[BC] Digital Storage

[RichardBJohnson at comcast.net]

From: "Alex Hartman" <goober at goobe.net>

>While consumer disk space is cheap, i wouldn't trust consumer grade drives to last much more than 5 years these days. Enterprise level drives about the same if they're running 24/7 but if you stick them in external cases and use them sparingly, they'll last virtually forever.

PC disk drives:

In the beginning... The first disk drives to go into a PC (actually an AT because PCs only had floppies) was named for its interface, ST-506. You could get these in any size as long as it was five megabytes! http://en.wikipedia.org/wiki/ST-506

Adaptec made adapter boards so PC users could also have hard disk drives. They, too, had the ST-506 interface.

The ST-506 interface treated the hard disk like a floppy disk. Software on the PC would actually seek to a certain track. These hard disks were quite reliable and eventually their size was increased to 50 megabytes as the number of cylinders, tracks, and heads were increased as technology matured. The PC/AT under DOS could only directly access 33 megabytes of raw disk data so the disk drives for PCs stagnated at that level for a while.

New recording formats were developed which increased the amount of data storage to about 85 megabytes. These drives used RLL or RLE encoding.

For industrial use, the SCSI interface came about. Adaptec made SCSI interface boards for the PC/AT machines. SCSI disks were reliable and the bit density improved over the years to reach gigabyte capacity. This improvement was because of VAX and Sun machines, which used them, not because of the "lowly" PC/AT.

While SCSI was improving for the industrial market, PCs were getting cheaper and an interface called IDE (Integrated Drive Electronics) reared its ugly head. This was a way to make disk drives and motherboards cheaper. IDE drives were slow and not very reliable mostly because "everybody" was making these junk drives for PCs. The IDE interface went on for about twenty years. It was cheap and the PC/AT, now cloned by the millions became a throwaway item. 

There was a need for a faster interface. The IDE drives were configured for DMA and UDMA, which made them very unreliable. The new "grown up" IDE interface was called "ATA." It did not last long. Somebody got smart and created "Fiber channel," a fiber-optic interface, for Sun Workstations and Adaptec made controllers for the PC/AT market. This interface was expensive, but allowed PC/AT clones to finally stream high-speed data to disk. Fiber-channel disks could record about 60 megabytes of data for every second of operation. This was a great improvement over even SCSI, which was limited to about 20 megabytes per second transfer rate. 

Then somebody got even smarter! They removed the expensive optical interface of fiber-channel and substituted a flat coaxial cable for the interface. They thus created SATA (serial ATA). This is a reliable interface, whose speed has improved year by year.

The physical disk drives are the same devices refined during the SCSI era and there are so many made that there are not any separate lines for "industrial" or "consumer."

You can read about the latest data transfer rates here: http://en.wikipedia.org/wiki/Serial_ATA After being involved in this evolution (I made embedded software for the Adaptec controllers as a sideline), I suggest that for a long life, do NOT turn off your disk drives! The mechanics will outlast the electronics if you keep the discs spinning.
 
Cheers,
Richard B. Johnson
Book: http://www.AbominableFirebug.com/