Chapter 9: Hard Disks and Floppy Disks^195
Disk Compression
Since hard disks have become large enough for most users, disk compression has passed
out of vogue. But on older systems with 1GB or smaller hard disks, disk compression
extends the capacity of a hard disk drive.Disk compressionuses data compression tech-
niques to reduce the amount of disk space a file uses. The effect is that more files fit into
thesamespace.Understandthatnothingisreallyhappeningtothedisk;thedataisactually
being compressed and stored in a special file. The compressed data must be translated in
and out of the compressed data store.
A disk compression utility must reside in memory and work between the operating
system and the disk controller. The compression utility intercepts any file read and write
actions sent to the disk. When the operating system saves a file to disk, the compression
utility intercepts the file and compresses it before it’s written to the compressed data
store. When the operating system reads a file, the compression utility intercepts the file,
decompresses it, and then passes the data on to the system memory. This utility does add
some overhead to the process and slows down all file access from the compressed disk.
A number of third-party disk compression utilities are available, all of which work
essentially the same. Windows has included disk compression software in nearly all of its
versions. Windows 3.x featured a routine called DBLSpace. Windows 95 included
DriveSpace, which could compress and uncompress data on floppy disks, removable media,
or hard disk drives. DriveSpace works by creating a new uncompressed logical drive,
called the host drive, where it stores the CVF (Compressed Volume File), a form of
VFAT for the compressed drive. The uncompressed drive also contains files that should
not or cannot be compressed, such as system files. Any unused space is available to the
user.TheWindows95versionofDriveSpacecreatescompresseddrivesofupto512MB.
Large disk drives usually can’t be compressed as a single volume. The version available
in Microsoft Plus! and Windows 98 can compress drives up to 2GB.RAID
ARedundant Array of Independent (or Inexpensive) Disks (RAID)is a technique applied to
disk drives as part of a high availability or fault tolerant program to protect the integrity
of the data stored on the disks. RAID employs two or more drives in combination to store
more than one copy of data or to spread the data over several disk drives to lessen the
impact of a disk drive failure. RAID technology is used frequently on network file servers
but isn’t generally used on PCs.
A fundamental concept in RAID systems isdata striping, in which data files are written
across several disks. Data striping retrieves and stores more data than a single disk can
supply or accept. As the first block of data is being written to or retrieved from the first
disk drive, the second block is being set u pby the second disk drive, and so on. Another
feature of a RAID system isdatamirroring, which involves writing duplicate data segments
or files to more than one disk to guard against a device failure.