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[ The PC Guide | Systems and Components Reference Guide | Hard Disk Drives | Hard Disk Performance, Quality and Reliability | Hard Disk Performance | Hard Disk External Performance Factors | File System Factors ]

Cluster Size

The FAT file system used by DOS and Windows divides all of the file data on the hard disk into clusters comprised of multiple sectors. A cluster is normally between 2 kiB and 32 kiB in size, in powers of two, containing between 4 and 64 sectors of user data. This is done to make managing the location of data easier. Clusters and related file system structures are discussed here.

The choice of cluster size has an impact on real-world performance,  though for most people it is not all that significant. In a nutshell, larger clusters waste more space due to slack but generally provide for slightly better performance because there will be less fragmentation and more of the file will be in consecutive blocks. This occurs because when clusters are larger, fewer of them are needed than when they are small. A 10,000 byte file would require three 4 kiB clusters but only one 16 kiB cluster. This means this file will always be in a contiguous block if stored in a 16 kiB cluster, but could be fragmented if stored in a 4 kiB cluster size partition. (The slack tradeoff is a waste of 4 kiB more storage in the case of the 16 kiB clusters.) Small cluster sizes also have a negative effect on partition because they require larger file allocation tables, to manage their much larger numbers of clusters. These tradeoffs are discussed in detail here.

Traditionally, most people have tried to use cluster sizes as small as possible in order to reduce slack and make more efficient use of disk space. This is of course a valid goal, but it has become increasingly irrelevant today as hard disks approach truly gargantuan sizes and the price per GB of storage drops to amazingly low  levels. Today, the large file allocation tables resulting from enormous FAT32 partitions means that balancing slack reduction with performance effects is also important, unless you are on a very tight budget. I certainly can't recommend forcing Windows to use 4 kiB clusters on a 30 GB partition "to reduce slack" as some people do, because I personally wouldn't want to take the performance hit of having 30 MiB file allocation tables--and I wouldn't want to have to wait for that puppy to defragment either! :^)

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