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Ethernet SAN and AoE
What is Ethernet SAN & AoE?
Definition
ATA over Ethernet (AoE) is an open standards based protocol that allows direct network access to disk drives by client hosts. Using disk storage arrays that support AoE shared storage networks (SAN) can be built that leverage the power of “Raw” Layer 2 Ethernet.
- AoE has been native in the Linux kernel since 2005
- AoE delivers a simple, high performance, low cost alternative to iSCSI and FibreChannel for networked block storage by eliminating the processing overhead of TCP/IP.
- Layer 2 Protocol which encapsulates ATA (the command set used by most commodity disk) in Ethernet Frames – An Ethernet request which has in it, give me block ‘00’ from disk ‘01’ on shelf ‘1’.
Protocol
AoE is a stateless protocol which consists of request messages sent to the AoE server and reply messages returned to the client host.
Messages have two formats:
- ATA Message
- Config/Query Messages
AoE utilizes the standard Ethernet MAC header for IEEE 802.3 Ethernet frames and has a registered Ethernet type of 0x88A2.
Legacy Fibre Channel and iSCSI protocols consist of several complex software layers see the diagram below. These layers force users through mandatory SAN point-to-point connection configuration procedures for each network path for all storage LUNs. Ethernet SAN is a connectionless protocol that connects servers and storage directly across layer 2 Ethernet. It does not require TCP/IP or user configured multi-path IO (MPIO) software. The use of layer 2 Ethernet represents a simpler approach for SAN.
With Ethernet SAN packets are automatically sent over every available network path between the server and storage (without MPIO configuration).
In the diagram below the initiator has four paths to the destination and all four paths are used in parallel, were as with iSCSI for example this protocol has to break up the IO into 8 separate serial writes. Ethernet SAN based AoE storage can accomplish the same task in just two writes using the four paths in parallel. Measurements with iSCSI products using 4 1GbE links compared to using the same 4 1GbE paths with Ethernet SAN storage show Ethernet SAN latency is up to 5 times less.
Ethernet SAN is robust also, every packet is checked for bit errors and all packets are acknowledged. Ethernet SANs low latency allows dropped packets to be retransmitted immediately insuring high throughput and reliability.
Ethernet SAN is a generational leap in SAN infrastructure that is an extremely fast and simple method for sharing disk drives through a low latency network. Since Ethernet SAN doesn’t need TCP/IP, it has less overhead, lower latency and automatically utilizes all network paths. Coraid’s EtherDrive storage products take full advantage of Ethernet SANs simplicity to deliver storage that will outperform Fibre Channel at faction of the cost.