ATA over Ethernet

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ATA over Ethernet (AoE) is a network protocol developed by the Brantley Coile Company[1], designed for simple, high-performance access of SATA storage devices over Ethernet networks. It gives the possibility to build SANs with low-cost, standard technologies.

AoE does not rely on network layers above Ethernet, such as IP and TCP. In this regard it is more comparable to Fibre Channel over Ethernet than iSCSI. While the non-routability means AoE cannot be accessed over the Internet or other IP networks, the feature makes AoE more lightweight (with less load on the host), easier to implement, provides a layer of inherent security, and offers higher performance. The AoE specification is 8 pages[1] compared with iSCSI's 257 pages[2].


[edit] Operating system support

The following operating systems provide ATA over Ethernet (AoE) support:

OS Support Third-party drivers
Linux Native (2.6.11+) Coraid [2]
Windows Third-party StarPort [3]
WinAoE [4]
Mac OS X 10.4 and up Third-party 2DegreesFrost [5]
Small Tree Communications [6]
Solaris Third-party Coraid [7]
FreeBSD Third-party Coraid [8]
OpenBSD Native (4.5-current)
Plan 9 from Bell Labs [9] Native

[edit] Hardware Support

The Coraid [10] company offers an array of AoE SAN appliances under the EtherDrive brand, along with diskless gateways that add network-attached storage functionality, using the NFS or SMB protocols, to one or more AoE appliances.

In 2007 LayerWalker [11] announced the world's first single-chip AoE hardware solution called miniSAN running at both Fast and Gigabit Ethernet grades. The miniSAN product family offers standard AoE server functions plus other management features that targets PC, consumer and SMB markets.

A commodity Linux server can function as an AoE target using a software AoE target. Three independently developed AoE target implementations exist: the userspace vblade[12], which is part of the aoetools package; one, kvblade, implemented as a Linux kernel module; and ggaoed, which runs in userspace but takes advantage of Linux-specific performance features.

[edit] Protocol Description

[edit] ATA Encapsulation

SATA (and older PATA) hard drives use the Advanced Technology Attachment (ATA) protocol to issue commands, such as read, write, and status. AoE encapsulates those commands inside Ethernet frames and lets them travel over an Ethernet network instead of a SATA or 40-pin ribbon cable. By using an AoE driver, the host operating system is able to access a remote disk as if it were directly attached.

The encapsulation of ATA provided by AoE is simple and low-level, allowing the translation to happen either at high performance or inside a small, embedded device, or both.

[edit] Routability

AoE runs directly on top of Ethernet, instead of an intermediate protocol such as TCP/IP. This reduces the significant CPU overhead of TCP/IP. However, this means that routers cannot be used to route a packet across disparate networks (such as the Internet). Instead, AoE packets can travel within a single local Ethernet storage area network (such as one created by a switch or VLAN).

[edit] Security

The non-routability of AoE is a source of inherent security (ie, an intruder can't connect through a router. He must physically plug into the local Ethernet switch). However, there are no AoE-specific mechanisms for password verification or encryption. Additional security may be implemented at the file-system level. Certain AoE targets, GGAOED [13] for example, support access lists allowing connections only from specific MAC addresses.

[edit] Config String

The AoE protocol provides a mechanism for host-based cooperative locking. When more than one AoE initiator is using an AoE target, they must communicate. The hosts need a way to avoid interfering with one another as they use and modify the data on the shared AoE device.

One option provided by AoE is to use the storage device itself as the mechanism for determining the access of particular hosts. The AoE protocol includes a "config string" feature. The config string can record who is using the device. (It can also record any other information.) If more than one host tries to set the config string simultaneously, only one succeeds. The other host is informed of the conflict.

[edit] Related Concepts

Although AoE is a simple network protocol, it opens up a complex realm of storage possibilities. To understand and evaluate these storage scenarios, it helps to be familiar with a few concepts.

[edit] Storage Area Networks

The purpose of Storage Area Networks (SANs) is often not to make files and directories available to multiple users, as is the purpose of Network Attached Storage (NAS). Instead, a SAN allows the physical hard drive to be removed from the server that uses it, and placed on the network. A SAN interface is thus similar in principle to non-networked interfaces such as SATA or SCSI. Most users will not use a SAN interface directly. Instead, they will connect to a server that uses a SAN disk instead of a local disk. Direct connection, however, can also be used.

When using a SAN network to access storage, there are several potential advantages over a local disk:

  • It is easier to add storage capacity and the amount of storage is practically unlimited.
  • It is easier to reallocate storage capacity.
  • Data may be shared.
  • Additionally, compared to other forms of networked storage, SANs are low-level and high performance

[edit] Utilizing Storage Area Networks

To utilize a SAN disk, the host must format it with a filesystem. However, unlike a SATA or SCSI disk a SAN hard drive may be accessed by multiple machines. This is a source of both danger and opportunity.

Traditional filesystems (such as FAT or ext3) are designed to be accessed by a single host, and will cause unpredictable behavior if accessed by multiple machines. Such filesystems may be used, and AoE provides mechanisms whereby an AoE target can be guarded against simultaneous access (see: Config String).

Shared disk filesystems allow multiple machines to use a single hard disk safely by coordinating simultaneous access to individual files. They are slightly similar to network filesystems. These filesystems can be used to allow multiple machines access to the same AoE target without an intermediate server or filesystem (and at higher performance). Examples of shared disk filesystems are GFS, GPFS, MetaSAN, and OCFS2.

[edit] See also

[edit] References

  1. ^
  2. ^

[edit] External links

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