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I've been using btrfs on Ubuntu 12.10 for both / and /home for several weeks. Through out the time I've come to the conclusion that ext4 performs better on my system and I'd like to switch back however I've found no easy way of converting both my btrfs partitions to ext4.

Today I've come up with an idea however I'm not sure how safe it is: I'm going to boot from a live environment and shrink my home partition. On the newly created space I'm going to tar my / and /home, reformat the old partitions and finally untar back onto the new ext4 partitions and update grub.

Technically this should be safe however I'm sure that there's some quirk that needs to be done after the whole process which I'm not aware of.

How safe is the process described above and is there a better way of doing it?

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This will not work for the root partition (special device files, kernel etc.). Unless you have very good reasons not to, I would install the system from scratch.

Also, I would not modify both partitions at the same time, but proceed step by step, always checking that everything works.

And I would start by doing a backup.

I would proceed as follows:

  1. Make a rigorous backup on an external disk
  2. Test that the backup works and that you can restore the data

Now, to the /root partition:

  1. Boot from live CD. Delete the root partition. Format ext4. Install system from scratch.
  2. Check that the new system works.

As for /home, rather than using tar, I would move it to the new ext4, check that it works and then delete btrfs and expand ext4. However, if disk space is an issue, do as follows:

  1. Boot from live CD. Shrink the home partition.
  2. Boot normally and test that the data is not corrupted.
  3. Boot from live CD. Create an ext4 partition in the empty space. Archive your home there.
  4. Delete the btrfs partition, create an ext4 partition, untar your home (as you have proposed).
  5. Boot normally and test that all worked as expected and that your /home is fully restored.
  6. Boot from live CD, remove the ext4 partition containing the archives, expand your /home partition back to the full size.
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Hmm..I'll try it this way. I'll come back after I'm done and post the results. –  Mihai Bişog Nov 9 '12 at 9:15
    
Just remember the backup thing and you will be fine :-) –  January Nov 9 '12 at 9:23
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There's nothing magic about the root partition - /dev, /proc etc. are just empty directories when booted from a LiveCD. Kernel is not magic too, although the bootloader will need to be re-installed. –  Sergey Nov 9 '12 at 11:25
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It worked, though I must admit I was a bit worried while doing it because I forgot to mention that my home was encrypted but in the end everything went well. –  Mihai Bişog Nov 9 '12 at 12:00
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