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The Two-Pronged Approach

Caching is King: Assessing Top PC Storage Options for Tomorrow
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Remember, mSATA drives aren’t always meant for caching. Options already range up to 480GB, making mSATA a perfectly viable 2.5-inch SSD replacement. As Ultrabook, tablet, and other ultramobile devices need high speed and capacity, the physical volume advantages of mSATA over SSD should become obvious. In fact, next generation form factor (NGFF) cards already stand poised to replace mSATA later this year. The mSATA design measures 51× 30×4.85mm while NGFF at its smallest measures just 42×22×2.75mm. (Longer, double-sided options reach 110mm in length and 3.85mm thick.) Despite the downsizing, NGFF designs like Intel’s SSD 530 Series will specify 6Gb/s SATA connections, 128-bit AES encryption, sequential read/write speeds of up to 550/520 MB/s, and capacities up to 480GB.

Read the fine print and you’ll see that Intel’s 530 Series uses single-layer cell (SLC) memory, while higher-capacity mSATA options use multi-layer cell (MLC), which stores two bits per NAND cell, or even tri-layer cell (TLC) memory. The more bits per NAND cell, the lower the cell’s endurance. The oxide layer that protects the cell’s integrity by keeping the stored electrons where they belong wears down more quickly. An SLC cell might endure 50,000 write cycles before it can no longer hold data. A TLC cell may last only 1000 cycles. When employed in a caching role meant for the most repetitive, high-use storage tasks in the system, this reliability issue should not be ignored.

Reliability aside, pricing remains a key issue. Amazon currently lists Intel’s 313 Series 20GB mSATA drive for US$99—nearly US$5 per gigabyte. And unless the drive is meant for some sort of embedded application, that US$100 investment still calls for the user to buy another drive for regular storage needs.

                                  A comparison of the mSATA and conventional SATA SSD form factors. Photo: Tom’s Hardware. A comparison of the mSATA and conventional SATA SSD form factors. Photo: Tom’s Hardware.

Solid state hybrid drives (SSHDs) offer an intriguing alternative. Hybrids are traditional hard drives with what is effectively an SSD mounted on their circuit boards. Functionally, the ideas behind hybrid design mirror those of mSATA combined with a hard drive. The onboard NAND serves as a high-speed cache for frequently accessed files.

The cost benefits of a hybrid approach are obvious. Newegg currently lists the latest 1TB Seagate Laptop SSHD (model ST1000LM4014) for US$130. The drive incorporates 8MB of MLC NAND, 64MB of conventional disk cache, a 5400-RPM spin rate, and a 6Gb/s SATA interface. Of the major drive vendors, Seagate has been the pioneer and leader in hybrid designs, so we’re confident in discussing the Laptop SSHD as a prime example of what’s in store for the 2013-2014 time frame.

Seagate incorporates NAND flash memory and spinning magnetic media in the same 2.5-inch form factor as conventional laptop hard drives.Seagate incorporates NAND flash memory and spinning magnetic media in the same 2.5-inch form factor as conventional laptop hard drives.