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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."
Wrong. Found in many places on the Internet: like here in this GC FQA: it's not true that GC disks spin backward. You can check it out youself. Instead the CD starts at the outer part of the disk and the laser then moves towards the inside. Normal CD-ROMs and DVDs start at the inside and go outside (and slow down while doing that).
There are probably lots of articles on the subject that can be bought up with a quick search, but my favourite broad explanation site has their definition here.
Well, there's an interesting article about hard drives here. Watch out, though, it's quite long and it's an easy way to waste some hours... But sure you wanted to know why we don't need to park the hard disk's heads anymore? =) Or, that in fact, there are holes in the hard drive, so it can "breathe".
On page 6 (ATA versus SCSI) Mr. Anderson (insert matrix joke here) references "ATA versus SCSI: More Than an Interface," by Dave Anderson, Erik Riedel, and Jim Dykes.
The pdf can he had here: ATA_vs_SCSI
I thought it was quite an interesting read, and an excellent companion piece to the "You Don't Know Jack about Disks" article.
RFC2119
Unfortunately, we're still quite a ways from developing solid-state drives cheaply that can finally compete against today's hard drives.
:-)
I think within the next 15 years we will finally see the breakthrough that will essentially turn our primary storage into physically-removeable cartridges of solid-state non-volatile memory storing over 250 GB of data on a cartridge somewhat smaller than the physical dimensions of a 3.5" floppy drive. And unlike today's non-volatile memory, the new solid-state memory can be erased many trillions of times without affecting the life of the memory.
The big advantage of solid-state storage is that since the delay in reading and writing data is a tiny fraction of that of hard drives, they could access data at extraordinarily fast speeds. Imagine loading something like Windows 2000 Professional in under two seconds from start to finish!
It spins the disk(it's not a CD, so it's not a disc) clockwise like normal.
It reads the disk from edge to center, which is the opposite of most other devices. There were a few very early CD players that did this too, and they weren't able to play CD singles.
There is no reason why the angular velocity should increase when the laser is pointed further away from the center of roation. The rate at which information is read is R * V where V is the angular velocity, and R is the distance from the center where the laser points to. The CD/DVD drive must slow down (less V) its spin as it starts to read furhter away from the center (more R) because the hardware can process information (R * V) only so fast.
Here are direct links to the figure images:
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Magnetic disk drives have been at the heart of computer systems since the early 1960s. They brought not only a significant advantage in processing performance, but also a new level of complexity for programmers. The three-dimensional geometry of a disk drive replaced the simple, linear, address spacetape-based programming model.
Whatever happened to cylinders and tracks?
Traditionally, the programmer's working model of disk storage has consisted of a set of uniform cylinders, each with a set of uniform tracks, which in turn hold a fixed number of 512-byte sectors, each with a unique address. The cylinder is made up of concentric circles (or tracks) on each disk platter in a multiplatter drive. Each track is divided up like pie slices into sectors. Because any location in this three-dimensional storage space could be uniquely identified by the cylinder number, head (surface) number, and sector number, this formed the basis for the original programming model for disk drives: cylinder-head-sector access.
This raises the question: If that is how data is stored on a drive, why don't we still use that as the programming model? The answer is not an easy one but has its roots in the fact that this geometric model endured until the advent of the intelligent inter-faces, SCSI and ATA. [The IBM mainframe world used a slightly different model, allowing tracks to be written with records (blocks) of user-defined length. An individual track could have sectors of different sizes. As one who programmed count key data (CKD) storage, I can attest that it offers the application wonderful flexibility, but the drive design challenges have relegated it to history. Also, a purist might point out that standards etiquette calls for SCSI to use blocks and ATA to use sectors, but I will use these terms interchangeably.]
Disk-interface protocols implement the programming model for disk drives. The earlier drive interfaces did little more than expose signals to let the host directly manipulate the drive mechanism and initiate a transfer of data at a target location. This put the task of dealing with all the low-level idiosyncrasies peculiar to drives on the programmer charged with developing the firmware or software support.
The introduction of ATA and SCSI fundamentally changed this. Table 1 describes the migration of intelligence from host to drive in the evolution of the more important interfaces. With these intelligent interface protocols, the task of programming the use of disk drives became much easier. Disk-drive designers also gained a freedom of action needed to design higher-capacity and higher-performance drives. I will look at just how drive designers used this freedom of action in their designs, but it is important first to understand the fundamental goal behind drive design: increasing areal density.
DAVE ANDERSON, director of strategic planning for Seagate Technology, has more than 20 years of experience in the computer field. His responsibilities include overall strategy for all disk interfaces. He has been involved in the architecture and planning of Fibre Channel since it was first proposed as a disk interface. He was also one of the principal architects of the disk XOR commands that are now a part of the standard
Another thing about CKD is that early disks were unreliable and you needed to make sure you read the location you thought you were reading (like for instance if seek calibration was off), so by putting a countfield containing cyl/head/record in front of each data field you got self-identifying data. If the read CCW said gimme a particular record and the count field you actually read didnt match you could recover by reissuing the read or otherwise hiccuping