What is a SAS Expander? Everything you need to know

Storage demands for personal and professional use have grown quickly in recent years. Take a video editing workstation, for example, where large quantities of raw footage must be saved, or a home media server storing high-resolution movies and shows for on-demand entertainment.

SAS expanders are a cost-effective plug-and-play solution for your home server or PC, whether you want to add many disks to a server or SAS drives to a consumer motherboard.

In this article, we’ll be going over what a SAS expander is, how it works, and how you can use it to add more storage to your system.

What is a SAS expander?

You can think of a SAS expander as a simple unmanaged network switch for storage devices. Like a network switch that takes a connection and distributes its bandwidth downstream, a SAS expander takes a SAS port and expands it to multiple SAS connections.

In our article comparing SAS expanders with HBAs, a key takeaway was that a SAS expander multiplies the available ports on an HBA, letting you populate it with the maximum number of drives the HBA can support.

The main benefit of a SAS expander is its ability to expand a server or PC’s storage capacity without needing to invest in expensive HBAs that take up PCIe expansion slots. Note that an HBA+SAS Expander setup might not always be ideal if you can find an HBA with enough connectors that are both cheaper and easier to install.

How do SAS expanders work?

SAS expanders present your drives directly to the upstream adapter, which may be an HBA or motherboard’s SAS port. A single cable connects the SAS expander to an HBA, but additional connections can increase bandwidth.

SAS expanders also can be daisy-chained to each other up to a theoretical maximum of SAS devices specified by the manufacturer of the SAS expander and its upstream device. However, as you keep adding expanders, you reduce the per-drive bandwidth, which can be inadequate for faster storage media like SSDs.

Do SAS expanders reduce drive bandwidth?

A SAS-2 interface supports a maximum theoretical bandwidth of 6 Gbps. As you add more expanders downstream, this bandwidth is divided by the number of simultaneous connections.

We’ve stressed simultaneous connections here because even if you have 128 drives connected to your motherboard through a network of HBAs and SAS expanders if only a single drive is being actively written to, the drive should have access to the complete 6Gbps bandwidth.

If a series of operations are being actively performed on multiple drives, you will see a reduction in bandwidth. With the fastest hard disks only capable of 524 MBps (around 4Gbps), you won’t run into issues with bandwidth while using SAS expanders with HDDs. SSDs have much faster transfer rates that completely populate the 6Gbps SAS-2 bandwidth, so we do not recommend using them with expanders.

Internal vs. External SAS Expanders

Based on the connection to the HBA, SAS Expanders can be classified as either external or internal. Apart from a difference in connection, both types are identical in form and function.

An internal SAS expander is helpful in scenarios where it resides with the HBA in the same chassis. It features only a single type of SFF-8087 or similar internal port, one or two of which needs to be wired to the HBA.

If you plan on adding drives to another chassis and wiring it to your primary server, an external SAS expander is what you need. It has an SFF-8088 or equivalent external port that can receive an external SAS cable from the HBA in another chassis.

External SAS Expanders are also helpful in wiring redundant SAS arrays.

Internal SAS expanders can also be used in an external chassis, provided an adapter can convert the external SFF-8088 cable to an internal SFF-8087 one compatible with the SAS expander.

How to power a SAS Expander?

Most SAS expanders are powered through PCIe, usually through the PCIe slots in your server or PC’s motherboard. Their PCIe pins are for power only and do not transfer data. Some SAS expanders also come with a Molex connector allowing you to run them off your power supply.

If your motherboard lacks sufficient PCIe slots for a SAS expander or you want to run one-off in a separate chassis, we have an entire guide dedicated to using a SAS expander without a motherboard.

SAS Expander pricing and availability

SAS expanders can be found pretty cheap in the used or refurbished market. The Intel RES2SV240 is a popular Internal SAS-2 Expander with six SFF-8087 connectors supporting up to 24 devices. This external HP SAS Expander is also a good choice if you run your SAS expander in a separate chassis.

SAS Expander Installation

Installation of an expander is pretty straightforward, but each setup is different, especially if you are planning on utilizing hardware RAID on your HBA. So, it is good practice to consult the manufacturer’s instructions before installation.

The general steps for installation can be summarized as follows:

1. Identify where you need to install your SAS expander. Most expanders have PCIe pins for power, so you may have to install them on your motherboard.
2. Once installed, connect the expander to the upstream device, which may be an HBA or your motherboard’s inbuilt SAS ports. Use the appropriate cable with suitable connectors on both ends. Generally, a single SAS cable is needed, but you may need to connect two for higher bandwidth.
3. Connect the downstream devices. According to your requirements, it may be a drive, another SAS expander, DAS, etc. Ensure suitable cables with the correct connectors are used. Consult manufacturer documentation for the maximum number of devices that can be connected.
4. When you start your server or PC, check if your operating system has detected the devices connected to the expander.
5. If you face any issues, double-check the connections and try with only a single device downstream of the SAS expander. Keep adding devices to isolate the problematic ones and check them for issues and compatibility with your expander and upstream devices.

SAS Expanders are plug-and-play devices that work with any server or PC, irrespective of their manufacturer. By saving you from purchasing multiple HBAs that can take up expansion slots and cost extra, they are a must-have for those looking to build a high-capacity storage server or PC.

If you found this article helpful, feel free to share it around. Please let us know in the comments how you plan to use a SAS expander in your storage project. We will guide you through any issues, so post them below.