Cluster, cluster size, and slack The filesystem does not keep track of the disk space allocation (what space is used for which file and what is free) down to the byte level. That would require too much maintenance. Instead, the disk space divided into equal blocks called clusters. The filesystem then allocates disk space cluster-by-cluster….
The reason the data takes up more space has to do with the cluster sizes used to store data. Microsoft operating systems using the FAT32 file system use varying cluster sizes depending on the size of the partition.
The following chart gives a breakdown of the partition/cluster size relationship using FAT32
| Partition Size | Cluster Size |
|---|---|
| 512 MB – 8192 MB (8 GB) | 4 KB |
| 8193 MB – 16384 MB | 8 KB |
| 16385 MB – 32769 MB | 16 KB |
| Greater than 32769 MB | 32 KB |
A cluster is the smallest unit used by the operating system to store data. Each piece of data, regardless of how small, uses at least one full cluster. For example, if you have a 6 GB partition in FAT32, it will have 4K clusters. If a file stored to that cluster is 3K, the entire 4K cluster will be used. On the other hand, with an 80 GB partition using 32K clusters, that same 3K file still uses one full cluster (32K). You can see that with larger cluster sizes there is the potential for more wasted space. In most cases, this is not a problem as most files will not be that small. If multiple clusters are used to accommodate a file’s size, the system will use as many clusters as necessary for the file, leaving wasted space on the last cluster used.
Clusters are sized in this way to balance speed and efficiency. If the larger partitions still used the smaller 4K clusters, utilities like ScanDisk, Defrag, etc. would take hours to complete.
