# 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.

```bash
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](https://www.man7.org/linux/man-pages/man1/iostat.1.html). 

```bash
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'](https://linux.die.net/man/1/fuser).

```bash
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)](https://linux.die.net/man/8/lsof).

```bash
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

```bash
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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