Allocation Unit/Cluster Size Information

What is an allocation unit/cluster?

An allocation unit/cluster, referred to as cluster throughout this page, is the smallest amount of space a file system entry can take up. Think of it as a block of data that has defined length. There are a variety of cluster sizes. The most commonly used sizes are 4096, 8192, and 16384.

Let’s say you have a file that is 4096 bytes in length. If your cluster size is 4096, the file will fit perfectly and you will be using all available space within the cluster. If the cluster is bigger, such as 8192, you will be wasting 4096 bytes of space. If it is smaller, like 2048, you will be using two clusters to store the file. The more clusters a file takes up, the more work needs to be done to retrieve the contents of the entire file, which decreases performance. Each cluster can only be used by one file. So if you only fill up half of it, you cannot use the wasted, also known as “slack”, space.

This problem is unavoidable and happens in nearly every well-known file system. It is a problem that cannot really be resolved without opening the door to other inefficiencies.

What size should I use?

Small (<=8192)

Pros

-Very little slack space and great for storage of smaller files.

Cons

-Depending on the size of the storage device, smaller sizes could greatly impact the speed in which certain operations are carried out. Ex: 500gb HDD with 2048 as a cluster size; takes nearly 3 minutes to load into FATXplorer.

Big (>=65536)

Pros

-Efficient for bigger files.

-Incredible performance gains.

Cons

-Lots of slack space, expect your storage device to fill up faster than normal.

Review

Basically, if all you care is about performance, go with the largest cluster size (65536). If you care about using all the space available to you efficiently, select a smaller cluster size, such as 4096. If you want a balance, select 16384, the system default.

*Note that the default size varies for each partition.

This page provides you with a basic understanding of allocation unit/cluster sizes. If you wish to learn more about this topic, click here.

What size do you use? Share your experiences in the comments.