Change NTFS disk allocation unit size from 4k to 64k

  • 26 September 2022
  • 3 replies

Userlevel 3
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We have a (older) Windows 2012 R2 VM with a OS Disk and a Data Disk (scsi.0 and scsi.1 respectively) running on AHV / AOS LTS

The data disk (scsi.1) was originally created with 4kb allocation unit size (no NTFS disk compression is utilized) and this disk has reached it’s maximum size under NTFS of 16TiB. 

It’s also 96% full… 😟

The custom application is such that mount points won’t work.


It was suggested to see if perhaps Acronis Disk Director could safely change the drive’s allocation unit size from 4k to 64k (and preserve the data). To my knowledge formatting a drive is the only way to accomplish allocation unit size changes...

Is changing the allocation unit a supported operation?

Has anyone successfully made such a change?


Best answer by JeroenTielen 27 September 2022, 15:14

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3 replies

Userlevel 6
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Is the application using large or small files? 

If there are a lot of small files, changing to a larger allocation unit will just increase the disk usage. 

Converting is not something you can do within windows, there are third party tools but I can assume this is not supported by microsoft (although Windows Server 2012 R2 is another story ;)). 

This is not something Nutanix can fix for you. What I would suggest is contact the application owner/developer and ask how to cleanup the data. Or how to archive old data of the data disk. 

Userlevel 5
Badge +6

The only safe way from my opinion is data backup and restore (Files and Folders) on new formatted disk.

Userlevel 3
Badge +6

Thank you both for your responses. It is what we’ve advised as well… or to migrate the data from within the application over time (without having to enumerate the entire folder structure.)