Ubuntu – have a swapfile on btrfs

btrfsswap

Modern Ubuntu versions are using a swap file instead of a swap partition by default.

Before the 5.0 Linux kernel it was not possible to place a swap file on a btrfs partition, btrfs file system could be damaged.

Now kernels support swap files on btrfs partitions. Can I use a swap file on Ubuntu installed on btrfs and what are possible problems?

Best Answer

It is possible to use a swap file on btrfs, but there are some considerations that need taking care of.

btrfs filesystem doesn't let to create snapshots if there is a working swap file on the subvolume. That means that it is highly recommended to place a swap file on a separate subvolume.

Lets assume that the current swap is already off, the / is on /dev/sda1 and Ubuntu is installed with / on @ subvolume and /home is on @home subvolume.

Mount /dev/sda1 to /mnt.
```
 sudo mount /dev/sda1 /mnt
```

If you run ls /mnt, you'll see @, @home and other subvolumes that may be there.

Create a new @swap subvolume.
```
 sudo btrfs sub create /mnt/@swap
```
Unmount /dev/sda1 from /mnt
```
 sudo umount /mnt
```
Create /swap directory where we plan to mount the @swap subvolume.
```
 sudo mkdir /swap
```

Mount the @swap subvolume to /swap.

 sudo mount -o subvol=@swap /dev/sda1 /swap

Create the swap file.
```
 sudo touch /swap/swapfile
```
Set 600 permissions to the file.
```
 sudo chmod 600 /swap/swapfile
```
Disable COW for this file.
```
 sudo chattr +C /swap/swapfile
```
Set size of the swap file to 4G as an example.
```
 sudo fallocate /swap/swapfile -l4g
```
Format the swapfile
```
sudo mkswap /swap/swapfile
```
Turn the swap file on.
```
sudo swapon /swap/swapfile
```

Now the new swap should be working.

You also need to update /etc/fstab to mount all this on boot. Add there two lines:

UUID=XXXXXXXXXXXXXXX /swap btrfs subvol=@swap 0 0
/swap/swapfile none swap sw 0 0

The UUID is the one of your /dev/sda1.

Comments and sugestions are welcome.

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Ubuntu – Trick installer to use btrfs root with compression

I came across this thread as because I was looking to install Linux Mint Debian Edition on a flash drive and installing on compressed btrfs from the get-go. Although these solutions were not directly applicable to reaching my results I used some of this information to reach my target.

The problem was that the point of the installer formatting and mounting the partition and starting copying files was right next to each other, as so I was unable to perform the "remount" option mentioned by others above.

The LMDE version of the installer script was in python (usr/lib/live-installer/installer.py). I'm not sure if it is the same with Ubuntu, but if it is, this will be directly applicable. This allowed me to edit the script and add this line right under the line that origionally mounted the partition for "/"

os.system("mount -o remount,compress /dev/sda3 /target -t btrfs")

of coures the "/dev/sda3" will vary depending on your device.

I understand this is an Ubuntu forum, but like I said it came up with the search and this solution would be directly relevant if the installer is python based. We are all Debian here anyways, right!?

Ubuntu – Message “Sparse file not allowed” after installing on a btrfs filesystem

OK after a bit of rummaging around I found a how-to too get rid of this problem at least temporarily it is fairly simple however I don't have my system set-up with btrfs so I can't confirm this fix.

either comment out or remove this line:

if [ -n ${have_grubenv} ]; then save_env recordfail; fi

if [ -n "\${have_grubenv}" ]; then if [ -z "\${boot_once}" ]; then save_env \
recordfail; fi; fi

in this file

/etc/grub.d/00_header

then run

update-grub

the reason for not editing /boot/grub/grub.cfg directly is that it will be over written every time grub is updated in this case you would only have to "re do" the fix if the grub common packages is updated.

This is the bug on launchpad if you want to add yourself bug #736743

Quoting Colin Watson from the bug report

This is actually a misleading error message: what's happening is that GRUB's btrfs implementation doesn't implement the file read hook interface for returning blocklists to calling code. I posted to grub-devel about this and the upstream maintainer pointed out that, even aside from multi-device problems, writing to btrfs from GRUB is fundamentally risky because:

the same block may be used by multiple snapshots every tree which uses a given block will contain its checksum, and so on recursively

However, btrfs reserves space at the start for the boot loader. This space is more than GRUB needs to embed itself, and so we could use 1KB of it for an environment block.

In any case, this is not a new problem that arose from using subvolumes, nor does it prevent booting (you get a spurious "Press any key to continue" prompt, but if you just ignore it it'll boot anyway). Downgrading to wishlist.

Hope this helps

Best Answer

Related Solutions

Ubuntu – Trick installer to use btrfs root with compression

Ubuntu – Message “Sparse file not allowed” after installing on a btrfs filesystem

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