Swapfile

A swapfile is file-backed memory that the system uses to temporarily offload the RAM. It is supported since kernel 5.0. Use swapon(8) to activate the swapfile. There are some limitations of the implementation in BTRFS and Linux swap subsystem:

  • filesystem - must be only single device

  • filesystem - must have only single data profile

  • swapfile - the containing subvolume cannot be snapshotted

  • swapfile - must be preallocated (i.e. no holes)

  • swapfile - must be NODATACOW (i.e. also NODATASUM, no compression)

The limitations come namely from the COW-based design and mapping layer of blocks that allows the advanced features like relocation and multi-device filesystems. However, the swap subsystem expects simpler mapping and no background changes of the file block location once they’ve been assigned to swap.

With active swapfiles, the following whole-filesystem operations will skip swapfile extents or may fail:

  • balance - block groups with swapfile extents are skipped and reported, the rest will be processed normally

  • resize grow - unaffected

  • resize shrink - works as long as the extents are outside of the shrunk range

  • device add - a new device does not interfere with existing swapfile and this operation will work, though no new swapfile can be activated afterwards

  • device delete - if the device has been added as above, it can be also deleted

  • device replace - ditto

When there are no active swapfiles and a whole-filesystem exclusive operation is running (e.g. balance, device delete, shrink), the swapfiles cannot be temporarily activated. The operation must finish first.

To create and activate a swapfile run the following commands:

# truncate -s 0 swapfile
# chattr +C swapfile
# fallocate -l 2G swapfile
# chmod 0600 swapfile
# mkswap swapfile
# swapon swapfile

Since version 6.1 it’s possible to create the swapfile in a single command (except the activation):

# btrfs filesystem mkswapfile swapfile
# swapon swapfile

Please note that the UUID returned by the mkswap utility identifies the swap “filesystem” and because it’s stored in a file, it’s not generally visible and usable as an identifier unlike if it was on a block device.

The file will appear in /proc/swaps:

# cat /proc/swaps
Filename          Type          Size           Used      Priority
/path/swapfile    file          2097152        0         -2

The swapfile can be created as one-time operation or, once properly created, activated on each boot by the swapon -a command (usually started by the service manager). Add the following entry to /etc/fstab, assuming the filesystem that provides the /path has been already mounted at this point. Additional mount options relevant for the swapfile can be set too (like priority, not the BTRFS mount options).

/path/swapfile        none        swap        defaults      0 0

Hibernation

A swapfile can be used for hibernation but it’s not straightforward. Before hibernation a resume offset must be written to file /sys/power/resume_offset or the kernel command line parameter resume_offset must be set.

The value is the physical offset on the device. Note that this is not the same value that filefrag prints as physical offset!

Btrfs filesystem uses mapping between logical and physical addresses but here the physical can still map to one or more device-specific physical block addresses. It’s the device-specific physical offset that is suitable as resume offset.

Since version 6.1 there’s a command btrfs inspect-internal map-swapfile that will print the device physical offset and the adjusted value for /sys/power/resume_offset. Note that the value is divided by page size, i.e. it’s not the offset itself.

# btrfs filesystem mkswapfile swapfile
# btrfs inspect-internal map-swapfile swapfile
Physical start: 811511726080
Resume offset:     198122980

For scripting and convenience the option -r will print just the offset:

# btrfs inspect-internal map-swapfile -r swapfile
198122980

The command map-swapfile also verifies all the requirements, i.e. no holes, single device, etc.

Troubleshooting

If the swapfile activation fails please verify that you followed all the steps above or check the system log (e.g. dmesg or journalctl) for more information.

Notably, the swapon utility exits with a message that does not say what failed:

# swapon /path/swapfile
swapon: /path/swapfile: swapon failed: Invalid argument

The specific reason is likely to be printed to the system log by the btrfs module:

# journalctl -t kernel | grep swapfile
kernel: BTRFS warning (device sda): swapfile must have single data profile