Maximum NTFS Volume Expansion

A peer recently had an issue when working on a client system: After adding a second shelf of SAS-attached drives to a physical Windows Storage Server and doubling the available capacity of the environment from ~20TB to ~40TB, he was unable to extend the existing NTFS volume after extending the SAS array group.

The error was "The volume cannot be extended because the number of clusters will exceed the maximum number of clusters supported by the filesystem."

The original volume was reportedly formatted "using the defaults," which under most circumstances would mean it was using 4K clusters. Why wouldn't it allow extending the volume?

Because NTFS (as currently implemented) has a cluster limit of 2³²-1 clusters per volume.

When you "do the math," that cluster limit does impose some hard limits on the maximum size of the NTFS volume, irrespective of the actual drive space that is available for the volume. And trying to use tricks like dynamic disks and software RAID won't help: those tricks modify the underlying disk structure, not the NTFS filesystem that "rides" on top of it.

Max NTFS Volume by Cluster Size
cluster size (B)	Bytes	KB	MB	GB	TB
512	2,199,023,255,040	2,147,483,648	2,097,152	2,048	2
1024	4,398,046,510,080	4,294,967,295	4,194,304	4,096	4
2048	8,796,093,020,160	8,589,934,590	8,388,608	8,192	8
4096	17,592,186,040,320	17,179,869,180	16,777,216	16,384	16
8192	35,184,372,080,640	34,359,738,360	33,554,432	32,768	32
16384	70,368,744,161,280	68,719,476,720	67,108,864	65,536	64
32768	140,737,488,322,560	137,438,953,440	134,217,728	131,072	128
65536	281,474,976,645,120	274,877,906,880	268,435,456	262,144	256

We knew that we had a functioning 20TB volume, so we verified my theory that the volume was actually formatted with 8K clusters (the smallest size that would support 20TB) using DISKPART's FILESYSTEM command. Sure enough: 8192 was the cluster size.

We gave the client several options for addressing the issue, including the purchase of software that could "live adjust" the cluster sizing. In the end, the client chose the "migrate->reformat->migrate" option, and while it took a long time to perform (20TB is a lot of data!), it was successful.