Hard Drives, How Do They Work?

About7Narwhal

A few of us at work are discussing the finer points of Hard Drive logistics and have hit upon a point of contention. In regards to data being written onto a platter, where does a new drive start writing data? I had always assumed in would write from the inside of the platter to the outside, similar to a record player. Anyone have a definitive answer?

Thanks!

aftereffector

Outside to inside for best performance.

OctalDump

It depends

Generally outside to inside, but also most drives have multiple platters, so writing to cylinders, not just tracks. The other thing to consider is if bad blocks have been mapped out, so might not be physically contiguous, there tends to be reserve, though, across the drive and not just "at the end" (ie the inside). Drives do a lot of clever stuff under the hood to optimise performance and the LBA that's given to computer is an abstraction. So you can never be quite sure where your data will end up.

Different drives, where the platters might be the same, can use different firmware to apply different optimisations for different applications. To the computer, they might look the same, but the drive firmware knows something different.

Interestingly, CDs and DVDs work the opposite way. CD players (the audio kind) had constant linear velocity - to give the predictable 1.5Mbps throughput needed for 16bit 44.1KHz stereo - so starting from the inside or outside would make no difference. Modern optical drives have go for constant angular velocity (like HDDs). CDs also have a spiral track, not the concentric arrangement of tracks you see on HDDs.

I found also this link which has quite a detailed explanation, and considers also seek times.