super-user-reserved blocks, which otherwise defaults to 5% of the
entire disk.
Rationale: Reserving a percentage of the volume is generally a neither
useful nor desirable feature for volumes that aren't used as root file
systems for Linux distributions, since the reserved portion becomes
unavailable for non-root users. For containers, the general case is to
use the entire volume for data, without running as root. The case where
one might want reserved blocks enabled is much rarer.
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