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Why the count is reported different by various tools/commands within same OS ?

1) Under Microsoft Windows MyComputer properties of C: capacity is reported as 27,454,861,312 bytes = 6702847 clusters (4K allocation unit. capacity / 4096) or 53622776 sectors (capacity / 512)

2) Chkdsk C: report 26811391 KB = 27,454,864,384 bytes = 6702847.75 clusters (4K allocation unit. capacity / 4096) or 53622782 sectors (capacity / 512)

3) fsutil fsinfo ntfsinfo c: report total clusters = 0x00000000006646ff = 6702847 [Decimal]

4) wmic partition get size report 27,454,865,408 bytes = 6702848 clusters (4K allocation unit. capacity / 4096) or 53622784 sectors (capacity / 512)

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1 Answers1

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1) Properties shows the actual usable space on the filesystem. The filesystem signature takes one sector at the start of the partition. Since the allocation unit is 4096 bytes, one full cluster is subtracted from the usable space. It also matches the physical layout of clusters on the partition, where clusters always start at four sector boundaries.

2) It seems that chkdsk doesn't do the full cluster subtraction in usable space for some reason. The minimum cluster size is 512 bytes, which is allocated for the filesystem signature. So, chkdsk subtracts only one sector and doesn't take into account the actual cluster size. This might be due to history, where first FAT filesystems had a cluster size of 512 bytes, and chkdsk was never updated to take into account larger cluster sizes.

3) fsutil shows the same information as Properties.

4) wmic shows the full partition size, and the cluster containing the filesystem signature is also included in the calculation. The partition could contain any filesystem, and therefore any filesystem specific data areas are included in the number wmic shows.

Tero Kilkanen
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