In The Lab With Seagate's Momentus XT 750 GB Hybrid HDD

Comparing Mechanical, Solid-State, And Hybrid Storage

In order to create our comparison, we're using Seagate's Momentus XT, which combines 8 GB of SLC NAND, a 32 MB cache buffer, 750 GB of magnetic media spinning at 7200 RPM, and a SATA 6Gb/s interface.

When we first reviewed Seagate's hybrid hard drive technology, the Momentus XT only had 4 GB of SLC NAND. Moreover, the model we tested was limited to a 500 GB capacity and a SATA 3Gb/s interface. In that first-generation implementation, along with the Momentus XT we're testing today, a small repository of SLC NANDis used to intelligently store hot (or repeatedly-requested) data.

This isn't strictly storage space where data goes once it's read more than once. Rather, the drive moves information that is determined would exceed a certain access time if it had to be pulled from the mechanical media. And it doesn't necessarily move complete files, either. If a read operation is slowed down by just a single fragmented block, then only that piece of the file will be copied to NAND. In theory, this should help reduce the access time penalties associated with conventional disks. On the Momentus XT, read seek times are rated at 11 ms on average, while write seek times average 13 ms.

Maximizing The Utility Of 8 GB

Although 8 GB may not sound like very much, it's not an insignificant chunk of space for storing small blocks of data that would otherwise affect access times in a negative way.

The NAND is primarily used to service read operations. Writes to the flash memory by the device controller occur following a read/write modification, or when more relevant data is transferred to NAND to help accelerate subsequent read operations. When a file (or a portion of a file) held in NAND is subject to a read/write modification, the revised data is modified on the hard disk platter first before it's copied back to the NAND. As a consequence, data integrity is reliant on the mechanical storage, rather than whatever is sitting in the solid-state space.

Writes to the NAND result in a program/erase cycle (P/E). The ability of a flash memory cell to endure many P/E cycles is dependent on transistor geometry, manufacturing process maturity, and the flash technology itself. Single-level cell (SLC) NAND, the type Seagate uses in its Momentus XT, can endure a much greater number of P/E cycles than multi-level cell (MLC) NAND. The trade-off, of course, is much higher cost for SLC NAND, which is why a vast majority of SSDs (even many of the enterprise-class offerings) employ MLC flash.

Despite its pricey chunk of SLC NAND, the 750 GB Momentus XT currently sells for about $150, translating to a cost per gigabyte of roughly $.20 cents. That's quite a bit more than what you'll pay for a conventional hard drive (Seagate's 750 GB Barracuda goes for $80, or about $.10 cents per gig). However, it's also a mere fraction of what you'd spend on an SSD of comparable size.

The SSD And Hard Drive Competition

We're comparing the Momentus XT to Samsung's 64 GB 830, which offers 59 GB of user capacity. The 830 employs multi-level cell Toggle-mode flash, a three-core MCX controller, and 256 MB of DDR2 SDRAM as a cache. Higher-capacity models include up to 1 GB of DDR2 SDRAM. Newer SSDs with Marvell's controller technology in them are also starting to use up to 1 GB of DRAM cache. Similar to how hybrid hard drives use flash memory for its strength in dealing with fast read operations, SSDs are increasingly turning to DRAM to deal with the weaknesses of NAND: write operations.

The 830 we're using is a fairly entry-level model. Its lower capacity does negatively impact performance. However, it's still able to achieve sequential read performance as high as 520 MB/s and sequential writes up to 160 MB/s. Random 4 KB read IOPS are in the 75 000 range, while random writes top out around 16 000. Samsung's 64 GB 830 sells for right around $90, resulting in a cost per gigabyte around $1.40.

Representing hard drive technology, we're using a Western Digital WD1500ADFD Raptor X 150 GB. It's not the company's most modern model; however, it still performs well compared to newer hard drives. The Raptor X is based on a twin-platter design with a 10 000 RPM spindle, a 16 MB buffer, and support for NCQ (Native Command Queuing). It boasts an average read seek time of 4.6 ms and an average write seek time of 5.2 ms. Track-to-track seek times are rated at .4 ms, on average, and the full stroke seek time comes in at 10.2 ms. When the Raptor X first came out in 2006, it sold for $350, it retailed for $349. Today, you can pick up a refurbished one for less than $60.

Test Setup And Benchmarks