The latest trend in internal computer storage are SSHDs, or Solid State Hard Drives, which combine a conventional hard disk with fast SSD cache. Until now SSHDs were primarily made for laptops in the 2.5-inch format, but now Seagate has a 3.5-inch SSHD for desktops. We tested the Desktop SSHD 2TB and compared it to other 3.5-inch disks.
SSHDs are marketed as being the best of both worlds, and based on our recent tests of 2.5-inch SSHD we tend to agree for the most part. You get the large volume capacity of conventional hard disks and the speed of SSDs. For laptops with limited space that is a great solution. In PCs you usually have the flexibility of fitting both inside the chassis, an SSD for the OS and main programs and the hard drive for storage. But if an SSHD isn't that much more expensive than a normal hard drive, then they could become interesting options for PCs as well, in combination with real SSDs.
The Seagate Desktop SSHD comes in two capacities, 1 TB and 2 TB, but it appears that a 4 TB model is on its way even if it's not available yet. We tested the 2 TB model, which has two 1 TB platters running at 7200 rpm. In that regard they're comparable with the Seagate Barracuda 7200.14 disks.
It has 8 GB of SSD cache provided by a single 24nm Toshiba MLC flash module with two dies. Reportedly, part of the chip runs in MLC mode with 2 bits per cell and part of it in SLC mode (1 bit per cell). The latter provides lower latencies and higher endurance. What the exact ratio is between MLC/SLC is unclear, Seagate kept that part under wraps.
If you're wondering about the lifespan of the flash chip, Seagate says that it's twice that of five years of 'normal' use, without specifying what that is.
The specs indicate that the maximum write speed to the cache memory is 190 MB/s. That's about a third of what SSDs are capable of and only slightly more than the average 156 MB/s that hard disk platters can achieve. The main advantage of the SSD cache isn't so much the higher transfer rate but the much faster access time of the flash memory.
The flash memory uses an SSD controller from eASIC. The standard SATA 600 HDD controller is from LSI and has 64 MB DRAM cache. These are the same components used in the Seagate Laptop Thin SSHD we tested in June.
Seagate doesn't disclose exactly how the caching algorithms work exactly but does indicate that both write tasks and commonly used data for reading is cached.
The 2TB disk we tested has an average price of £92 / €111, which is about 30 percent more expensive than the conventional Seagate Barracuda 7200.14 2TB for example. It's still a fraction of the cost of 2 TB of SSD storage.