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Low-Level Characteristics

It’s our pleasure to introduce to you another development of ours (besides FC-Test) – a test that allows gathering precise information about the most interesting characteristics of hard disk drives. The utility isn’t yet finally released, but we can already use it to get some results. For example, we can measure the average seek time.

The seek time is calculated basing on the average time it takes to access a random sector and the spindle rotation speed, which are measured experimentally. The “Measured Buffer” column contains our measurements of the read buffer size, which sometimes doesn’t correspond to the size reported by the device itself. Segmentation is the number of independent data sequences which are stored in the HDD’s read buffer. Leak-ahead shows the maximum size of the data blocks the device can perform anticipatory reading with; a dash in this column means we couldn’t measure this parameter.

Reading this table carefully you can notice the record-breaking average access time of the two senior models with the classic ATA interface. The measurements are half a millisecond better than the specification, which is an unprecedented thing, because the measured time also includes the command overhead Hitachi specifies to be about 0.5 milliseconds for seek and read operations! None of the manufacturers hasn’t yet achieved such a low seek time, and this record also belongs to the three-platter models with the heaviest actuator! On the other hand, the Serial ATA version of the same drive turns to be somewhat slower. That’s strange. The dual-platter models have a typical speed, but also better than their own specs say. The junior models, like the manufacturer declares, are slower yet.

We should give an explanation of the results marked with an asterisk. The truth is that the first sample of the HDS722580VLAT20 didn’t pass the full test cycle – its performance began to degenerate:

After its data-transfer graph took this shape, the participant dropped out of the race. The substitute was first of all passed through the WinBench tests, but its average access time was much worse than the norm. Moreover, this second sample didn’t endure the whole marathon, either. Thus, the “7K250 80GB PATA 2MB” model we will be talking about is a collective image of two samples of this hard disk drive, each of which couldn’t make it through all of our tests.

Now, let’s do the promised check of the read buffer segmentation. The measurements correlate well with the reference data on the previous models: where 12 segments are specified, 11 are measured; where 21 are specified, 20 are measured. For the new series, however, the specified value is as high as 128, but we don’t see anything close to that in our measurements. Yes, there are more buffer segments in the senior models, but not much more, while the number of the segments in the junior models remained the same!

But how do we check out the availability of the look-ahead feature in our test? If after reading a sector with the address N and a short pause, the drive returns the next, N+1, sector in a time interval shorter than the time it takes the platter to make one revolution, then read look-ahead is really supported by the drive. And here we notice one curious detail about the 7K250 models with an 8MB buffer: they do not perform forced look-ahead reading at each access of the disk. They behave more like SCSI drives which emphasize streaming operations rather than a fast response to a random request. Since a modern hard disk drive just cannot live without anticipatory reading, we can only suppose that the firmware of the models with an 8MB buffer contains some new adaptive look-ahead algorithm – something like the hardware prefetch logics in the Pentium 4.

Although the measurements of the look-ahead distance are not ideally precise (well, the measurements produce stable results with small values, like with the Deskstar 120GXP), we can state that the Vancouver 2 (180GXP) and the Vancouver 3 (7K250) have a more aggressive read look-ahead as they read 6 times more data per access that the Deskstar 120GXP.

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