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Linux - unmount a busy target safely

Goal - removing target without data loss

Unplugging or unmount -l (lazy unmount) can cause data loss. I want to share a way o avoid data loss.

Side note: unmount -l will let you unmount the device, but as far as I know only 'hides' the mountpoint, and active processes can still write on said device.

The problem

Error unmounting /dev/sdc1: target is busy

So, there are now different ways to unmount the target safely.

Side note: the most common case is that you are still in a directory of said target. It happened way too often to me.

Preparation

Those steps are not necessary, but help you troubleshoot.

Finding the mount point

We are going to use df -h to find the mount point of the busy target. It is often not necessary, but it can be helpful.

kuser@pleasejustwork:~$ df -h
[...]
/dev/sdc1        59G   25G   35G  42% /media/kuser/hdd-target
Check if the device is still actively in use

Additionally, you can check the activity of said device with iostat.

kuser@pleasejustwork:~$ iostat 1 -p sdc

vg-cpu:  %user   %nice %system %iowait  %steal   %idle
           1,14    0,00    0,63    2,66    0,00   95,56

Device             tps    kB_read/s    kB_wrtn/s    kB_dscd/s    kB_read    kB_wrtn    kB_dscd
sdc              13,00         0,00         7,50         0,00          0          7          0
sdc1             13,00         0,00         7,50         0,00          0          7          0

iostat is powerful, but in this case the most important columns here are kB_read/s kB_wrtn/s. If there is anything but 0,00, the device is in use.

If there is any activity and you unplug or unmount the device forcefully, data loss will most likely occur.

Finding the process

Using 'fuser'

More information can be found in the manual of 'fuser'.

kuser@pleasejustwork:~$ fuser -vm /dev/sdc1
                     USER        PID ACCESS COMMAND
/dev/sdc1:           root     kernel mount /media/kuser/hdd-target
                     kuser     43966 F.c.. kate
                     kuser     44842 ..c.. kate

I prefer 'fuser' since it is installed on most OS and does the job too.

Using 'lsof'

More information can be found in the manual of 'lsof' (list open files).

kuser@pleasejustwork:~$ lsof /dev/sdc1
COMMAND   PID  USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
kate    43966 kuser  cwd    DIR   8,33    32768    1 /media/kuser/hdd-target
kate    43966 kuser   24w   REG   8,33      142 2176 /media/kuser/hdd-target/.busybusy.txt.kate-swp

or

kuser@pleasejustwork:~$ lsof /media/kuser/hdd-target
COMMAND   PID  USER   FD   TYPE DEVICE SIZE/OFF NODE NAME
kate    43966 kuser  cwd    DIR   8,33    32768    1 /media/kuser/hdd-target
kate    43966 kuser   24w   REG   8,33      142 2176 /media/kuser/hdd-target/.busybusy.txt.kate-swp

Kill process / close program

Kill process by PID:
sudo kill -9 43966

or simply close the program that is using the file in the GUI.

Unmount

Try to unmount again.

Some other methods

I have had no problems with those yet, but some notable mentions.

Some other things to look into:
check the swap partition: cat /proc/swaps
stop nfs-kernel-server
stop samba/smb server
check for symbolic links

There are more scenarios in which a target can be busy, but this should cover 95% of cases.





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