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Comparing the Two Approaches

Caching is King: Assessing Top PC Storage Options for Tomorrow
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On the surface, the hybrid and mSATA+HDD approaches sound very similar: NAND flash up front with magnetic media in back. However, there are some fundamental differences that bear examining to fully understand the trade-offs between the two options.

With flash cache (mSATA+HDD), software residing on the host system controls where data gets stored. This is principally a function of the operating system and drivers monitoring LBA accesses. The flash cache approach allows for some design flexibility. For example, the flash memory could reside on the SATA bus or on the PCI Express bus. It could sit on an easily upgradeable plug-in card or, if size concerns are paramount, be mounted right on the system motherboard.

In contrast, the host does not have direct control over data in a hybrid’s flash resources. Whereas the host serves as the storage controller for flash cache arrangements, hybrid drives carry their own storage controller within the drive. The controller—and specifically the algorithms running within it—dictates what data should reside in the cache and whether incoming read requests should seek from the NAND or magnetic media. Recent advances by Microsoft and its storage partners allow the host to offer the hybrid controller hints based on the LBA accesses that it is still monitoring, but it remains the SSHD controller’s prerogative to make the final choice on whether data on the platters needs to be copied into cache for faster accessing.

In considering these storage alternatives, all judgment calls boil down to: it depends. Consider system integration. A hybrid is one component rather than two. This means easier installation and a smaller physical volume requirement. In cases where drive images need to be copied onto new systems, there is more chance of complications and incompatibilities when flash cache components change.