New Technique Promises Much Faster Hard Drive Write Speeds

It's time to dust off the old concept of hard sectored discs;) Realistically, of course, it's a bit more complex than that.

First of all, modern hard drives have a servo track that's used to maintain radial position of the head servo. Instead of each hard drive having a very accurate (and expensive) radial and axial head position sensor, you pay for it once, install it in the factory, use it to accurately guide a hard drive to write the servo track. Its cost is amortized over thousands of drives made. This is probably the reason for a covered up radial slot in many hard drive enclosures: I guess it's used for the sensor to couple with the head system while the drive writes the servo track. Or perhaps the servo platter is prewritten outside the disc? Someone familiar with how it's made please chip in!

The servo track can be also used to provide angular position feedback. A rough estimate of angular position of the spindle is available first from the Hall sensors in the spindle motor. A somewhat more accurate estimate can be had from back-EMF from the spindle motor windings. This still is methinks a couple orders of magnitude away from what's needed to pack sectors tightly on the drive -- thus the feedback can come from the servo track. Not having to read the data tracks helps with packing the sectors: there's no read-write switchover overhead (if it were significant -- perhaps it isn't nowadays). The servo head is always reading, and the data heads can be kept in write/erase standby. It'd be nondestructive, but read amplifiers are disconnected to prevent saturating them -- amplifier overload recovery is slow. Heck, if you want an amp that recovers from overloads quickly, you have to split it into more stages, and you need fast clamps between each stage. There are other similar approaches to this problem, too, and perhaps modern read amps are designed to deal with overloads gracefully -- I never tested a recent one. Stuff from a decade ago was painfully slow on overloads (tried to reuse a head amp from a hard drive for a non-drive-related project).

Alas, this ultra-fast-writing drive would unfortunately need very accurate position sensors -- both angular and radial. It's an engineering issue to make those affordable, as is the design of the optochip with femtosecond laser and its driver and serializer. The latter would probably take a couple serial lanes and multiplex them -- I presume it's not all that easy to push 10gbit/s data between external chips and the laser driver/laser combo. I think that to make it all practical you need an on-chip serializer, write precompensation, driver, and the diode. Perhaps the diode would be "tacked on" later to a substrate that has everything else. I only imagine that bond wire parasitics, even over a couple mm, become kinda important when the laser waveform has a 100GHz bandwidth...

Source: http://rss.slashdot.org/~r/Slashdot/slashdotScience/~3/b8l_off01og/new-technique-promises-much-faster-hard-drive-write-speeds

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