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Electronics Manufacturing – M528

Western Digital's Raptor X - Spinning ahead of the competition.


Introduction.

There is a wide range of desktop PC hard-drives on the market today but one set of drives has managed to stay ahead of the game for nearing four years now, the Western Digital Raptor series. These SATA drives were originally aimed at the small server market, as a cheaper and more flexible alternative to SCSI drives. However, the drive has become extremely popular with the performance and gaming community. Over recent years technology has advanced immensely and as such the games have increased in complexity. This in turn led to the games being larger in terms of size and nowadays it is not rare for “levels” or the equivalent to take up gigabytes of space. The faster this can be loaded the better and as such a speedy hard drive is a must for any respectable gamer’s system. Western Digital has realised the value of this market and has even started catering drives purely for it. The Raptor X Drive even features a transparent panel through which the read/write head can be seen buzzing about carrying out its tasks. The pictures below are sourced from The Tech Report (2005, Western Digital's Raptor X Hard Drive) and show the Raptor X in all of it's glory.

Raptor X Raptor X

Below is a table sourced from Extreme Tech (2006. Western Digital Raptor X 150GB), containing the various performance specifications of the Raptor X, and pitting up against two “Standard” drives allowing the differences in speeds to be observed. Unfortunately the hefty price tag can also be seen, this is especially high considering the relatively small capacity of the drive.

Comparisons

Although there are many factors which affect a drives performance, the revolutions per minute, or RPM, is generally considered one of the most relevant. Only until recently the Raptor was unmatched as the only desktop SATA drive to operate at 10 000 RPM. The current standard revolutionary speed is generally 7200 RPM. The aim of this article is to dissect Western Digital’s flagship drive, the Raptor X, and see the components involved in achieving its impressive results.

There is no point in taking apart a hard drive unless it is understand how it works. So the first task is to cover how the Raptor, and indeed most hard disks operate in general. Hard disks are so called because the part that actually stores the data is a solid metal platter, there are usually multiple platters in a single drive. The other main components of a disk drive are the motor, the spindle, the read/write heads, and finally the various chips that drive and control the processes. These components are covered in detail further on in this article. A very basic overview of how a hard drive functions is as follows: The read/write head magnetises specific parts of the platter when writing to the disk. The patterns of the magnetised areas stay stored on the platter until modified. These magnetised areas represent the binary data stored. The combination of the platter spinning and the head scanning from the centre of the disk outwards allow the read/write heads to access every part of the platter.

The Platter.

The Raptor X consists of two identical platters, each capable of storing 75GB of information. This gives a total of 150GB of storage. The platters are made from aluminium, and are coated with a thin layer of a cobalt based magnetic material. This is done by a vacuum deposition process known as Magnetron Sputtering. The coating has a complex layered structure consisting of various metallic alloys. These under-layers are optimised for the control of crystallographic orientation and grain size of the actual magnetic media layer on top of them. The magnetic surface of each platter is divided into many small sub-micrometre-sized regions. Each one of these regions is able to be encoded to store a single bit of information. Inside these regions are hundreds of tiny magnetic grains.

The final layers consists of a protective carbon-based overcoat, this is applied again by Magnetron Sputtering. A nanometre thick polymeric lubricant layer is deposited on top of the sputtered structure by dipping the disk into a solvent solution. Eventually the platter buffed by various processes to eliminate small defects. The image below is sourced from Wikipedia (2007, Hard Disk) and shows how the charge is stored on a platter.

Magnetic Media

The main reason grains are used to store the magnetic charge instead of a continuous medium is that they reduce the physical space required for a magnetic region. With a continuous magnetic material, formations known as Neel Spikes appear. These are ‘spikes’ of conflicting magnetisation. Imagine two magnetic regions next to each other, each containing opposing magnetic fields. The area in between the regions would contain spikes from both of the fields, effectively cancelling each other out and forming a grey area. Grains counter this problem because in theory, each single grain is seen as its own magnetic domain. This in turn means that the magnetic domains are set, and they are unable to grow and form Neel Spikes. With a smaller transition width more regions are able to be created on a platter. The image below is sourced from Wikipedia (2007, Hard Disk Platter) and shows the Neel Spikes and how grains prevent them.

Grain

Read/Write heads.

These are the mechanisms that actually sweep over the platters at a high speed and read or write to them with amazing accuracy. The Raptor X consists of four heads. These heads fly about the surface of the platter with a height of just 3 nanometres. This is controlled by something called an air-bearing which is etched onto the disk facing surface of the read/write head. The air bearing tasks is to maintain a constant flight height throughout operation. If the height is too high data will be read and written corruptly, if it is too low a head crash can occur. This will result in permanent data loss and possible irreparable hardware damage. The image below is sourced from The Tech Report (2005, Western Digital's Raptor X Hard Drive) and shows the read/write heads through the transparent window of the drive.

Head

The heads their selves consist of a tiny C-shaped piece of highly a magnetisable material called ferrite. This is wrapped in a very fine metal coil. The Raptor X hard drive uses a technology which is widely in known in disk storage called Giant Magnetoresistance, or GMR. This means that a separate head is used for reading and writing. The read head uses the GMR effect which changes the resistance of a material in the presence of magnetic field. This difference can then be measured to interpret the disks surface. These GMR heads are able to read very small magnetic features reliably, but cannot be used to create the strong field used for writing.

Interface and Electronics.

The two previous generations of Raptor drives did not use a true SATA interface. Although on the outside the drives had SATA physical connection protocols, a chip was used inside to bridge the PATA-style electronics of the drive with the SATA protocols. The Raptor X is a true SATA drive through and through, this is why no bridge chip can be seen on the PCB inside. The Raptor X uses the first implementation of SATA with a 150MB/sec interface. A newer 300MB/sec SATA interface exists but Western Digital did not feel it necessary to use this, as no SATA drive even comes close to fully utilising the 150MB/sec limit of the primary SATA design.

PCB

The image above is sourced from X-bit Labs (2006, Raptor X HDD from Western Digital). The three chips which can be seen on the PCB are the processor, the cache memory and the motor controller chip. The cache its self is fairly standard memory, 16MB of SDRAM. However, uncovering information on the two other chips is a thankless task. Western Digital is reluctant to release information about them, partly because the electronics play an important part to the drives impressive performance.

Time Limited Error Recovery.

The Raptor X also features a Western Digital unique feature called Time-Limited Error Recover (or TLER). This allows SATA drives to be used with RAID controllers. This feature limits the amount of time that the drive spends on correcting and detecting errors caused by vibrations during high loads. This helps to prevent data loss which can happen when multiple drives fail while the RAID volume is in degraded mode.

Rotary Acceleration Feed Forward.

RAFF stands for Rotary Acceleration Feed Forward. This is a method of sensing something which is known as rotational vibration of the other drives in a multi-drive installation. This is then compensated for. The actual actions of RAFF are controlling drive head position and keeping within a safe tolerance during read and write operations. This feature has the ability to increase performance significantly. It does this by reducing the amount of retry read/write efforts where the head is ever so slightly bumped off the track. There has been little statistical proof to show the performance increases that RAFF can provide, but it does not slow down the drive in any way so it is generally welcomed as a feature, even if the effects are near invisible.

Native Command Queuing.

By far one of the most important features of the Raptor X in terms of multi-user speed is Native Command Queuing (or NCQ). This technology is not unique to Western Digital. NCQ gains performance by allowing the drive to internally organise and optimise the order in which read/write commands are received. Quite literally the drive may receive the commands to write, write, read, and write. The drive can re-arrange this as it sees fit to any other combination to improve performance. The image below is sourced from Wikipedia (2007, Native Command Queuing.) and shows how NCQ can reduce head routes.

NCQ

Although this sounds relatively straight forward it can have noticeable benefits. It reduces the amount of unnecessary movements the read/write head has to make in terms of going back and forth. Not only does this increase the performance but it also reduces the physical ware of the drive. Unfortunately this technology is a double edged blade as it can also slow down performance slightly in some cases. If no changes are made to the scheduled order of tasks ahead, then obviously no time is saved. However, a small amount of time would have been added on from the NCQ logic, the actual task of checking to see if it can increase performance.

Conclusion.

As benchmarks continue to show, the Raptor X remains the fastest single SATA drive on the market today. There are two obvious drawbacks to the drive however which are visible from the offset. These are the cost and the capacity. The Raptor X only stores 150GB of data, which in the modern world is very little. There are single SATA drives on the market today capable of holding 750GB. The other drawback is the price, there is a noticeable performance leap from other drives, but you certainly pay for it. The addition of the transparent window on the Raptor X alone adds £30 to its price tag. The window leads to the drives final drawback. The Raptor X contains identical components to the WD1500. The only difference is the casing of the drive, the Raptor X is aimed at the gaming and performance enthusiasts who want their PC to look good as well as run fast. Adding the window actually halved its mean time before failure down to 600 000 hours. Obviously this is still a very impressive figure though. All in all the Raptor hit the nail on the head in terms of delivering for its target market. Performance hardware generally involves cutting edge technology and this drive is no exception.



Reference list

All information has been sourced from the sites bellow. Information has been referenced when used in text.