Seagate and SSDs: A Cautious Approach Is Nothing To Sneeze (At)

Hybrid hard drives, how long has been their gestation! A memory refresher courtesy of Google’s search archive reminds me that I first covered the topic in conjunction with the late April 2005 Microsoft WinHEC. After publishing several other follow-up blog posts, I tackled a more in-depth online treatment which appeared in January 2007 coincident with that year’s Consumer Electronics Show. And I’ve continued to revisit the subject both online and in print since then.As a quick refresher, a hybrid hard drive augments a conventional rotating magnetic HDD platter structure with a solid-state flash memory array. In a hybrid HDD, the flash memory locates within the drive, versus in the system as is the case with Intel’s long-delayed and repeatedly renamed Robson aka ‘Turbo Memory’ aspirations. As such, the flash memory in a hybrid HDD supplements the RAM buffer that’s long been within the drive, with several key differences:

• Much slower write speeds, especially with random access patterns, but
• Nonvolatility; i.e. stored data survives reboots, system power transitions, etc.

The intent of a hybrid HDD is to blend the best attributes of a SSD and HDD. Since a SSD is completely flash memory-based, it delivers:

• High ruggedness
• Low power consumption, and
• Fast read performance, especially with random access patterns

However, magnetic storage has notable benefits of its own:

• Low cost/bit at high capacities, and
• Fast write performance, especially with sequential access patterns

Last December, I suggested that Seagate was easing into the embryonic HDD-to-SSD transition with a two-pronged strategy; ultra-thin 2.5″ HDDs as a means of preserving near-term business, along with a first-generation SSD engagement with the relatively price-insensitive enterprise storage market. The company’s 2.5″ Momentus XT drives, introduced last Monday, act as a HDD-to-SSD bridge of another sort. They’re actually not the first hybrid HDDs that Seagate has experimented with; witness the Momentus 5400 PSD (Power-Saving Drive) first publicly discussed in mid-2006 and in production 1+ year later. Befitting the era of their existence, their capacities were moderate (160 GBytes, later increasing to 250 GBytes), as was the amount of on-drive flash memory (256 MBytes). This first-generation hybrid HDD design was also closely wedded to Windows Vista and therefore was negatively impacted by the underwhelming adoption of that particular Microsoft O/S iteration.

Now contrast that situation with the second-generation Momentus XT approach. First and foremost, it’s claimed to be operating system-agnostic, presenting itself to the tethered system as a conventional 3 Gbps SATA HDD. Magnetic media capacities are substantially larger, befitting the latest PMR (perpendicular magnetic recording) areal density capabilities, as are the sizes of the flash memory (4 GBytes, sourced from Micron Technology judging from the internal PCB shots) and RAM (32 Mbytes) buffers. The magnetic media rotational speed is 7200 RPM, versus 5400 RPM in the first-generation hybrid HDD approach. And Seagate claims that Momentus XT prices will track those of conventional HDDs to within a capacity-dependent $50-90 mark-up:

• 500 GBytes: $156
• 320 GBytes: $122
• 250 GBytes: $113

The flash memory implementation in the Momentus XT design is relatively elementary. Writes to the HDD completely bypass the NAND component and go directly to the rotating magnetic media. Conversely, reads from the magnetic platter are simultaneously transferred to the system and (if deemed appropriate by the memory management unit) cached to flash memory. If the cache controller (which Seagate brands as ‘Adaptive Memory’) has done an effective selection job, that same information will be resident in faster-access NAND the next time the system requests it. And interestingly, Seagate decided to go with more expensive but more reliable SLC (single-level cell) NAND flash memory versus a more economical (or if you prefer, dense) MLC (multi-level cell) device, presumably to dispense with the need for elaborate wear leveling, error detection-and-correction and erase block retirement media management algorithms. As such, I suspect that the single component package installed on the PCB has two NAND flash memory die inside it.

The implementation may be simple, but the results are still notable. Admittedly, a hybrid HDD doesn’t improve ruggedness over a pure HDD alternative, since the bulk of the stored data still locates on the vulnerable rotating media platter(s). But it can make a notable dent in the storage subsystem power consumption budget, if the flash memory caching scheme is efficient and the platter(s) can therefore be spun down a high percentage of the time. Since system writes bypass flash memory, they aren’t hampered by its performance limitations. Run comparison prices on 250-to-500 GByte SSDs and the price advantage of the Momentus XT (in spite of its current sole-sourced status) will be quickly evident to you. And as for comparative performance results, I’ll defer to AnandTech’s thorough testing study, which benchmarks a 500 GByte Momentus XT against two SSDs and two HDDs (one with 5400 RPM rotating speeds and the other spinning at 10,000 RPM).