“mount: / is busy” when trying to mount as read-only so that I can run zerofree

Question

I am trying to run zerofree on Ubuntu 11.04 so that I can compact the VirtualBox vdi image using:

VBoxManage modifyhd Ubuntu.vdi --compact

In order to run zerofree the disk image has be mounted as read-only. I'm following these instructions which says to use this to remount as read-only from the recovery mode (Drop to root shell prompt):

mount -n -o remount,ro -t ext2 /dev/sda1 /

But when I do this I get the error:

mount: / is busy

Any ideas on how to do this?

Follow up: Following Jari's answer and this post by running these commands resolves the issue.

service rsyslog stop
service network-manager stop
killall dhclient

Answer 1

Some processes are keeping files open for writing. These could be, for example, programs that write logs, like rsyslogd, networking tools, like dhclientor something else. Shutting these down one by one and trying the remount might work.

You can find processes that use certain files by using the program fuser. For example, fuser -v -m / will return a list of processes. However, I am not sure if it is one of these which keeps the file system busy.

Answer 2

You can only remount the filesystem read-only if there is no process that has a file open for writing. Run lsof / to see what processes have files open on the root filesystem. Files open for writing will be indicated in the FD column. You can filter these with

lsof / | awk '$4 ~ /[0-9].*w/'

To filter the process IDs automatically, parse the output of lsof -F pa:

lsof -F pa /home | awk '/^p/ {pid = substr($0, 2)} /^a.*w/ {print pid}'

Answer 3

Improving Peter answer:

I could not kill processes that uses my mount point. So, I did this:

• Edit /etc/fstab to starts in read-only in next boot. Ex: /dev/sda1 / ext2 ro 0 1
• Reboot and open a shell (Host Key + F2, for instance)
• Run zerofree

To "recover" your system:

• Mount with read-write option: $ mount -o remount,rw /dev/sda1
• Edit /etc/fstab again, restoring your original value

Extra: if necessary open recovery mode in any step:

• Reboot
• Hold shift while VM is loading
• Advanced Options > Kernel option with Recovery Mode > Drop to Root Shell Prompt

Answer 4

There are sometimes so many processes keeping the mount busy, that it might be easier to simply reboot with the mount read-only.

Change the entry for the root filesystem in /etc/fstab, for example:

/dev/sda1 / ext2 relatime,rw,errors=remount-ro 0 1

becomes:

/dev/sda1 / ext2 noatime,ro 0 1

On reboot, the filesystem will be mounted read-only, so you can run zerofree on it.

When you're finished, remount the filesystem read/write again (mount -o remount,rw /) and undo your changes to /etc/fstab.

Inspired by https://wiki.debian.org/ReadonlyRoot#Enable_readonly_root

Answer 5

If you use systemd then disk writes are stopped by stopping systemd-journald.

# systemctl stop systemd-journald.socket
# systemctl stop systemd-journald.service
# mount -o ro,remount /

Answer 6

For anyone else who's just looking for a quick fix to paste into your terminal...

(Before following the commands below, make sure you have completed the other steps like telinit 1.)

Found this:

fuser -mv /dev/sda1 > /tmp/sda1.pids
kill `cat /tmp/sda1.pids`

Commands may kill your session, but after you come back, your partition is ready to be remounted.

Answer 7

I assume you try to run the mount command in the VM. Anyway you probably have a different root partition than the author of the forum post.

Try:

mount -o ro,remount /

Which does not depend on the fs or mount option. If this does not work you can also try to determine your root fs with grep " / " /proc/mounts or cat /proc/cmdline. If your root filesystem is for example /dev/mapper/system-rootfs you can use:

mount -o ro,remount /dev/mapper/system-rootfs /

Answer 8

I had this issue which kept me from putting my machine in read only mode. Solving the problem took me a longer than id like to admit... I believe my issue was that I left Apache-Zeppelin running and forgot about it. Make sure that you are not doing the same with another hosting type service. To solve the problem I used fuser -kill / to terminate all of the processes that could be causing the issue and it allowed me to go back into ro mode.

Answer 9

If you still can not unmount or remount your device after stopping all services and processes with open files, then there may be a swap file or swap partition keeping your device busy. This will not show up with fuser or lsof. Turn off swapping with:

sudo swapoff -a

You could check beforehand and show a summary of any swap partitions or swap files with:

swapon -s

or:

cat /proc/swaps

As an alternative to using the command sudo swapoff -a, you might also be able to disable the swap by stopping a service or systemd unit. For example:

sudo systemctl stop dphys-swapfile

or:

sudo systemctl stop var-swap.swap

In my case, turning off swap was necessary, in addition to stopping any services and processes with files open for writing, so that I could remount my root partition as read only in order to run fsck on my root partition without rebooting. This was necessary on a Raspberry Pi running Raspbian Jessie.

Answer 10

Ok, there is what I've done

First, my goal was to reduce my / (ROOT) partition in order to add another one on my disk.

• From running session once everything cleaned and backuped:

  init 1
  

  My desktop go away and I'm now on Linux console...

  ...
  Give root password for maintenance or press CTRL+D to continue
  TheRootPassword_SomethingLike1234
  

• Now, trying to mount / in read-only mode:

  mount -o remount,ro /
  mount: / is busy
  

• Ok, from now, I'm in theorically single-user mode, but ps ax show a lot of other process!!

  Killing them all tis not really possible, or dangerous... ( kill 1 is forbiden... I don't have time to play psdoom :-)

  The only thing I can do is a System Request, for this, I know two ways: (see Documentation/sysrq.txt file in kernel docs):

  1. first using magic SysRq key keyboard kernel trap:

     • hold down AltGr, then maintain then pressed,
     • hit PrtScn just once, but don't release AltGr,
     • hit s just once, this will send Emergency sync request to kernel and
     • hit u, this will send Umount all request, this will remount all mounted filesystems read-only,
     • then release AltGr

  2. Or by command line:

     echo s >/proc/sysrq-trigger
     echo u >/proc/sysrq-trigger
     

• Then now, I could

  fsck -fC0 /dev/mapper/MyDisk-ROOT
  

  ....

...