Raid Expander Cards

What is a SAS Expander?

A SAS expander is a module that allows a SAS controller card to use more SAS/SATA drives than it would be able to use on its own, because of the limited number of SAS connectors on the card. A typical SAS card may have just one or two connectors on it, and on its own would only be able to support up to four or eight drives.

A setup using SAS expander enclosures can allow a controller to use up to the maximum devices it can support, which is typically 128 or 256 devices depending on the card.

For example, a SAS controller card with two external connectors can support up to eight drives if using a regular 8 bay SAS enclosure with two connectors. However, one can purchase an enclosure with a built-in expander and use just one MiniSAS cable to connect to the enclosure. The enclosure could be as little as 8 bays or as big as a 24 bay enclosure.

And you have room for future expansion by way of the second connector on the controller card, or by daisy chaining addtional expander chassis to the first one.

What does 12G, 6G and 3G mean?

Also seen as 12Gb/s, 6Gb/s and 3Gb/s. The Gb/s stands for Gigabits per second. This is the throughput speed the SAS 3.0, 2.0 and 1.0 specs have. 12G = 4800MB/s, 6G = 2400MB/s and 3G = 1200MB/s. These speeds are theoretical and real world results are often less depending on which controller card, sas expander, drives and raid level are used. 

12G SAS expanders and controller cards came out recently and they proivde huge performance gains over 6G products.

Does using just one MiniSAS cable limit my bandwidth?

In most cases, no. MiniSAS cables are multi-lane. Depending on the cable, controller and expander being used, each cable supports four lanes of 12G or 6G or 3G. That yields you 12G x 4 lanes = 4800MB/s, or 6G x 4 lanes = 2400MB/s, etc.

Since standard hard drives' max transfer speed is maybe around 135MB/s, it will take many of those drives to saturate the bandwidth of a single cable. SSDs on the other hand are much faster, and it takes fewer SSDs to use up the bandwidth of a single cable. In some cases running two cables from your expander enclosure to your controller card will give you more speed.

Variables to be aware of when trying to get maximum speed from your SAS expander setup:

•   ::  Use a PCIe 3.0 controller card in a PCIe 3.0 slot.
•   ::  Make sure the x8 or x16 slot is an x8 signal slot. Some x8 slots are only x4 signal, thus cutting your bandwidth in half.
•   ::  When using a 12G controller card, using a 12G expander chassis is ideal.
•   ::  SSDs yield outrageous speed vs hard drives. Use SSDs if max speed is what you need.
•   ::  Use SAS hard drives instead of SATA. 7200rpm SAS drives have been found to be around 10% faster than equivalent SATA drives, at a very minor cost difference.
•   ::  Use two hd minisas cables for optimal performance. Some expanders and controllers don't yield better performance with two cables. Some do. You'll want to try two cables to see.