Re: swapon: swapfile has holes

["Austin S. Hemmelgarn" <ahferroin7@gmail.com>]

On 2017-11-28 07:31, Cristian wrote:
> Hello,
> 
> Report in:
> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1723449
> 
> dmesg:
> [ 3.782191] swapon: swapfile has holes

As mentioned in the linked bug report, this is a known issue.  To use a 
swap file on BTRFS, you need to:

1. Create an empty file.
2. Mark it NOCOW with `chattr +C`
3. Use fallocate to pre-allocate to the appropriate size.
4. Hook it up to a loop device.
5. Use the loop device for swap.

This is because the kernel needs a consistent block mapping for a swap 
file, since it bypasses the filesystem and accesses the block device 
directly.  However, since BTRFS has copy-on-write semantics, blocks move 
around on each write, which violates the requirement for a consistent 
block mapping, you can't use a file on a BTRFS filesystem directly as a 
swap file.

Of the above steps, you technically don't need to mark the file NOCOW or 
pre-allocate it, but those are good practice to remove random latency.