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Google proposes significant revisions to disk drive design

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February 26, 2016

Google just finished writing a white paper in which it recommends some substantial revisions to overall disk drive design. The paper was started in November 2015 ans is quite elaborate.

Titled “Disks for Data Centers” the document is Google-centric as it calls for disk-makers to reconsider and redesign their storage products to better adjust to Google's needs.

Some in the IT industry think that this is a bit far fetched to ask that of major disk makers such as Seagate and Western Digital but considering the size of Google and its footprint in terms of hardware used, it may not be as bas as it looks from the outset.

At any rate, as the paper explains, those needs are very substantial. Just YouTube requires a petabyte of new storage every day. That's a lot of storage area just for one user site.

The whitepaper says that Google meets that requirement with disks designed for servers, not cloud-scale storage. The document also argues that the rise of cloud computing means cloud-specific disks need to be invented sooner rather than later, because plenty of disks will never see a conventional server in the first place.

“The IT and storage industry is relatively good at improving gigabytes per dollar (ratio) as an equation, but less so at IOPS/GB [input-output-per-second per gigabyte],” the paper says.

A desire to see the industry improve both informs Google's shopping list for a dream cloud disk. Let's explore that a bit.

The first item on Google's wish list is new form factors. Taller drives make a lot of sense to Google as such devices allow for more platters per disk, which adds capacity, and amortizes the costs of packaging, the printed circuit board, and the drive motor/actuator.

The paper also makes you guess if mixing platter sizes inside a disk could help, as the smaller platters would offer a lower GB/$ ratio but better IOPS/GB. But of course that would be balanced by the presence of larger and higher-capacity platters.

Later in the paper, Google also ponders the concept that individual disks could mix shingled magnetic recording and conventional recording technologies, so that disks could get the best of both worlds.

The result could be a disk drive suited to both fast writes for transactional workloads and archival storage. All within the same chassis. The best of both worlds.

Overall, disks with more than one IO source is another concept Google would like on its wish list. The paper imagines disks with more than one actuator arm, or one arm capable of reading more than one track at a time.

Google's third consideration in a new design is what it calls “group disks”, IE: pre-packaged clusters of disks that are nowhere near as smart as a NAS device but offer fine low-level control of disks.

Each individual disk in the group might still have its own SATA or PCI-E interface, but Google thinks buying them in groups with some shared components might be less costly. It does have a good point there.

Lastly but not least, on the form factor, Google calls for a standard 12 volts DC power supply to power the disks instead of the different various voltages used with conventional designs.

Google's next idea to disk engineers is to strip the cache out of disks and centralize it. “From a TCO perspective it makes more sense to move RAM caching from the disks to the host or tray, as a single big cache will be both a lot cheaper and more effective,” the authors suggest.

Plenty of added suggestions also concern how to make disks more efficient at the many background tasks they need to perform today to secure data. Google seems to want those tasks made a lower priority, so that the disk can spend more time doing productive I/O.

Spreading data across multiple redundant disks picks up the slack on the data protection side of things.

There's also a call for disks to understand the kind of IO demands a host makes and to be able to change the stream of data they offer to suit the times. Google wants APIs to make this happen. And it is confident that it can.

Many of the ideas and recommendations above concern the spinning disk itself, the disk medium commonly assumed to be on the way out as solid state disk gets cheaper.

Google says magnetic disks will be around for several more years, at least in its data centres, because “the growth rates in capacity/$ between disks and SSDs are relatively close (at least for SSDs that have sufficient numbers of program-erase cycles to use in data centers), so that cost will not change enough in the next 10 years.”

This would mean better wear-levelling and durability for SSDs are also on Google's wish list. We'll see what happens with the responses of the various makers of drives. We'll keep you posted, as always.

Source: Google.

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