When the filesystem goes read-only

Writes start failing with Read-only file system and the mount shows ro.

Nobody changed it. The kernel remounted it read-only after an I/O error — common on cloud disks when the backing store hiccups.

dmesg -T | grep -i -E 'I/O error|remount|ext4'

If you see Remounting filesystem read-only, do not immediately remount read-write; that hides the cause.

Check SMART (smartctl -a /dev/vda), look for a provider-side storage event, and get readable data off first. Remount only once you know the disk is healthy, or the next write may corrupt metadata.