Re: btrfs and hibernation to swap file on it?

[Andrei Borzenkov <arvidjaar@gmail.com>]

On 27.09.2022 18:45, Christoph Anton Mitterer wrote:
> Hey.
> 
> Maybe someone could help me with that question:
> 
> I'd like to set up hibernation, using a swapfile on btrfs (which itself
> is on dm-crypt).
> 
> Now I'm aware of the section on swaptfiles in btrfs(5) and their
> restrictions... and I think these should be fine for me.
> 
> 
> What I don't quite get is:
> 
> a) Hibernate seems to want an offset parameter for the swapfile. How is
> that used respectively why is it needed.
> 
> Does that mean that hibernation directly writes/reads to/from the block
> device?

Correct.

> And wouldn't that be kind of fragile (if something internally moves in
> the fs... or if it's split in several extents and not one big
> contiguous space)?
> 

There are quite some restrictions for using swapfile on btrfs, in
particular, it must be preallocated and btrfs will refuse relocation of
extents in this file.

kernel supports swapfile with multiple extents.

> Guess I would be kinda scared to get some corruptions if something
> writes directly to the btrfs' block device - other than btrfs.
> 
> Are there any suggestions with respect to btrfs? Like not using a
> swapfile for hibernation, or is it considered safe?
> 
> 
> b) Internet resources say that for btrfs one cannot use filefrag to get
> the offset.
> I found Omar's tool, but wondered whether the same had been directly
> integrated into btrfs-progs in the meantime?
> 
> 
> 
> Thanks,
> Chris.