Fragmentation
When the blocks of a file are stored in non-contiguous (separate) places on a disk. It happens over time as files are saved, deleted and edited.
The hard disk
Simplified view of a hard disk surface. Each square is one block; the small number is its address (0 → 39).
Defragmentation progress0%
Blocks moved: 0
Key terms
Contiguous
Stored next to each other, one after another, with no gaps between the blocks.
Block (cluster / sector)
The smallest unit of storage the operating system uses on a disk. A file is stored in one or more blocks.
Read/write head
The part of a hard disk drive that moves across the spinning platters to read and write data magnetically.
Seek time / access time
The time taken for the head to move to the correct position. More head movement means longer access times.
Free space
Blocks that are not being used by any file and can store new data.
Defragmentation
Rearranging the data on a disk so each file is stored in contiguous blocks and the free space is grouped together.
Utility software
System software that helps maintain the computer, e.g. defragmentation, anti-virus, backup and disk formatting tools.
Why defragment a hard disk drive?
- The read/write head moves less, so files are accessed faster.
- Files open and programs load more quickly.
- Less head movement means less wear on the mechanical parts.
- Free space is contiguous, so new files are less likely to be fragmented.
Exam tip: defragmentation does not increase the storage capacity of the disk.
What about SSDs?
A solid state drive stores data in flash memory and has no moving parts.
- There is no read/write head, so data can be accessed equally fast from any location — fragmentation does not slow it down.
- Flash memory can only be written a limited number of times. Defragmenting causes lots of unnecessary writes and reduces the lifespan of the SSD.
Instead, operating systems optimise SSDs with a command called TRIM.
Check your understanding
Exam-style question
Explain how defragmenting a hard disk drive can improve the performance of a computer. [4 marks]
- The blocks of each file are moved so they are stored contiguously / together.
- Free space is gathered together into one area.
- The read/write head needs to make fewer movements (fewer seeks) to read a file.
- So files are accessed / loaded faster.
- New files are less likely to be fragmented when saved.
- Less wear on the read/write head / mechanical parts.