How does disk defrag work?

The word "disk defrag" is typically used to refer to the
Microsoft Windows utility called Disk Defragmenter. It is
designed to solve a problem that occurs because of the way
hard disks store data.

If you have read the article How Hard Disks Work, then you
know three key facts about hard disks:

1. Hard disks store data in chunks called sectors. If you
imagine the surface of the disk divided into rings (like the
rings of a tree), and then imagine dividing each ring into
pie-slices, a sector is one pie-slice on one ring. Each
sector holds a fixed amount of data, like 512 bytes.
2. The hard disk has a small arm that can move from ring
to ring on the surface of the disk. To reach a particular
sector, the hard disk moves the arm to the right ring and
waits for the sector to spin into position.
3. Hard disks are slow in computer terms. Compared to the
speed of the processor and its memory, the time it takes for
the arm to move and for a sector to spin into place is
an eon.

Because of fact #3, you want to minimize arm movement as much
as possible, and you want data stored in sequential segments
on the disk.

So let's imagine that you install a new application onto
an empty hard disk. Because the disk is empty, the computer
can store the files of the application into sequential
sectors on sequential rings. This is an efficient way to
place data on a hard disk.

As you use a disk, however, this efficient technique becomes
harder for a disk. What happens is that the disk fills up.
Then you erase files to reclaim space. These files that you
delete are scattered all over the surface of the disk. When
you load a new application or a large file onto the disk, it
ends up being stored in hundreds or thousands of these
scattered pockets of space. Now when the computer tries to
load the scattered pieces, the disk's arm has to move all
over the surface and it takes forever.

The idea behind the disk defragmenter is to move all the
files around so that every file is stored on sequential
sectors on sequential rings of the disk. In addition, a good
defragmenter may also try to optimize things even more, for
example by placing all applications "close" to the operating
system on the disk to minimize movement when an application
loads. When done well on older disks, defragmenting can
significantly increase the speed of file loading. On a new
disk that has never filled up or had any significant number
of file deletions, it will have almost no effect because
everything is stored sequentially already.

As you might imagine, the process of indivdually picking up
and moving thousands of files on a relatively slow hard disk
is not a quick process -- it normally takes hours.

For the defragmenter to properly run, ensure you have no
applications running. Typically, SYSTRAY and EXPLORER are
all you need to have running to run this application. You can
see the active tasks you have running by doing a
"three-finger-salute" (Ctrl+Alt+Del). Disable any screen
saver in use, too. The defragmenter will fail to stay running
if your system is constantly accessing some other
application like Findfast.exe, a resource user that
automatically gets installed with Microsoft office. To
prevent Findfast.exe from running at every system boot,
simply delete it from your Windows STARTUP folder, or look
for the Findfast icon in control panel and change its
setting.

The defragmenter can take a considerable time to run, so
start the Defragmenter before going out for the evening or at
the end of the day, before going to sleep.