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1992-07-16
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TIP #: 077
KEYWORDS: Hard Drive, Partition, Disk Space
UPDATED: June, 1992
PROBLEM: There is not as much disk space available as there should be when
using large hard drive partitions. How should a large hard drive
be partitioned to make optimal use of available space?
SYMPTOMS: After re-partitioning a 320 Megabyte hard drive on my BBS computer
system, there did not seem to be as much free space available as
there was previously. Originally, this drive was partitioned into
ten 32 megabyte partitions. To make it easier to locate files, I
created two 32 megabyte partitions for programs, and one large
partition for ZIP files. The large partition was just over 256
megabytes in size. When I finished restoring the files, there was
not nearly as much free space as there had been before I began this
project. I did some calculations, and figured that I had lost
about 7 megabytes of space! Where did all the space go?
SOLUTION: The solution to this problem lies in understanding the way DOS
allocates space on a hard disk. Since the DOS FAT (File Allocation
Table) is limited to 16 bits, the maximum number of entries is
65,536 (64K). DOS automatically adjusts the cluster size
(allocation unit), depending on the size of the partition.
The maximum size partition that can be created using 2K clusters is
128 megabytes (2K * 64K = 128MEG). The maximum size partition that
can be created using 4K clusters is 256 megabytes (4K * 64K =
256MEG). The maximum size partition that can be created using 8K
clusters is 512 megabytes (8K * 64K = 512MEG).
The problem in my case, was that I had gone from using 2K clusters
(32MEG partition), to using 8K clusters (>256MEG partition). Even
the smallest file would use at least 8K of hard disk space, since
DOS could not allocate a smaller portion of the available space.
Disk space was wasted on most files, because DOS had to allocate
space in 8K increments, even if a smaller amount would have been
sufficient. Hard disk defects were also removed from service in 8K
increments, wasting even more space.
In order to use hard drive capacity most efficiently, I realized
that 2K clusters should be used. I backed up the hard drive,
verified my backups, and partitioned the drive into three 32
megabyte partitions, and two 112 megabyte partitions. This gave me
the flexibility I needed, and allowed DOS to use 2K clusters for
all partitions. I now have substantially more space, without
purchasing a larger hard drive.
NOTE: If you decide to re-partition your hard disk to reclaim this
wasted space, backup all data and verify the integrity of
your backup!