Re: [Bug-ddrescue] Feature Suggestion: Automatic Cooldown mode

[Franc Zabkar]

It is not always possible to determine the number of platters and heads 
from the published specs. You can sometimes see the number of heads in a 
short-stroked HD Tune read benchmark graph, but obviously this is not 
something that you would do to a sick drive.

For example, I have seen a case where a 500GB drive was created from a 
750GB drive that had been short-stroked. Instead of a single 500GB platter 
and two heads, the drive had two platters and three heads, and the number 
of zones had been reduced accordingly.

Another example is Seagate's 2TB ST2000DM001 which can have 4, 5, or 6 heads.

http://www.users.on.net/~fzabkar/HDD/GRCC4CD9.TXT

In the past I have used a rule-of-thumb for estimating the platter density 
from the maximum sustained data transfer rate measurements. It works for 
older drives, but is not reliable for modern ones.

If you assume that a 5400RPM Green WD drive is equivalent in every respect 
to a similar 7200RPM Black model, except for the rotational speed, then the 
ratio between their transfer rates should be 3:4 (= 5400:7200). I know that 
7200RPM 500GB-per-platter models typically have max transfer rates of 125 - 
130MB/s, so the 5400RPM models could be expected to perform at 94 - 98 
MB/s. However, WD's datasheet for the WD20EARS specifies 110MB/s, so that 
would suggest a higher data density.

Let's assume that everything scales up equally when going to higher platter 
densities. For example, if we wish to quadruple the data density (bits per 
square inch), then we would need to double the number of bits per track, 
and double the number of tracks per inch. The transfer rate is dependent on 
the bits per track, so therefore ...

 (transfer rate A) / (transfer rate B) = sqrt[(data density A) / (data 
density B)]

Applying this formula to the subject drive we have ...

 data density = (110 / 96)^2 x 500 = 656GB per platter

I stress that this relationship is not very reliable for current models (in 
fact it appears to be unreliable).

There are definitive ways to determine the actual number of heads, plus 
there are ways to prevent the drive from reallocating bad sectors, but 
these involve the use of Vendor Specific Commands. If someone would be 
prepared to work with me, then perhaps we could incorporate such features 
into ddrescue.

See http://forum.hddguru.com/viewtopic.php?t=27905&p=191573#p191573

-Franc Zabkar



At 08:05 PM 6/02/14, you wrote:
> Hi Scott,
>
> Thanks for the details. The Model number of the disk is WDC
> WD20EARS-00MVWB0. The reason I assumed it would be 4 platters with 8
> heads is that it's more than two years old and at the time was the
> largest capacity you could get. A web search for head count is
> somewhat inconclusive, with some sources claiming that the same model
> might have different configurations. This here seems quite thorough
> and would suggest it's actually three platters with 6 heads:
>
> http://rml527.blogspot.de/2010/10/hdd-platter-database-western-digital-35_1109.html