Allocation Unit Size And Disk/SSD/USB Speed


Do you want to know allocation unit size speed difference? how large would the speed difference between a 4096 b and a 64kb allocation unit size after formatting? What allocation unit size should I use?

Andrew Wright


by Andrew Wright

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Effect of allocation setting size on disk reading speed

Question: Does the allocation unit size effect the disk speed or usb flash drive or sd/cf card speed?

Answer: It depends. If it's the drive that stores large video/movie files, 64k or larger would be better; if it's for files including the application files, system files, 4k is better.

Resolution: Allocation Unit Size is the size of the hard disk partition cluster. The computer's hard disk storage organization is based on clusters.

Assuming that the file size is 4097 Bytes, if the size of the cluster is 4096 Bytes, the file needs to be stored in two clusters, that is, the actual occupied space of the file is 8192 Bytes; if the size of the cluster is 8192 Bytes If it is 512 Bytes, 9 clusters are required for storage, and the actual occupied space is 4608 Bytes.

choose block size

Question: Does allocation unit size effect the OS speed?

Answer: When the OS is installed, usually the disk was set to 4k which is the best for OS disk, if you set it to bigger unit size, it won't speed the OS.

More factors that would involve in the OS speed: HDD or SSD (SSD is faster); Windows 7 or Windows 10; cpu, memory etc...

Question: Does the allocation unit size effect the sd card or usb stick speed?

Answer: It depends on the file size

Also read: allocation unit size for usb drive

Conclusion: The smaller the cluster, the more space is saved, but when reading files, the smaller the cluster, the more time it takes to search, and the performance will be poor.

The data loading speed is similar. When creating a table space, the larger the AU, the faster the speed. That is to say, the larger the AU, the faster the data file resize. However, setting a too large AU will reduce the space for storing data on the disk. From the experimental point of view, starting from AU=4, the sequential read speed does not increase significantly due to the increase of AU, so AU=4 [system default size] is an ideal setting.

How can I test my disk speed?

How do I check my disk speed, usb speed or memory card speed on Windows 10/11?

Utilities can do the speed testing job on your storage devices. You can test Hard Drive speed on your Windows 11/10 computer by installing free HDD Read/Write speed testing software. We have the following HDD speed testing software on this list:

  1. Novabench
  2. ATTO Disk Benchmark
  3. DiskMark
  4. HD Tune
  5. DiskCheck
  6. SysGauge
  7. CPUID HWMonitor

How to test disk/ssd/usb speed using cmd DiskSpd in Windows 10/8/7

DiskSpd download link:


Q: Which is the best allocation unit size for Windows 10 system volume to get the best performance?

A: The default 4k is the best option for a Windows computer hdd/ssd that install the OS.

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