Today we have the first 12Gb/s SAS HDD in our labs for testing, Seagate's 6TB nearline Enterprise Capacity 3.5 HDD v4. A few months ago, Seagate brought an entire family of 7,200 RPM drives in capacities of 6, 5, 4 and 2TB drives to the familiar 3.5" form factor, and these new drives have the option of a dual-port 12GB/s connection or SATA 6Gb/s.
Seagate's 8th generation 6TB Enterprise Capacity v4 HDD's has 6 platters spinning at 7,200 RPM. The drive features a jump in areal density to 643 Gbits/sq. inch, the new advanced format, and 2 additional platters and 2 additional heads. This boosts capacity to 6TB, a 50% increase.
Nearline SAS HDDs provide an enhanced feature set for a small price premium. Nearline SAS drives feature the same media, heads, and rotational speed of typical SATA HDD. The addition of the SCSI command set adds enhanced management features, and dual port functionality provides multi-path and failover capabilities. 12Gb/s SAS also supports TCQ and a larger command queue. The 6Gb/s SAS interface is actually faster than SATA (by 20%) because there is no need for the STP (Serial ATA Tunneling Protocol) overhead. Seagate chose to provide even more performance with a 12Gb/s connection.
Many have questioned the wisdom of 12Gb/s SAS for hard disk drives that lack the speed to push the interface. In fact, HGST stayed with 6Gb/s SAS on the He6, which is currently the only other 6TB drive on the market. One reason to make the immediate move to 12Gb/s is to ensure forward compatibility with new RAID controllers and HBAs, and another reason is if you plan to deploy an SSHD.
The SAS v4 we have in the labs has mounting points for a NAND package on the PCB, denoting a future SSHD product. There is also another mount point that is very likely for an SSD controller. This type of implementation will require more bandwidth for situations where the HDD is communicating directly from NAND and 12Gb/s SAS fits the bill nicely. The advent of new dense NAND solutions can cram up to 128GB of flash into one package, and future 3D NAND technologies will pack up to a terabyte on a single chip. Increasing NAND density provides Seagate with a path for even more performance for their SSHD initiative, which provides huge performance benefits as noted in our Seagate Enterprise Turbo SSHD Review.
Seagate will not officially confirm a pending SSHD revision, but the striking similarity to SSHD designs we tested in the past is unavoidable. We have a more detailed analysis on the following page.
The Seagate v4 supports Super Parity, which adds an extra parity bit to improve data integrity for data at rest. In conjunction with enhanced error correction the v4 features a standard error rate of 1 per 10^15. The v4's MTBF is rated for 1.4 million hours (an AFR of 0.63%) while handling nearline workloads of 550TB.
The maximum sustained transfer rate of the SAS Seagate v4 is 226 MB/s, an increase of 10 MB/s over the SATA variant. The 12Gb/s SAS v4 features the same seek time of 4.16ms, and features a 128MB multi-segmented cache buffer.
One of the more notable changes in the SAS-powered v4 is the increase in power consumption. 12Gb/s SAS is a much faster interface with dual ports that requires more power for operation. The typical operating power of the SATA v4 is 11.27W, the same as their previous generation Constellation ES.3. The SAS version bumps that up to 11.87 Watts, but also matches the power consumption of the previous generation ES.3 SAS HDDs.
The SAS Seagate v4 exhibits a marked improvement of 1.98W-per-TB, compared to 2.96W-per-TB for the ES.3. The only slight increase over the ES.3 is idle power; from 7.8W to 7.97W.
The Seagate Enterprise Capacity v4 with the SATA interface was a great entrant into the high-capacity space. Let's take a closer look at the SAS variant.