Taking into account everything we have just said, and keeping in mind that the previous Deskstar 120GXP HDD generation included similar models with lower per-track data density, it makes sense to compare both modifications:

You see that the new model with 40GB storage capacity came to replace the IC35L040AVVN: the same case, the data transfer rate remained almost unchanged, although the data seek time grew up a lot. Let’s continue comparing:

The specs promise us 17% linear speed growth, which strikes as very strange keeping in mind that the physical (momentary) speed and the number of sectors per track grew up by only 11%. However, there is an explanation to this phenomenon: the track and heads switch time got notably reduced. You probably wonder how the track switch time affects the sequential read speed? I will try to explain it now.

Look at the Sectors per track parameter in the table, which indicates the maximum number of sectors per track. For 180GXP this density value equals 1092 sectors, which is a little over half a megabyte of data. If the HDD can read the first half megabyte at the maximum speed, then after that it will need to switch to another head or maybe even move to the next track. The time it spent on these operations is included into the overall sequential read time thus reducing the “linear” (or sustained) speed. The ongoing formula has been taken from IBM documentation:

(Sustained Transfer Rate) = A / (B+C+D)
where:
A = (Number of data sectors per cylinder) * 512
B = ((# of Surface per cylinder) - 1) * (Head switch time)
C = (Cylinder change time)
D = (# of Surface) * (One revolution time)

As you see, the track switch time directly affects the linear read speed. And the successful attempt of IBM engineers to reduce this time by 1/3 could have been called another brilliant breakthrough, if it hadn’t been for one thing…

Having checked the specifications of the previous IBM HDDs, starting from DTLA, I was surprised to find out that Head and Sector Switch Time had been constantly growing in every new model: