[Ti] Need new HD for my TiBook

[Scott Strehlow]

On Jan 26, 2007, at 10:13 PM, David Brostoff wrote:

>>>  The high speed is definitely a user-advantage.  If budget is a
> This what I would have thought, but this article has given me a new 
> perspective on speed vs. size: <http://www.barefeats.com/mbcd7.html>. 
> It seems to make the point that given the same amount of data, 
> lower-speed, larger drives may be about as fast as higher-speed, 
> smaller drives.

If the limiting factor for speed is getting bits onto and off of the 
platter, then it is the combination of rotation speed and bit density 
that determine speed.  If a 5400 RPM disk has a bit density at least 
1.33 times higher than a 7200 RPM, then it will move data faster.  
Larger drives get the added capacity either by adding platters and 
heads, or increasing the density of the data on the same area of the 
platter.  This increased density will naturally increase the data rate 
also.

The two speed factors that most affect the perception of speed is the 
sustained data rate and the seek time:

Sustained data rate is a measure of how many bits can be pulled or 
pushed per second.  It is determined by the platter speed, bit density, 
buffer size, and is bounded by the interface.  In most cases, the 
interface will be substantially faster than the drive hardware.  If 
using an external drive in a USB or Firewire enclosure, the interface 
might limit it.  Especially if other devices are sharing the serial 
bus.  The buffer size doesn't affect the burst speed but will affect 
the sustained speed.  Having a large buffer allows the drive to jump 
around a bit looking for the next piece of the file without 
interrupting the stream of data already read.

Sustained data rate is most critical for applications like capturing or 
playing video or burning a CD/DVD.  The drive must keep up with the 
data stream or video frames will be lost.  When burning a disc, the 
buffer in the optical drive must never be empty or the disc becomes a 
drink coaster.

Seek time is determined by how fast the head can move from track to 
track, and the rotational speed.  Once the head gets to the desired 
track, it must wait for the chosen sector to pass under the head.  This 
will add on average half the rotation period for each seek.  The faster 
the disk spins, the less time is required to wait for the sector to 
come around.  This can be minimized by reading multiple sectors 
out-of-order.  If a file occupies several sectors in a track, it can 
grab the first that passes the head and place it in the buffer, then 
get the other parts from the track.  The data packets get read from the 
buffer in the proper order.

Seek time is most critical during processes like booting the computer 
or searching operations where many different files need to be "seen" in 
a short period of time.  It is also important for things like database 
queries from a large database file that spans a significant area of the 
disk, or is highly fragmented.

I put a Travelstar 7K60 7200 RPM drive in my G4 a year or so ago, 
replacing a 20 GB 4200 RPM stock drive.  They both have the same number 
of heads so the bit density is about three times higher and the 
rotational speed is almost twice, resulting in a burst rate of almost 
six times higher.  The buffer is also bigger.  I think this drive has 
an 8MB buffer vs. 2MB in the old drive.

I noticed a significant decrease in boot time from the old drive.  Of 
course, I've forgotten how long it used to take to boot so now it still 
takes too long...  It does seem to run hotter than it used to.  Though 
I am having some problems with the fan in my TiBook now too so that may 
be a factor.

Scott
-------------------------------------------------
Scott Strehlow, RDA, CDA, BS

Life is for service.