NTFS allocation unit sizes for large volumes

The NTFS file system has allocation unit options that usually don't matter unless there is a limitation. Rick Vanover explains how this matters for large volumes.

All Windows Server administrators have done this: quickly format an NTFS volume and select the default allocation unit size without giving it much thought. It turns out the unit size is quite important when it comes to the layout and practicality of the drive. I do this for non-boot volumes. I'm not a fan of putting everything on the C:\ drive, but in turn, I prefer to create different drive letters for programs or data on a server.

Let's start with the format task for a new drive on a Windows Server, which is the point where administrators can make a decision about allocation unit size. Figure A shows the allocation unit menu. Figure A

When it comes to formatting larger NTFS volumes, you'll find that the default allocation unit size increases from the default 4 KB when you cross the 16 TB thresholds. A single NTFS volume of 16 TB is quite large, but there are use cases for drives this large. The issue is that the minimum allocation unit goes from 4K to 8K when the NTFS volume exceeds 16 TB. There is another threshold at the 32 TB level and more as the file system scales up. This TechNet article explains the scaling points of NTFS for large volumes.

Windows is adaptive in the size of the volume and displaying the available allocation unit sizes for a volume based on its size. Consider this example with a single drive that was initially 13 TB and then expanded to 19 TB. Once the drive passed to being larger than 16 TB, the 8K allocation unit is the smallest option (Figure B). Figure B

The allocation unit is very important, as it represents the smallest unit of consumption on disk. For example, if you have a small text file that is 1,350 bytes, it will consume a full allocation unit of 8K on disk. Larger files that span multiple allocation units will have the remainder available, but smaller files can quickly consume disk space on large volumes; this is the difference between the Size and the Size On Disk display options in Windows Explorer. This example is shown in Figure C. Figure C

Click the image to enlarge.

When it comes to venturing into larger volumes, NTFS will do better with larger files rather than a large number of small files.

How do you provision large storage and consider the allocation unit size? Let us know in the discussion.


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