Ensure writing has completed after `umount -l / --lazy`

Question

One of many issues with umount -l is that there's way to know when it has completed.

Is there a way to:

• Know all writing processes have finished and all data has been written to disk.
• Prevent future writes

Answer 1

Mount the filesystem's device again at a new location, say /mnt:

mount /dev/device /mnt

Then use:

mount -o remount,ro /mnt

This will attempt to remount the filesystem on the device read-only.

If this succeeds, there is no more data being written to the disk because mount -o remount,ro is guaranteed to fail if there are files open for writing.

If the remount,ro fails, then use:

lsof +f -- /dev/<devicename> | awk 'NR==1 || $4~/[0-9]+[uw -]/'

to list processes which are blocking the remount.

If you are paranioid and want to prevent any future remount read-write, you can:

blockdev --setro /dev/device

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Note: It seems that even mounting the device ro the first time may be sufficient. I get mount: /tmp/mountpoint: /dev/loop0 already mounted or mount point busy. if there are processes writing.