Disk usage was computed as used/size, where size is the raw device size.
ext4 reserves 5% of the filesystem for root — 92.4 GiB of this node's
1.8 TiB — which size includes but nothing can allocate. Two consequences,
both live on archi-dev-box today:
The dashboard advertised 251 GiB free when only 159 GiB could actually be
written, and reported 86.2% usage against df's 90.8%.
Worse, disk_monitor triggers automatic cleanup (podman image prune) at
90%. The disk has been genuinely above that threshold while this returned
86.2%, so the cleanup never once fired — which is exactly how ~72 GB of
dangling images accumulated unnoticed, and why deleting apps appeared to
free nothing.
Both call sites now ask df for avail and use used/(used+avail): the same
figure df itself prints, and the space an operator can actually spend.
Callers deriving free as total - used now get avail.
Note this shifts disk_total_bytes in the analytics series down by the
reserve; historical samples are not comparable across this change.
Tests updated for the three-column output, plus a regression test built
from this box's real numbers asserting the corrected math crosses the 90%
threshold the old math missed. 15/15 disk_monitor tests pass.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
