Harddisks: Seek, Read, Write, Read, Write, Slow ?

On Thu, 15 Jul 2004 00:38:50 +0200, "Folkert Rienstra"
wrote:
snip
Is this a hardware issue ? ( Harddisk Read Head and Harddisk Write Head
can't work together like this and an extra seek is needed ? )

There are NOT two heads

Yes there are, you mighty clueless one.

Actually, the document you quote seems to prove you wrong.
A bit of sloppy writing in the document that confused you probably.

"The head consists of a thin film inductive write element and an MR read element.

"The head consists..." Now, english is not my native language, but I
think "the head" clearly indicates ONE head. Otherwise they would
have said "the heads"

But that single head has two elements: a write element and a read
element.

The read element is typically narrower than the write element to improve the
off-track performance. In practice, there is an offset between the center of
the read and write elements due to the longitudinal separation of the elements.
When used with a rotary actuator, the head is skewed with respect to the tracks
as the actuator moves across the disk. The result is a lateral offset between the
read and write head centerlines.

This statement is completely logical when you have one head with two
elements. It doesn't make sense when you would have two seperate
heads.

Optimum performance is achieved by centering the read head over the data track
for read operations, and centering the write head over the data track for write
operations. This operation will cause the read head to be partially off-track
during a write operation. "

Here the confusion starts. The document suddenly speaks about "read
heads".
But that doesn't make any sense when you compare it to the first part
of the document. But if you replace the words "read head" and "write
head" with "read element" and "write element" it makes perfect sense
again.

Source: IBM/HGST http://www.hgst.com/hdd/ipl/oem/tech/noid.htm

Looking at that document it becomes crystal clear that there is only 1
head with a write and a read element, but that the author sometimes
uses "read head" where it should really be "read element".

but just a single head per disk surface. That head both reads and writes
No, it doesn't.

Yes it does. It's clearly shown in that info from IBM/HGST.

but can do only one or the other at a given instant.
There is a finite switch time between read and write mode.

Because of the 2 heads, one aligned behind the other.

One head, with 2 elements. One behind the other.
As you can clearly see in figure 4 on the webpage you have pointed at
yourself.


Marc