[RESOLVED] block vs sector vs cluster



New Member
Im looking for clear definitioins of what a block, sector and cluster are.

My current understand is that a block is the smallest unit that can be read/written to on a physical disk.

A sector is a number of blocks.

Im not sure what a cluster is though. i have saw cluster sizes bigger and smaller than the block size so im unsure where it fits in.

I also believe that applications can read in blocks of varying sizes ( e.g. sql server reads blocks of 64kb and sometimes512 kb). so where do these blocks fit into the scheme of things?

can anyone clarify this for me and correct any inaccuracies in my understanding.

Last edited:
Steve R Jones

Steve R Jones

If you Google-> hard drive terminology
You'll find sites like: http://www.dewassoc.com/kbase/hard_drives/hard_drive_glossary.htm

A group of bytes handled, stored, and accessed as a logical data unit, such as an individual file record.

A group of disk sectors. The smallest allocatable unit of disk storage allowed; each FAT entry represents one cluster. Under FAT16, an average cluster is 16K; under FAT32, clusters are only 4K on partitions up to 8GB.

A sector is a logical segment of information on a particular track, and is the smallest addressable unit of storage on a disk. Tracks are divided into sectors, with each sector 512 bytes long. They contain data, but also contain information as to where the data is located, among other useful bits of information. Modern drives use ZDR - Zone Density Recording, where there are more sectors per track on the outside of the disk where there is more surface area, and fewer and fewer sectors as you go in toward the center of the drive. Newer drives have about 16 zones now. This allows more data to fit on the drive. The outer zones therefore have a higher data transfer rate that those closer to the center of the drive.


New Member
In Linux manuals, blocks and clusters tend to be synonyms. They mean a group of disk sectors.