Information Technology Reference
In-Depth Information
As this description indicates, erasure involves an initial step not
present for simple file deletion, but thereafter the two processes are
the same.
In deleting a file, the space is no longer allocated for file use, and
that space is recycled for use by another application.
However, when a file is only deleted, the actual data from that
file are still physically present on the disk. This means that, at least
in principle, the old data might be accessible to a new application
that happened to reuse this space. This deletion process is quite
analogous to the disposal of papers by placing them in a wastepa
per basket or trash bin. Although the material is gone from the
standpoint of the original user, the information might be accessible
to a nosy neighbor who is willing to take time going through the
trash.
Erasure of the file, on the other hand, physically removes the in
formation (analogous to the use of a shredder in our paper anal
ogy), so a nosy neighbor cannot make use of the old information,
even if she or he might find an old disk.
should I care?
As noted during the discussion of file storage on disks, informa
tion is divided into pieces, which are stored in tracks and sectors
throughout a disk. When processing disk data, the disk must spin so
that the relevant sector may pass under the read/write head.
However, when the pieces are widely scattered, the read/write head
must move considerable distances (e.g., between the center of the
disk and the outside edge) to get to the required sector. Such moves
can take considerable time, producing noise on the disk drive some
what akin to what one hears during thrashing. In the terminology
of computing, we say that files whose pieces are spread over much
of a disk are
fragmented
. When a disk contains many fragmented
files, we say the disk is
badly fragmented
.
At the other extreme, an ideal placement of files on a disk oc
curs when sectors of a file are located on contiguous sectors. Such a
contiguous arrangement allows for easy file access, primarily
through only an initial movement of the read/write head. Once the
