Detective Story Begins
Before we pass over to the specifications comparison of the two latest Deskstar generations, I would like to tell you a mystical story about IBM’s behind-the-scenes game. The thing is that 180GXP announced in September and its today’s version differ from one another. Firstly, the specifications now list additional 40GB models marked as (sic!) IC35L060AVV2 40GB. Why “models” in the plural? Well, because there are two of them, and we can only guess what is the sacral idea behind this dualism. :) Anyway, there is a regular 40GB solution and its “optimized” version, though there is no mention at all about the details of this optimization. Thanks goodness they listed the specifications of the two, so that we could somehow figure out the changes that have been made:
I suspect that the non-optimized version is the already familiar cut-down product with a few density zones that have been disabled. However, cutting off one third of the storage capacity will inevitably lead to significant decrease of the average access time. This situation threatened to ruin the hierarchy in the product line that is why they decided to give up the densest cylinders and “thin out” the tracks. As a result, the per-track density in the optimized model has been reduced from 76 to 72 thousand per inch. Stop, how is that possible?
The previous specification version has never heard of 76 ktpi data density; all models features 72 ktpi density. If this is not just a misprint in the product description (which is hardly possible, because the total data density has also mysteriously increased from 45.5 to 46.3GB/sq.inch), then the whole thing appears extremely interesting. After the mass production of Deskstar 180GXP had been started, the engineers found a way to increase the per-track density by another 5%. At the same time they had to start producing 40GB models, that is why all newly manufactured platters with lower data density per track were used for the 40GB models manufacturing. This way, it becomes absolutely clear where the pretty big idle part of the platter in a 180GB model comes from (see our article called Western Digital WD2500JB HDD: More than Drivezilla?!, where we mentioned this fact): the tracks became somewhat denser.
As you remember, we discovered an unusually wide circle on the external “fastest” part of the platter, which was not used for data storage. If they had utilized this area too, they could have got 66GB platter capacity instead of 60GB and design a 200GB model (Maxtor and Western Digital seen to have done exactly what I am talking about, unfortunately, these companies keep silent about the number of cylinders in their solutions). Why did IBM decide to sacrifice maximum linear speed and storage capacity? Maybe it is the limited capabilities of the existing GMR heads or the insufficient bandwidth of the DSP read/write channels.