Inaccurate Disk Usage Reporting for ext* Filesystems Due to Reserved Blocks

[ernolf]

Description:

• The current implementation inaccurately reports disk usage for ext* filesystems by not accounting for reserved blocks. The df command’s Used-value does not include these reserved blocks, leading to an underestimation of the actual used space. When calculating the file system usage in percent, the calculation is also based on this incorrect basis, which means that the percentages provided by df are inaccurate and therefore actually unusable. Since the 'Blocks' (aka 'Size') value is not used for the actual size of the disk, but the computed sum of 'Used' and 'Available', the same inaccuracy also occur when displaying the actual disk size.
  This affects ext* file systems where a significant part of the disk (default 5%) can be reserved for system and root use.

Expected Behavior:

• The disk usage should reflect the sum of all blocks that can not be used, which includes reserved blocks, to give an accurate representation of the available space in relation to the real disksize. Users know the size of their devices and get confused, when 5% of disksize is 'missing'.

Ref: https://help.nextcloud.com/t/why-is-my-1tb-ssd-showing-only-890gb-of-usable-space-in-nextcloud/202375/4

[kesselb]

Thanks for the report and your debugging work 👍

A pull request that extends the disk object with another value for the total blocks is okay for me.

Taking the reserved blocks into account is out of scope.