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