Comparing the Two Approaches Part II
Caching is King: Assessing Top PC Storage Options for Tomorrow
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Generally speaking, hybrids will be more efficient. Because of its tighter integration with HDD resources, the SSHD’s controller knows which data accesses require the longest rotational and seek latencies. Such factors are taken into account when deciding which data to keep in flash, thus making better use of available resources. More importantly, data transfer between the flash media and platters is done solely within the hybrid drive and is controlled via onboard ASICs. In the flash cache model, the CPU must handle such transfer loads, and either the SATA or PCI Express buses must shoulder the traffic bandwidth, leaving fewer resources for other system tasks.
Also, with only one component in play, hybrid solutions can offer lower cost. For OEMs, there’s no need to redesign systems to accommodate the additional flash modules. Hybrids have the flash controller integrated into the HDD controller, so there are fewer chips involved.
Furthermore, if a user needs to upgrade the storage subsystem, he or she can now simply replace the HDD and leave the mSATA SSD in place. This makes for a lower total upgrade cost. However, this advantage presumes a notable price disparity between standard HDDs and hybrids. If there was only a US$10 difference between the drive types, would the case for upgrading cost advantage be as strong for flash cache? Probably not. On the other hand, if a system’s use-model changes and more NAND cache suddenly becomes advantageous, then an mSATA-based approach could be preferable. Of course, this presumes that the user remembers to ensure that the flash cache’s data is mirrored before upgrading. Failure to do this may result in significant data loss. Meanwhile, SSHDs are designed such that all data in flash also resides on the HDD.
One of the key ways to address these questions is by considering just how much NAND is needed for storage acceleration, which is, after all, the whole point of this pursuit. Those interested in a deeper dive into this aspect should read our previous article on hybrids at http://www.tomshardware.com/us/sponsored/Seagate-winning-boot-drive-battles-214. Suffice it to say that Seagate researchers have determined that the average office worker needed considerably less than 8GB of flash in order to realize the majority of storage acceleration benefits. More NAND may increase endurance, but the average user is unlikely to gain more performance for the added cost, and even with just 8GB, users are unlikely to meet their NAND endurance thresholds within ten years. This may be part of why Intel hasn’t bothered to release mSATA products with more than 24GB of capacity—there is simply no practical need to do so in a NAND-based caching environment.
