Monday, April 4, 2011

SAN protocols for dummies

Just doing a brush up on the basic storage concepts. Noobs will appreciate the simplification in this post, experts will likely find it too oversimplified. More detail can be found in the links or more recent posts!

For reference, the OSI model:
7. Application
6. Presentation
5. Session
4. Transport
3. Network
2. Datalink
1. Physical

Quick hits*1:
SCSI, SATA, FC, and SAS are layer 1 and 2 protocols. iSCSI is a layer 5 protocol. Ethernet is a layer 2 protocol. FCP is a layer 1, 2, and above protocol. FCoE is a layer 1-6 protocol.

Photo Courtesy of

Local protocols: these are how the CPU communicates to the hard drives.  They are all layer 1 and 2 protocols specifying the hardware and electronic signals needed to send data between the drive and the CPU.  This has trended toward serial protocols (away from parallel) for performance and cost reasons.

1. SCSI (Small Computer System Interface): a high performance parallel standard that specifies hardware level communication over a local BUS.

2. SATA (Serial Advanced Technology Attachment): Slow, inexpensive.  Used mainly for unimportant, low change data.

3. Fibre Channel: Fast and expensive, this serial protocol is commonly used in enterprise SANs.

4. SAS (Serial attached SCSI): The SCSI protocol was modified to take advantage of cost and speed improvements in serial technology.  This is also commonly found in enterprise SAN's.

5. FATA (Fibre Attached Technology Adapted): Slow, inexpensive.  Really is SATA wrapped in a FC interface to gain from shelf technology.   This is less used than the other types.

Network Protocols: This is how a server can communicate over your SAN to its storage, essentially virtualizes the relationship between a computer and its local drives, allowing your server to think a virtual drive in a datacenter somewhere is actually directly plugged into it.

1. iSCSI:  Protocol simulates the SCSI protocol by wrapping it in ethernet-friendly packets.
Advantage: Works over existing ethernet networks (if given enough bandwidth).
Disadvantage: Some risks involved with having all your traffic on the same cables. Theoretically high overhead since it's higher up on the stack.

2. Fibre Channel Protocol: protocol that requires special fibre cabling and an entire alternate network to support communication.
Advantage: Can be fast, separates traffic and enhances stability.
Disadvantage: Expensive, requires special cabling.

3. Fibre Channel over Ethernet: FCoE. Protocol simulates the FC protocol by wrapping it in ethernet-friendly packets.
Advantage: Can be very fast for cheaper than FC.
Disadvantage: Theoretically more overhead. Doesn't separate traffic.

You can find a great performance discussion here:

*1: FCOE Discussion:

FC Discussion

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