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Next: Disk Defragmenter Up: What is Fragmentation? Previous: An Example

What's Wrong With Fragmentation?

Considering that fragmentation helps increase disk utilization, what is wrong with fragmentation?

Unfortunately, fragmentation decreases disk performance. When files are fragmented, the disk head must move from one area of the disk to another to read all of the fragments belonging to a file. The more fragmented a file is, and the more files there are on the disk, the worse disk performance gets as the disk head moves back and forth over the disk. [Pro95]

John Ruley, in an article for WINDOWS Magazine, ran a hard disk performance test on his NCR 486/33 running NT Server 3.5 using WINDOWS Magazine's NTHell version 2.0 benchmark program. He conducted a test of low-level, uncached performance using a 4 megabyte file and a test of cached performance using a 1 megabyte file. His results showed that while fragmentation has little effect on the uncached 4 megabyte file, it did have a significant effect on the cached 1 megabyte file. When severely fragmented, disk throughput was under 1.5 megabytes per second. After defragmentation, disk throughput improved to almost five megabytes per second. [Rul95]

Fortunately, there are solutions to this problem. The HPFS and NTFS file systems, used by IBM OS/2 and Windows NT respectively, try to find a contiguous block of free disk space big enough for a file. If there isn't a large enough block of free disk space anywhere on the disk, then HPFS and NTFS fragments the file. [Pro95] Another solution is to use one of the available disk defragmentation programs. The next few sections describe some of these programs.


next up previous contents
Next: Disk Defragmenter Up: What is Fragmentation? Previous: An Example
Barnett Hsu
1998-10-31