Disk Drives Explained

CowboyRobot writes "Magnetic disk drives are one of those things I usually take for granted without thinking about, but I recently realized how little I understood about how they really work. ACM Queue has an article from their 'Storage' issue titled, 'You Don't Know Jack About Disks', which does a very good job of explaining exactly how magnetic disks have evolved since the 70s and how they work today."

Comments

Re:This is a great paper.

[Ella the Cat]

If someone asks me which computer to buy, I'd like to think I could assess their level of technical understanding and their needs in under two hours, provide encouragement, explanation, and make a useful recommendation. Your attitude alienates people.

Magentic disk drives are bad for environment

[Krapangor]

The magenic layers contain very large amounts of chrome-oxides and other heavy metals and rare elements. This makes disk drives a huge problem in the disposal of old computers.
Furthermore the rare element production takes often place in very anarchic countries like Kongo or Liberia. Usually warlords and local terrorists use the money from the disk drives rare elements to finance their blood raids and terrorship.
That's btw the reason why the US were setting up Kabila in Kongo. This guy was killed, but only because the French were more clever.

So, instead of this old technology which is going to be phased out in 5 years anyway, you should use more modern flash/ram disks and DVDs for data storage, just for moralic reasons.
Think about it: If you refuse to buy bananas or big name brands because of the cruel, inhumane exploitation of the third-world workers, then you should do the same in IT and avoid disk drives.

Re:A bit more history

[Rick.C]

When you consider that the average mainframes of the early '70s had around 512K (yes, "K") of memory (the big ones had a meg or two), you can understand the need to conserve memory. Most programs ran in 60K regions. You just didn't have room for a lot of large data buffers. CKD format allowed you to write whatever size records made sense for your application.

The real beauty of CKD was the "K" or "key" field. If you wrote data blocks with keys, you could then ask the disk controller to search for a given key while your program was executing other code. The controller would find the matching record, read it into storage and interrupt when it was done!

Nowadays most mainframe DASD is really RAID-1 or RAID-5 SCSI arrays that emulate CKD under the covers. With gobs of RAM and the introduction of "dataspaces", the usefulness of CKD is debatable, but like other legacy interfaces, CKD will be a long time dying.