Cad Guidebook: A Basic Manual for Understanding and Improving Computer-Aided Design (8 page)

Computer Hardware Basics 19

turing operations, then changes to those drawings may be restricted until the
manufacturing operation is complete. In this case, the drawings (contained in a
CAD program’s computer files that are on a disk drive) may be marked or identi-
fied as Read Only. The granting of these privileges to access data on disk drives
may be controlled by the operating system or the CAD program, or a combina-
tion of the two.

Another important issue to consider is how the disk drive finds those tiny
regions of magnetic space. Obviously, the computer must have a way of mapping
or translating from the name of the data file the user is interested in to the actual
physical location on the disk. This is generally accomplished by have a sort of
master file on the disk drive. This file is not really accessible to the user, but it is
accessible to the operating system which is controlling the computer. For PCs,
this master file is often referred to as the FAT (File Allocation Table). On work-
stations, this sort of information is related to the “inode table” (an inode is a
unique file identifier number), or perhaps as the “superblock” data (a “block” is
another unit of space on the disk, it is usually 512 or 1024 bytes). This data is
referred to as a table since its function is as a lookup table. The system “knows”
what data file the user wants to access, so then the table is used to find or “lookup”
the proper location on the disk (or following directory structures). The exact work-
ings of this master file and how it leads to data storage and retrieval is beyond the
scope of this work, but it is important to know that the master files are there.

2.5.3 Backups

One of the most important issues with respect to the master file is that it can be
corrupted. The disk drives can and do fail; they wear or are damaged to the point
that they make a “mistake” in reading or writing the users data. If this mistake
occurs on the region of the disk where the master file is located, then ALL the
data on the disk drive can be lost. The computer can no longer determine the lo-
cation of all the other files. Obviously, this is a very bad situation that must be
avoided. However, since there is no way to absolutely predict when the failure
may happen, the only practical recourse is to back up all the disk drive’s data.
Backup in this case means to copy all the data on the disk drive to some other
device or devices so that a backup or reserve copy is available. In many cases, the
backup copy is to a more stable or static magnetic material such as magnetic tape
in tape drives. These devices are similar to the disk drives in that they use mag-
netic film to store the data, but there is no spinning disk. Instead, there is mag-
netic tape. These tapes are then stored in cartridges and filed in places that are
protected from physical damage. Since the master file on the disk drives as well
as the data on the disk drives must be synchronized or be in agreement, this activ-
ity of writing the tapes is often done at times of day when the computer system is
not in use.

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In addition to the creation and keeping of backup tapes, many CAD sys-
tems will rely on redundancy. In this case, the data created and used by CAD
users are actually written to 2 physical disk drives. This provides a redundant
source of the data. These kinds of disk drives often come in an array or drawer
that contains a complete set of disk drives. These sets of disk drives may also be
referred to as a RAID array. RAID is an acronym that relates the level of redun-
dancy achieved by the set of disk drives; a number may be used in connection
with the RAID array, such as RAID Level 5. Other terms that have to do with
redundancy are mirroring and striping.

2.5.4 Formatting

Another issue with respect to the behavior of the disk drive device is formatting.
When a disk drive is first made available to a computer system, the master file
needs to be created on the device, and the master file needs to know where the
available space exists. This is the process known as formatting. There are often
different levels of formatting. A low level format would actually have the heads
scan and/or check every region of the disk. A normal level format usually just
creates a brand new master file. Of course, creating that new master file means
the old master file is gone, and thus all the data that was on the disk drive is basi-
cally gone (it can’t be located any longer).

The use of formatting is generally confined to setting up new computers,
but it can be used to improve performance as well. If the master file has been
used on a computer system for a long time, the file may become large. Then, even
when all the files that are residing on the disk drive are removed, the master file
may still be large. In this case, reformatting the disk drive may speed up the rate
at which files can be looked up in the master file. Another performance improve-
ment that may be realized from low level formatting has to do with marking
physical regions of the disk as unreliable or unusable. In this case, the master file
is able to know that certain parts of the disk should no longer be used.

In addition to the performance aspects of the master file, the master file
concept can also be used to undelete files. When a file is removed or deleted from
a disk drive, the only action that generally happens is that the data in the master
file about the deleted file is removed from the master file. However, the physical
region (or regions) of the spinning disk that actually stored the data is unchanged.
Obviously, if the information in the master file is not completely removed during
the delete process, but simply put aside for future use, it would be possible to
restore the file that was deleted. Of course, if the physical region of the disk
(which would then be considered available for other files) is re-used for new data
by the master file, then the “undelete” process is not going to work very well. In
general, PCs have had the undelete capability for many years; however, the work-
stations have never had this capability. One reason not to include this capability is

Computer Hardware Basics 21

the potential for slowing down the overall speed of data access. If the master file
completely “forgets” the deleted file information, then the master file can remain
more compact.

Another issue with respect to disk drive behavior is the importance of how
the data in a computer file are actually located on the spinning disk. This is espe-
cially true for the very large files, and CAD files, particularly 3-D models, tend to
be rather large (as mentioned earlier with respect to the memory system it could
be 100s of megabytes). With these large files the data is going to far exceed the
capacity for storage of one particular region of the disk (that is a particular cylin-
der or block). In this case, the data is going to be written to a large portion of
these regions, and the master file is going to have to keep track of where all those
regions are located.

Clearly it would be best if all that data is written to regions of the disk that
are “next to each other” or contiguous. In this case, the master file will only have
a small amount of information to store; i.e., where the large file starts on disk and
how far to go to read all the data. On the other hand, if the large amount of data in
a big CAD file is broken into many small pieces, and these small pieces are in
regions of the disk that are not contiguous, then the heads above the spinning disk
have to move many times (perhaps 100s or even 1000s of times) to reassemble
the entire file into the memory system. Clearly this situation is going to be pretty
devastating to the performance of the disk drive and thus the entire storage sys-
tem. This situation is sometimes called “thrashing,” although this term may be
best considered to apply to the situation of the memory system having to unload
data to the storage system, and the storage system has to rapidly attempt to shut-
tle data to and from memory (or swapping).

One important method to prevent this breakdown in performance of the
disk drive is to not let the disk drive capacity become too near to its maximum.
As the used regions of the disk grow, the available regions not only become
smaller (since less and less free space is available), these regions become spread
apart as the larger files create smaller gaps. As a rule of thumb, no disk drive
should exceed 80 percent of its maximum capacity. On the other hand, if a disk
drive is 90 or 95 percent used, one can be certain that performance is suffering as
the heads attempt to find and assemble files.

It was mentioned earlier that reformatting a disk drive will improve perfor-
mance, and indeed, this will help alleviate the situation of the heads not getting
data efficiently due to the poor organization of the data on the disk. However, it
was also mentioned that the reformatting will erase all the data in the disk drive.
Obviously, this is not always acceptable. In this case, a technique called “defrag-
mentation” can be used. In this case, a special computer program is used to
“shuffle” the data around on the disk (without doing the more drastic reformat-
ting). The program reads a file into the memory system, then it deletes enough of
the file to create some contiguous space on the disk. Then it rewrites the file from

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the memory system back to this improved region. This process continues, file by
file, until the all the files on the disk drive are in a more efficient arrangement. Of
course, one must be certain that all the data on the disk drive has been backed up
(as mentioned earlier) before anything like a “defrag” program is used. If the
computer system should shut down for some reason during the running of the
program, the result could be somewhat catastrophic.

In the chapter on software, the concept of directories and subdirectories for
files is presented. This technique of organizing larger numbers of files on a disk
drive has some bearing on the performance, as well. It is the master files that
need to keep track of the directories, and thus the same arguments of spreading
data too thinly on the disk, or making the master file too large and cumbersome
can apply to directories. For instance, if there are 10,000 files on a disk drive and
they are in a single directory, then the master file for that directory will be rela-
tively large. On the other hand, if these files are spread into 10 different subdirec-
tories, and there is one master file for the top level directory, and then 10 smaller
master files for each of the directories, then these master files will be much
smaller, and therefore more efficient. Unfortunately, not all operating systems
take complete advantage of this sort of efficiency, but it is still to be considered
very beneficial for CAD programs, in particular, to keep files in a well-organized
system of directories.

2.5.5 Disk Cache

The final concept to be mentioned with respect to the storage system is the disk
cache. As already explained, the memory system (which is closely tied to the
CPU) operates at a much higher rate of data transfer than the storage system. One
way to improve the speed of data transfer for the storage system is to dedicate
some memory chips to just communicate with the storage system. This relatively
small amount of memory (perhaps a few percent of the overall memory system
size) is called a cache or disk cache. It forms a buffer between the main opera-
tions of the computer and the disk drives. And, special software may be running
within the computer to attempt to keep the data “most likely” to be needed from
the disk in that special memory area. This can certainly improve the performance
of the storage system, and generally, the more memory that can be made avail-
able for the cache, the better the performance. Also, many systems will use this
cache technique not only between the disk drive and the memory, but also be-
tween the memory and the CPU. These memory-type of caches may also be
found on the CPU of the system.