pastilhasandClaude Opus 5
6947284f1b
New first section, before anything else that changes the machine, because the
swapfile and the ballast both size themselves from free disk.
Ubuntu Server's installer on its defaults gives the root logical volume a fixed
size and leaves the rest of the drive as unallocated extents in the volume group.
On a 2TB disk that is a ~100G root with nothing to indicate a problem: lsblk
shows the whole drive, df shows 100G, and the two are never seen side by side
until the day it fills. Growing a virtual disk at a provider leaves the same
shape one layer down, and so does resizing a partition without telling the
filesystem inside it.
Three layers, any of which can be the short one, so all three are measured and
printed together:
drive: 76.3GB /dev/sda
volume: 76.1GB /dev/sda1
filesystem: 76.1GB ext4, mounted at /
Seeing them in one place is most of the value. The fix is then whichever layer is
short: lvextend for free extents, growpart for a partition that stops early
(followed by pvresize and lvextend when LVM is in the way), or resize2fs alone
when only the filesystem is behind.
Only ever grows. Nothing here shrinks, creates or deletes a partition, and ext4,
xfs and btrfs all grow while mounted — so no unmount, no reboot, and a failure
part-way leaves a smaller filesystem on a larger container, which is the state it
started in.
growpart is the authority on whether a partition can move — it exits 1 with
NOCHANGE when the partition already reaches the end — but it comes from
cloud-guest-utils, which is not on every image. Installing a package purely to
ask a question is too eager, so plain arithmetic on the device sizes decides
whether it is even worth looking, and only then is growpart fetched.
A gigabyte of slack before anything is reported: a filesystem is always slightly
smaller than its container, and reporting journal and reserved-block overhead as
reclaimable space would make this section cry wolf on every machine.
Verified on this host: plain ext4 partition filling its disk, correctly reports
nothing to reclaim, and every helper returns the right device and size.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>