[ryao]

Are you sure that disabling CoW solves btrfs' fragmentation issue when you pre-allocate the image? If the volume is snapshotted regularly, CoW should be active once on each extent once until the next snapshot reactivates it. That should mean that the fragmentation would still occur, although more slowly. Is that correct or am I misunderstanding something?

I am only superficially familiar with btrfs internals, but I do not see any way to implement snapshots without either doing CoW or duplicating the data in its entirety. If you are checking to see if the data is part of a snapshot, then you should be doing CoW.

[josefbacik]

Yeah nocow only works if there are no snapshots, which is why we have to check, so if you use snapshots it falls back to COW.

[MertsA]

Disabling CoW disables snapshotting.

[versteegen]

(I'm not an expert, just a user) Snapshotting effectively disables NOCOW. Snapshotting always works, and needs COW to do so. Which means ryao is right that fragmentation will occur. btrfs doesn't copy a NOCOW file in its entirety if it's in a snapshot, or I would have run out of disk space by now. (I really need to move my VM images to a non-backed-up subvolume.)

[josefbacik]

No it doesn't.