A swapfile, when active, is a file-backed swap area. It is supported since kernel 5.0. Use swapon(8) to activate it, until then (respectively again after deactivating it with swapoff(8)) it’s just a normal file (with NODATACOW set), for which the special restrictions for active swapfiles don’t apply.
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
subvolume - cannot be snapshotted if it contains any active swapfiles
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 extents of any active swapfiles are skipped and reported, the rest will be processed normally
resize grow - unaffected
resize shrink - works as long as the extents of any active swapfiles are outside of the shrunk range
device add - if the new devices do not interfere with any already active swapfiles 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 --size 2G 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.
Once activated the file will appear in
# 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
From now on the subvolume with the active swapfile cannot be snapshotted until the swapfile is deactivated again by swapoff. Then the swapfile is a regular file and the subvolume can be snapshotted again, though this would prevent another activation any swapfile that has been snapshotted. New swapfiles (not snapshotted) can be created and activated.
Otherwise, an inactive swapfile does not affect the containing subvolume. Activation creates a temporary in-memory status and prevents some file operations, but is not stored permanently.
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
The command map-swapfile also verifies all the requirements, i.e. no holes, single device, etc.
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