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 | | | | Aug31Written by:D. Glen Cardenas
Thursday, August 31, 2000 6:00 PM  In comparing IDE and SCSI it is important to understand that both types of drive are, from a "between the shells" point of view, the same.
Inside the Drive
Hard disks have a sealed case with one or more platters of magnetically coated media, a small synchronous motor designed to rotate the platters at a precise speed, and an actuator with one or more arms attached, each with a read/write head at the tip. The platters hold the data in the form of concentric tracks, each split like a pie into many sectors. Each sector will hold 512 bytes of user data as well as error correction information and other alignment information.
The actuator is designed like a speaker voice coil, extending or retracting along its throw path depending on the strength of an electrical signal in the coil which will force it very precisely to any location. The arms attached to the actuator are thereby positioned to various places above the spinning platters where the heads can pick up or lay down streams of magnetic information.
The heads float on a cushion of air at a distance of about 10 microns above the platter surface. The platter's rotation produces that cushion of air. In contrast, a particle of smoke is about 100 microns in size, or 10 times the head gap. For this reason, these drives are manufactured in very closely controlled "clean room" conditions, are sealed at the factory against any interaction with the outside environment and sold with the expressed condition that the user never, for any reason, open the drive casing.
The drive also has a circuit board to control the mechanism and coordinate the transfer of data to and from the platters in a specific format. Aside from the data and power connectors, that's about the whole story. It stands to reason, therefore, that the physical properties of these moving parts hold the key to a drive's access speed and data throughput.
In reality, this is more the case than is commonly believed, and for that matter, commonly disclosed by the drive manufacturers. So many drives are advertised with little more than their data storage capacity and interface burst transfer speed. Neither of these factors relates directly to a drive's usability as a DAW storage system. To get the real story, you must dig into the drive specifications, usually available only on the maker's web site and even then only after linking past several pages of ad hype and chest pounding.
The drive actually performs two distinct operations in order to read or write data, those being head positioning and data transfer. Let's start with head positioning. To perform this act, the drive must:
1) receive a request to position the heads to a specific location on the platter.
2) select the proper head to access the requested platter.
3) wait for the requested sector on the track to rotate into position for access.
All of this positioning and the buffering of the data to be written or that is finally read must be controlled by the drive electronics. Although the electronics is quite fast by all accounts, there is still a certain amount of overhead associated with this activity. It is referred to as... you guessed it, Controller Overhead. Sometimes this spec will be listed for the drive and is usually the same over a given product line or at least a given model range. It is expressed in milliseconds (thousands of a second), or "mSec".
Understanding the Specs
The act of locating and positioning to a specific track is called "seek" and is likewise measured in mSec. It can come in three flavors. A seek can cause the heads to ramp from one end of the drive, say the outer most track, to the other end, the inner most track or vice versa. Obviously, this end-to-end movement represents the worst case as far as seek time. It is specified as the "full stroke" seek time. Another case is the head having to move only one track over, most common in reading data from a large file that extends from one track to another. This is called "track-to-track" seek time and represents the best case. The last is a measure of the average time required to perform a series of random seeks to various tracks and is referred to as "average" seek time.
Here is another factor, but one that is seldom specified. Given that the heads are at the end of long arms that are being swung along an arc by the actuator, and considering that the track being hunted is very narrow and separated from the adjacent tracks by fractions of microns, once the actuator has stopped, the head will require a finite amount of time to stop jiggling around and hover precisely over the target track. This time is called "settling" time. If you look at a drive's track-to-track seek time and then its full stroke seek time, it will be obvious that it takes a long time for a head to move just to the next track as compared to the head moving across a thousand tracks. In other words, it doesn't take a thousand times longer to move the head across a thousand tracks. This is because a good portion of that seek time is really settling time and is determined by a pre-programmed delay in the drive electronics.
Once the seek has taken place, the drive must wait for the target sector to rotate into position under the head. This delay is called "Rotational Latency", and "average latency" is given to be one half the time it takes the platter to make one full rotation. It will be the same for all drives running at the same rotation speed. There isn't much you can do about it. However, the faster the rotation speed of a drive, the less time it takes the target sector to rotate into position. Faster is better.
It is important to note that drives are formatted with what is called "track skewing" where by the sectors of adjacent tracks are not laid out next to each other, but are offset along the arc of the track. It's designed so as to be more likely that the next sequential sector in a read or write operation will be ready to rotate under the head after the amount of time it takes the head to move to the next track has elapsed. Because the rotational latency number is specified for new, random accesses, we can't speak properly of the latency spec as it simply doesn't apply to the sequential access case.
As the drive reads a large file that extends beyond the capacity of a single track, the drive will switch heads to the same numbered track of the next platter to continue reading or writing. This wastes less time than doing a track-to-track seek after filling every track. In technical terms, all tracks of the same number on all sides of all platters is referred to as a single "cylinder".
Therefore, if a drive has 4 platters making for a total of 8 "heads" or sides of platters and each side has 1400 tracks, then the entire drive has 1400 cylinders and each cylinder is then made up of 8 tracks that all line up under each other. The more platters there are in a drive, the more often the heads will be switched from platter to platter during a long read or write, exactly the sort of thing that will happen in a DAW. The heads are switched electronically and thus are not subject to mechanical delays except for rotational latency, and track skewing helps here too.
You might think that the more platters, the better. In rough terms, yes.
However, it is even better if each platter is so large that a track has many more sectors and thus will hold more data before the heads need to be switched or sent seeking the next track. Therefore, the important spec in this area is lowest number of heads for the same amount of storage space. Drives with higher capacity platters are the clear winners here. This spec is sometimes given as the "areal density" or the number of bits per square inch that the magnetic material can hold. Even if this number is not given, you can make a good guess by taking the storage capacity of a drive and dividing by the number of heads in the drive (that is, the number of PHYSICAL heads, not the number reported by DOS). If you compare this figure among drives, you will have a good guide even if areal density isn't listed.
Now let us review and prioritize:
1) In DAW streaming access, we must read or write small chunks of data for each audio track, and so random seeks are the majority. The average track seek time holds a lot of weight, although it will be reduced if all data accessed is inside a relatively small part of the disk. The lower this average seek time, the better.
2) The higher the areal density, the more data will be throughput before the head will have to switch platters or move to another track. Therefore, the higher the capacity-to-head ratio, the better.
3) Finally, rotation speed is a big factor in increasing throughput. If other factors are held constant, the faster a drive can spin, the better.
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