Monday, February 06, 2012

modern HDD's: No more 'cylinders'

Another rule busted into a myth. How does this affect File Systems, like the original Berkeley Fast File System?

There's a really interesting piece of detail in a 2010 paper on "Shingled Writes".

Cylinder organisations are no longer advantageous.
It's faster to keep writing on the one surface than to switch heads.

With very small feature/track sizes, the time taken for a head-switch is large. The new head isn't automatically 'on track', it has to find the track... "settling time".

Bands consist of contiguous tracks on the same surface.
 At first glance, it seems attractive to incorporate parallel tracks on all surfaces (i.e., cylinders) into bands.

 However, a switch to another track in the same cylinder takes longer than a seek to an adjacent track:
 thermal differences within the disk casing may prevent different heads from hovering over the same track in a cylinder.

 To switch surfaces, the servo mechanism must first wait for several blocks of servo information to pass by to ascertain its position before it can start moving the head to its desired position exactly over the
desired track.

 In contrast, a seek to an adjacent track starts out from a known position.

 In both cases, there is a settling time to ensure that the head remains solidly over the track and is not oscillating over it.

 Because of this difference in switching times, and contrary to traditional wisdom regarding colocation within a cylinder, bands are better constructed from contiguous tracks on the same surface.

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