How to Fix "Device Busy" Errors During Unmounting on FreeBSD

When working with filesystems on FreeBSD, you may encounter an error message stating that a device is “busy” when trying to unmount it. This error occurs when processes or resources are actively using the filesystem, preventing it from being safely unmounted. This article explores various methods to diagnose and resolve “device busy” errors on FreeBSD.

Understanding the “Device Busy” Error

The “device busy” error occurs when you attempt to unmount a filesystem using the umount command, and the system refuses to proceed because something is still accessing the device. This can be due to open files, active processes, or system daemons using the mount point.

Common Causes

1. Open Files – A process has a file open on the filesystem.
2. Working Directory in Use – A shell session or process is in a directory within the mount point.
3. Active System Services or Daemons – Background services, such as NFS or database services, are using the filesystem.
4. Kernel Modules and Devices – Some mounted filesystems have kernel dependencies preventing them from being unmounted.
5. Swap Usage on Device – If the device is being used for swap, it cannot be unmounted.

Step-by-Step Fixes

1. Identify Open Files

Use the fstat or lsof command to find which processes are keeping the device busy.

fstat -f /mnt/mountpoint

or

lsof +D /mnt/mountpoint

These commands list the processes using files within the mount point. If you identify a specific process, you can terminate it using:

kill -9 <PID>

Replace <PID> with the actual process ID.

2. Check for Processes Using the Mount Point

Sometimes, a shell or another process might be inside the directory. Use who or ps to check.

who
ps aux | grep /mnt/mountpoint

If a user session is in the directory, ask them to exit or use pkill to terminate their session:

pkill -u username

3. Use umount -f (Force Unmount)

If a process refuses to let go, forcing an unmount may be necessary. The -f flag can help:

umount -f /mnt/mountpoint

However, forcing an unmount can cause data loss if files are still being written. Use this option with caution.

4. Check and Stop Services

Certain services may be using the device. Common ones include:

• NFS (rpc.statd, rpc.lockd)
• Databases (MySQL, PostgreSQL)
• Background daemons

To check services using the mount point:

service --status-all | grep <service-name>

Stop them using:

service <service-name> stop

For NFS, use:

service nfsclient stop
service nfsd stop

5. Use umount -l (Lazy Unmount)

If you cannot immediately unmount but want to detach the filesystem safely when no processes are using it, use the lazy unmount option:

umount -l /mnt/mountpoint

This marks the filesystem for unmounting as soon as it is no longer in use.

6. Check for Swap Usage

If the device is being used as swap space, it cannot be unmounted. Check with:

swapinfo

If necessary, turn off swap:

swapoff /dev/<device>

Replace <device> with the actual swap partition.

7. Identify and Unload Kernel Modules

Sometimes, kernel modules associated with a mounted device prevent unmounting. Check for loaded modules:

kldstat

If you see a module related to your device, unload it:

kldunload <module_name>

8. Remount as Read-Only Before Unmounting

If the filesystem is busy, remounting it as read-only can help release locks:

mount -o ro,remount /mnt/mountpoint

Then attempt to unmount:

umount /mnt/mountpoint

9. Check System Logs

If none of the above methods work, checking system logs might provide clues:

dmesg | tail -20

or

tail -f /var/log/messages

Look for any errors related to the mount point.

10. Use fuser to Identify and Kill Processes

The fuser command helps identify and kill processes accessing the mount point:

fuser -m /mnt/mountpoint

To kill all processes using the mount:

fuser -km /mnt/mountpoint

Conclusion

The “device busy” error on FreeBSD typically occurs due to open files, active processes, or system services using the filesystem. By systematically identifying the root cause and applying the appropriate fix, you can safely unmount the device without risking data corruption.

To summarize:

• Use fstat or lsof to check for open files.
• Terminate processes using the mount point.
• Stop active services.
• Try umount -f or umount -l if necessary.
• Ensure the device is not being used as swap.
• Check for kernel module dependencies.

By following these steps, you can efficiently resolve “device busy” errors and maintain smooth filesystem operations on FreeBSD.