The one
major disadvantage of the standard
ISO 9660 format for creating CDs is that it requires a lot of overhead. If
you wish to copy several small sets of data to a CD-R or CD-RW over a certain
time period, making daily backups for example, the system will need to create a
separate session for each set of data.
As each session requires its own lead-in which contains the
table of contents and data needed to synchronize the reader for the data to
follow, and lead-out space, the overhead for creating a session equates to 13MB of
wasted space. The UDF file system is an alternative method of storage, created to
allow small files to be efficiently added to CD media without wasting
space.
In effect, the CD will appear as a form of
removable media like a floppy disk, once formatted with the UDF file
system.
This
method does have some drawbacks of
its own though; any information written to the disk has a minimum size of
32KB, meaning that files smaller than this will take up a 32K block of space regardless,
resulting in a fair bit of wasted space. Originally, the UDF file system
required that the whole CD be erased before space created by deleting files
could be used again, but newer revisions of the format allow for random deleting.
This modification allows individual sectors of the disk to be erased and reused
on the fly.
UDF disks still require
formatting before use, which can take a long time. Also, compatibility can be a
real issue, as older CD-ROMs and early CD-R drives will not read CDs created
using packet writing, and so software must be installed to create the disks. Newer
packet writing technologies such as the Mount Rainier standard
provide faster formatting and less wasted space, but still need additional software
to read the created CDs. More on Mt. Rainier in a moment.
Understanding CD-R/RW
media Read and Write speeds
There are a variety of types
of CD-R and RW media on the market, and it can be bewildering at times, though
concerns about the physical makeup of CD-R disks have become somewhat
marginalized as the technology has become commonplace and the price of
individual disks has plummeted.
There are some things that the
buyer should look out for still, the most important of these being the rated
speed of the disks.
As you will have
noticed, CD burners are rated for the speed in which they can create and read
CDs. These speeds are generally listed in the format (CD-R write speed)/(CD-RW
write speed)/(CD read speed); for example 48x/24x/48x. The speed is calculated by taking
the 150KB per second transfer rate of a 1X CD-ROM drive and multiplying it.
The trouble is that the media you use may or may not be able to keep up with the
burner, depending on the process used to manufacture it.
Using a CD-R disk made a few
years ago on a modern 48X or above CD-RW drive, for example, is just not likely
to work well, resulting in errors or outright failure unless you reduce the
write speed of the drive considerably. Modern CD-R media is produced to take the
higher write speeds into account, so the best practice is to make sure the
advertised acceptable speed of the disks you are buying matches the speed of
your burner.
Note that
this is not a hard and fast rule.... media that is not advertised as 48X
compatible may in fact work at that speed, it's just not guaranteed
to, so
proceed at the risk of making lovely drink coasters. Using higher speed rated
media on a lower rated burner will work fine, of course.
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