Allocation unit size refers to the cluster size. You need to select an allocation unit size while formatting a new hard disk. Usually, the default cluster size while formatting is 4096 bytes. If you are interested in knowing your hard disks’ cluster size then you can find it by looking at ‘How to Find the Cluster Size of Hard Disk’.
Why is Allocation Unit Size important?
The selection of cluster have an impact on the performance! For instance, a 10,000 byte file would require three 4 kiB clusters but only one 16 kiB cluster. Here, this file will always be in one block if stored in 16 kiB cluster, but it can be fragmented if stored in a 4 kiB cluster size partition. This means that there is a wastage of space, i.e.; a loss of 4 kiB in the 16 kiB cluster. But, selecting larger clusters results in faster access times.
Large clusters or Smaller ones?
If you are storing large files like game files, HD movies, videos, etc, larger clusters will be good. But, if you’re storing small files then smaller clusters are nice.
What is the best allocation unit size?
Here, I have taken a pen drive and formatted it using two types of allocation unit sizes:
Allocation unit size of 4096 bytes
Allocation unit size of 16kb
Here you can see the differences in the allocation units available on disk. The higher the allocation unit size, the lesser will be the allocation units available on disk. If you aren’t sure of what you should choose then simply select the default cluster size, i.e; 4096 bytes.