Background
BTRFS warning (device XXXX): space cache v1 is being deprecated and will be removed in a future release, please use -o space_cache=v2
Creating v2 on an existing file system involves either
mount -o space_cache=v2
mount -o remount,rw,space_cache=v2
Additional notes on v2 creation
Because Fedora initially mounts root file systems RO, we can't create v2 with a kernel command line option.
We can modify /etc/fstab which in effect becomes mount -o remount,rw,space_cache=v2
/etc/fstab
Questions
Why wouldn't we automatically convert on mount? It seems reasonable to me if we're going to rip it out eventually that we need a step where detected filesystems are automatically converted.
I don't think upstream will automatically convert because of the potential for hours to days for creating free space tree for very large file systems - during which mount is in-progress but not completed so we don't even have read-only capability let alone read-write.
For us, it should be fast unless someone is using a single Btrfs for boot and for a lot of file storage. But we'd still need a way to trigger it and all I can think of is modifying /etc/fstab twice. That too is a big tricky.
Anyway I asked about another option upstream and we'll see what they say about all of it.
lazy free space tree creation, deprecation of space cache v1
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