12Gb SAS: What the 12Gb/s Figure Means for Drives, HBAs, Cables and Expanders

What 12Gb SAS means: a link rate, not a drive speed

The 12Gb/s on a SAS drive, controller or cable is a link rate, counted in gigabits per second. Seagate's Exos X24 data sheet has a specification table headed SAS 12Gb/s, and in that table the maximum sustained transfer rate at the outer diameter is 285 MB/s, or 272 MiB/s. The 12Gb/s is bits per second on the link; the 285 MB/s is bytes per second off the drive, so the two do not measure the same thing.

A piece of arithmetic shows the gap, and it is ours rather than a maker's: 12 gigabits divided by 8 is 1.5 gigabytes per second, before any protocol overhead. We have not found a document that states the overhead, so we do not net it off. Even unadjusted, the sheet's 285 MB/s is under a fifth of that figure, so a single hard drive does not come close to filling a 12Gb/s link on its sustained rate.

Makers sell one drive at two link rates. WD's HC580 sheet lists its 24 TB and 22 TB drives in a SATA 6Gb/s and a SAS 12Gb/s configuration, and the HC560 sheet reads 7200 RPM, SATA 6Gb/s and SAS 12Gb/s. Toshiba's MG11 SAS models list interface speeds of 12.0, 6.0, 3.0 and 1.5 Gbit/s, under an interface line that reads SAS-3 and later, and its MG09 overview gives the same four speeds under SAS-3.0.

So when a listing says 12Gb/s SAS, read it as the SAS version of a drive and the fastest link it can use, not as a promise of throughput. The interface decides what the drive plugs into; the sustained rate on the sheet tells you how fast it actually moves data.

12Gb SAS hdd: what the maker sheets print

Each of those lines ties the SAS figure to a named drive on the maker's own document, and that is the evidence that a drive is 12Gb/s SAS. A listing title is not. Our pages for 16TB SAS, 20TB SAS and 24TB SAS drives set out the part numbers at those capacities, which is where the SAS and SATA versions of one drive part company.

12Gb SAS hard drives on Amazon.co.uk

The table shows 12 listings right now. In all, the table draws on 20 listings in stock. Prices are pounds sterling inc VAT, and each row marks whether the offer is new or used.

One rule for reading this table: the 12Gb/s on each row is the seller's title, not a measurement. Where a row's drive is one of the lines on the sheets above, the maker's document backs the title; where it is not, treat 12Gb/s as the seller's claim until you have found the maker's sheet. Synology-branded drives are not listed here.

Will a 12Gb SAS drive work on a 6Gb/s controller, or a SATA one?

Between SAS speeds, Broadcom's user guide describes negotiation. Its 94xx Tri-Mode adapters run SAS at 12Gb/s, 6Gb/s and 3Gb/s per lane, and the guide says the adapters negotiate between the speeds and the protocols to recognise and work with each type of device. On the drive side, Toshiba's MG11 and MG09 SAS models list 6.0, 3.0 and 1.5 Gbit/s beneath their 12.0 Gbit/s top speed. That rests on what one adapter maker and one drive maker print, so check your own controller's documentation for the rates it supports.

Between SAS and SATA, the rule runs one way. Seagate states that SATA drives may be plugged into SAS controllers, and that SAS drives cannot be plugged into SATA controllers. Its installation note adds that connecting a SAS drive to a SATA controller will not work. The physical difference Seagate points to is the connector: on SATA drives power and data are separate, while on SAS drives they are unified.

A SATA drive on a 12Gb/s controller still runs at SATA rates. Broadcom's guide gives SATA transfer rates of 6Gb/s and 3Gb/s per lane on the same adapters. A SNIA post by a SCSI Trade Association vice president explains that SAS carries SATA devices through the Serial ATA Tunneling Protocol (STP), alongside the Serial SCSI Protocol (SSP) it uses for SAS devices. The SAS vs SATA page compares the two interfaces in full.

12Gb SAS HBA, card and controller: Broadcom's 9400 series

Broadcom's 9400 series is the example used here. The 94xx family covers MegaRAID and HBA Tri-Mode adapters, and Tri-Mode means that a single controller can operate in all three modes concurrently: SAS, SATA, or PCIe/NVMe.

The slot is the easy thing to miss. Broadcom says the HBA requires an x8 PCIe slot, so confirm a free slot of that size before ordering a card. The 8i and 16i differ in connector count; for whole enclosures, Broadcom's guide describes connecting through SAS expanders, covered below.

12Gb SAS cable and connector

The connector on Broadcom's cards has its own specification. SFF-8643 Rev 3.5 defines the Mini Multilane 4/8X 12 Gb/s Unshielded Connector (HD12un), and its abstract notes that this connector is popularly referred to as the Mini-SAS HD (High Density) connector system. That is the same Mini SAS HD name Broadcom uses for the internal ports on the 9400-8i and 9400-16i.

For SAS and SATA drives, Broadcom's installation guide says to connect standard 12Gb/s SAS cables with an internal Mini SAS HD connector at the controller end, and the appropriate connector at the other end for the backplane or the drives. So a 12Gb SAS cable is specified by its two ends: Mini SAS HD at the card, and whatever your backplane or drive needs at the far end.

Do not reuse one for NVMe. Broadcom warns that using standard 12Gb/s SAS cables for NVMe connections might result in damage to the PCIe adapter or the drive, which matters on a Tri-Mode card where NVMe is one of the modes.

Seagate describes the basic SAS layout as each drive having its own cable that connects directly to a SAS host adapter. The wider SAS cabling family uses Mini SAS HD and QSFP, and the December 2019 SNIA post names SlimSAS and MiniLink as definitions then added to the SAS standard. Lengths and cable part numbers for a SAS cable or an SFF-8643 lead depend on the cable maker; check its own listing.

12Gb SAS expander: more drives than ports

An expander is how a card with two or four connectors reaches a full enclosure. Broadcom's guide says its Tri-Mode adapters support enclosure connectivity through SAS expanders and PCIe switches, and it lists addressing of multiple SATA targets through an expander, so SATA drives can sit behind one too.

SAS also builds redundancy in at this level. The SNIA post describes native failover and dual-port features that let multiple domains be configured and then take over automatically in the case of component failures, broken cable connections or domain failures.

12Gb SAS SSD

SAS is not only for hard drives. WD's Ultrastar DC SS530 data sheet describes the SSD as designed with a dual-port 12Gb/s SAS interface, and gives its interface as SAS 6/12Gb/s, supporting wide port at 12Gb/s. So a 12Gb SAS SSD exists in the same terms as a 12Gb SAS hard drive: the link rate belongs to the interface.

The table on this page lists hard drives only. The controller and cable sections above describe SAS devices in general, as Broadcom's guides do, rather than hard drives alone, and the SAS SSD page goes further on solid-state drives with this interface.

Is 12Gb SAS still worth buying?

12Gb/s is not the newest SAS generation. In December 2019 a SCSI Trade Association vice president wrote that SAS and the SCSI protocol were transitioning to 24G SAS. That post looked ahead, and we do not treat 24G as what is on the shelf: the drive sheets cited here, including Toshiba's August 2024 MG11 overview and Seagate's Exos X24 sheet, still print 12Gb/s SAS.

For a hard drive, the link is rarely the limit: the X24's 285 MB/s sustained sits well inside a 12Gb/s link on our arithmetic above. The reasons to pick the SAS version lie elsewhere, in a controller that takes both SAS and SATA drives and in SAS's dual-port and failover design. If your controller is SATA-only, none of that applies, because a SAS drive will not work on it.