Some things you should be aware of:
- Linux Kernel 5.x supports swapfiles natively on btrfs
- This is a bit of a kludge.
Summary:
Btrfs does not natively support swapfiles as the kernel wants them to be contiguous blocks on disk.To work around this, we create a loop-mounted filesystem that is formatted ext4, and in that filesystem, we create a swapfile.
Background:
Btrfs as a filesystem is much more similar to a modify on write filesystem like Netapp’s wafl or ZFS. It’s a copy-on-write filesystem, so it’s not quite the same, but under the hood, a files blocks aren’t necessarily stored next to each other. Historically, the kernel wanted the blocks in a swapfile to be next to each other and contiguous, because on spinning disk, seek time is what kills you.
Workflow:
Create a 2.5G swapfile:
dd if=/dev/zero of=/var/loopdisk.img bs=1024 count=2500000
Format the file with ext4:
mkfs.ext4 /var/loopdisk.img
Mount the file and make it permanent:
mkdir /mnt/loop-ext4fs mount -t ext4 -o loop /var/loopdisk.img /mnt/loop-ext4fs/ && \ grep '/mnt/loop-ext4fs' /etc/mtab >> /etc/fstab
Create the swapfile:
dd if=/dev/zero of=/mnt/loop-ext4fs/swapfile bs=1024 count=2000000 chmod 600 /mnt/loop-ext4fs/swapfile mkswap /mnt/loop-ext4fs/swapfile swapon /mnt/loop-ext4fs/swapfile
Verify that it worked:
free -h --si
total used free shared buff/cache available
Mem: 4.0G 153M 123M 966K 3.8G 3.6G
Swap: 2.0G 0B 2.0G
Add an entry to the /etc/fstab for the swapfile:
/mnt/loop-ext4fs/swapfile none swap sw 0 0
Verify that it will be used as swap when the sytem boots:
swapoff -a
free -h --si
total used free shared buff/cache available
Mem: 4.0G 152M 122M 966K 3.8G 3.6G
Swap: 0B 0B 0B
swapon -a
free -h --si
total used free shared buff/cache available
Mem: 4.0G 153M 121M 966K 3.8G 3.6G
Swap: 2.0G 0B 2.0G
It’s not a terrible idea to reboot to make sure that everything will work on startup.
