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Western Digital VelociRaptor
by Marc Prieur
Published on August 7, 2008

Since 2003, Western Digital is the only one to offer Serial ATA drives that function at a speed of 10.000 rpm. This rotation speed up until now was reserved for SCSI drives that could even go as high as 15.000 rpm, gives an additional boost to performances compared to classic models that function at 7200 rpm.

The history of the Raptor started in February 2003 with a 36 GB version while 7 months later its capacity was doubled. We then had to wait until December 2005 for the new generation with a capacity of 150 GB, NCQ and a doubled cache size of 16 MB. Western Digital is now back in the spotlight with the successor to its Raptors, the VelociRaptor.
What are the 10.000 rpm for?
Rotation speed has a double impact on the performances of modern hard drives. The first is in terms of speed. At equal data storage densities, the faster the platters turn, the higher the speed.

However, this is compensated for by the fact that it is difficult to use the same density on faster turning platters. Therefore, 7200 rpm drives make up for their slower speed with platters that generally have higher densities.

The second effect is in terms of latency, an essential characteristic that is added to pure access times. Actually, to access specific data on a platter’s track, the reading head has to properly align itself (what manufacturers call access time) and also the platter has to turn in a way that the data is found under the reading head.

Depending on the situation, the reading head may have to wait a 1/4 or 1/2 turn or more in order to read data and this is the latency which is shortened depending on the rotation speed of a drive. In fact, at 7200 rpm, 8.3ms are needed for a complete rotation versus 6ms at 10.000 rpm.
Le VelociRaptor

While the speed always increased with each new Raptor generation (aided by increases in density), it wasn’t the case for access time which remained more of less the same. So how can this value be decreased without using a rotation speed of 15,000 rpm which isn’t necessarily compatible in terms of sound levels with what we expect from modern hard drives?

Western Digital in fact adopted a simple idea which is already used in professional drives, notably with Seagate models. To decrease the time needed to position reading heads, why not reduce the distance they have to move by decreasing the size of platters?

So here the drive is actually in 2"1/2 format but it comes attached to a 3"1/2 drive radiator, the WD IcePack. Note that it will not fit into a laptop as its height is 15mm versus 9mm for classic drives in this format. Moreover, this wasn’t the only option available to Western Digital as Seagate kept a 3"1/2 format on its Cheetah professional drives despite the smaller platter size (the bigger casing notably enabling better sound isolation). Otherwise, Western’s choice means a non standard placement of connectors and for this reason it is denied access to certain NAS hotswap chassis, for example.

On the inside, we find two platters with a density of 150 GB or a surface density that is roughly 10% less than that found on recent 3"1/2 drives with 333 GB per platter. Compared to the previous Raptor generation, Western announces an access time reduced from 4.6 to 4.2ms and speed which goes from 88 to 120 MB/s.

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