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
swapfile - must be nodatacow (ie. also nodatasum)
swapfile - must not be compressed
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 blocks once they’ve been attached 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 (eg. 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
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
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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