Re: Btrfs default on Fedora?

[Andrei Borzenkov <arvidjaar@gmail.com>]

12.07.2020 00:18, Chris Murphy пишет:
> On Sat, Jul 11, 2020 at 12:44 PM Ken D'Ambrosio <ken@jots.org> wrote:
> 
>> * Swap files.  At least last time I checked, it was a PITA to take a
>> snapshot of a volume that had a swapfile on it -- I wound up writing a
>> wrapper that goes, does a swapoff, removes the file, creates the
>> snapshot, and then re-creates the file.   Is this still "a thing"?  Or
>> is there a way to work around that that isn't kludgey?
> 
> Put the swapfile in its own subvolume and don't snapshot it. One way
> is to create a (nested) subvolume named "swap" inside of the "root"
> subvolume created at installation time; use chattr +C on it; now
> create the swapfile per 'man 5 btrfs'.
> 
> Since btrfs snapshots aren't recursive, making a snapshot of 'root'
> will not cause a snapshot to be taken of 'swap' or its swapfiles.
> 

swap file requires calculation of absolute disk offset (that is what
kernel works with) and this is not supported on btrfs with by systemd:

https://github.com/systemd/systemd/blob/d67b1d18fcda2c8c5aacfc50f9591c8dc7a4a8a1/src/shared/sleep-config.c#L240

I believe with current systemd if you manually compute and provide
resume=btrfs-partition and resume_offset=absolute-offset-of-swapfile it
should take it, but using resume=file-on-btrfs will fail.