The bootloader (grub in this case) will pass the boot parameters to the Linux kernel in a memory buffer called the kernel command line. The kernel command line, along with other console messages output during boot, should be in the system log, retrievable with the dmesg command.
A copy of the kernel command line is also in the file /proc/cmdline.
Following advice from the Server Fault community, I checked my Block Size:
stat --printf='%s' -f .
which was "normal" at 4096
Then I checked how many deleted files were still held open by processes:
lsof | grep -c DEL
which reported 143 files which might account for all the lost space, but I think it is unlikely
Then I rebooted my box and voila! All my disk space was back:
df -h
Filesystem Size Used Avail Use% Mounted on
udev 487M 4.0K 487M 1% /dev
tmpfs 100M 388K 100M 1% /run
/dev/vda1 30G 17G 12G 61% /
Reflection
The fact that I regained so much of my disk space after rebooting means that the volume's block size was not the main culprit. So, still not 100% sure what caused the discrepancy, but happy to have my space back!
Best Answer
At a command line, type
This will list each device and usage information.
You might find
more readable, since it gives quantities in Gb or Mb, etc.