when you consider location—i.e., with Fibre Channel and iSCSI, you can scale your storage throughout the organization. This is primarily due to the inability to route AoE traffic.
  • ATA disks are not as reliable as their SCSI counterparts. However, many iSCSI implementations use ATA disks, too.

The future of AoE?

What do I think the future holds for AoE? While, on the surface, it appears to be a low-end technology, I think it definitely has a great niche. You won’t see AoE displace EMC or IBM in powerhouse data centers, nor will AoE damage sales of iSCSI gear. Fibre Channel and iSCSI customers are often looking for more than just storage. For example, snapshots and replication are two big add-ons for many SAN customers. However, for the organization looking to move from direct-attached storage to something central and more easily extended and managed, I think you will see AoE do very well. In my research, I’ve seen a similar trend with AoE that iSCSI went through in its infancy. At first, it was barely a blip on the radar, with few new stories and even fewer vendors behind it. Today, however, entire organizations (mine included) depend on iSCSI for their data storage needs. AoE will likely follow a similar trend.