I want to force a read-only remount of /, so I can make a full hard disk backup while continue working.

There is the command
but it refuses to work because it is busy.

If I use to find and kill all processes using /, it does work. But it also kills the X server, and then I cannot browse the Internet anymore, which is even worse than a live CD.

So how can you remount / without killing anything?

It must be possible, because recently I had some hard drive corruption, and whenever it noticed them, it just remounted / as read-only as I want it to. Except that happened completely randomly

You can't. Or more correctly you really don't want to do this.

What's more important - compromising your backup, or that browsing.
Do it from a liveCD.

No, I think I want to do this.


The internet. Always the internet

Usually I end up making no backup at all because it is such a hassle with the live cd

You can do backup using single user mode.

Do you want to backup the OS data or private files or the whole?

I want a full backup of everything

Boot from a Live DVD.

A full backup will be much quicker if you use rsync.
This will avoid copying bits of empty disk or files which haven't changed since the last backup.

But I want to be sure that every thing is there and exactly the same. Links, extended file attributes...

Also, I have set the append-only flag on most of my important files, and now I do not think rsync can modify them in the backup.

"I can make a full hard disk backup while continue working"

From all the things you are describing, I am starting to wonder if not a simple mirrored raid 1 is more what you are asking about?

If you want to use 'dd' the way you said then LiveCD is the one. You can use systemRescuecd or make you own one for your setup.

Perhaps

But it is a laptop, I do not think you can have a raid there

I am doing it with a live CD (backtrack)

But rebooting is very annoying. I do not want to do that anymore

Especially since I have needed the backup anyways so far.

Unless you have a read-only mounted filesystem your partition would be in flux of data change. So if you run ''dd' then the image may be in corrupted state. You can use LVM device snapshot to do the same without rebooting if you setup LVM. Also btrfs has snapshot features. And ZFS too has it.

That is why I asked on how to remount it read only...

Can I setup an LVM on the existing partition without deleting the files there?



I use ext3

No can't setup LVM on partition with data. Read the LVM HOWTO for more info(Web Search).

If you change all partitions to read only, you won't be able to run anything.
You will also need to terminate anything that writes to a log file.

LVM is really of use for multiple disks shared by multiple users.

Time to revisit this

Anyone has found a solution to do this, remounting without doing a reboot to a liveCD?

Because I accidentally spilled a water over my laptop. Now the keyboard is stuck. Which means I cannot reboot anymore. GRUB's timeout stops, because some key is stuck, and neither Enter nor Ctrl+X nor F10 work. Especially I cannot start from a LiveCD, because I had to press F12 to choose the device and that key is also broken.

I also have no recent backup, because the rebooting was already too annoying.

So now the only option is to remount it from during normal use.