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What is Cluster Size in Rufus?

Published in Rufus Cluster Size 3 mins read

The default cluster size in Rufus is typically 4096 bytes, serving as a common standard for most bootable USB drives. However, this value is not static and can automatically adjust depending on the specific ISO file being used. For instance, when creating a bootable USB drive for certain operating system versions like Ubuntu 18.04.2 LTS amd64, Rufus automatically sets the default cluster size to 8192 bytes.

Understanding Cluster Size

Cluster size, also known as allocation unit size, refers to the smallest block of data that a file system can allocate for a file. When you format a drive, whether it's a hard drive, SSD, or a USB stick, you set its cluster size. Even if a file is smaller than the cluster size, it will still occupy an entire cluster.

  • Smaller Cluster Size: More efficient use of disk space for small files but can lead to more fragmentation and potentially slower performance for very large files.
  • Larger Cluster Size: Less efficient for very small files (as they waste more space per cluster) but can improve performance for large files due to fewer access operations.

Default Cluster Sizes in Rufus

Rufus intelligently selects a default cluster size based on the requirements of the operating system image (ISO file) you are writing to the USB drive. This ensures optimal compatibility and performance for the resulting bootable media.

Here's a quick overview of the default cluster sizes observed in Rufus:

Scenario Default Cluster Size
General Default 4096 bytes
Ubuntu 18.04.2 LTS 8192 bytes
(and potentially other specific ISOs)

Why Does Cluster Size Matter in Rufus?

When creating a bootable USB drive, the cluster size plays a role in how the operating system image is written and subsequently read from the drive. Rufus's automatic selection of an appropriate cluster size helps in:

  • Compatibility: Ensuring the bootable USB is compatible with the system you intend to boot from, especially for older BIOS or UEFI systems.
  • Performance: Optimizing read/write speeds, although for most bootable USB uses, the difference is often negligible for typical users.
  • Disk Space Utilization: While important for storage drives, for a bootable drive that will be overwritten or used primarily for installation, this is less critical.

Practical Insights

While Rufus generally handles cluster size selection automatically and effectively, there might be rare instances where you might encounter issues or wish to manually adjust it:

  • Troubleshooting Boot Issues: If a bootable USB created with Rufus doesn't work as expected, checking or changing the cluster size (though rarely the primary cause) could be a last resort.
  • Specific Requirements: Some very specific or custom OS images might explicitly require a non-standard cluster size.
  • Disk Formatting: If you use the USB stick for general storage after its bootable purpose, reformatting it with a different cluster size might be beneficial depending on the type of files you plan to store. For general storage, a 4096-byte cluster size is often a good balance.

In most cases, users can rely on Rufus's default settings for cluster size, as it is designed to choose the most suitable option for successful bootable media creation.