via google search for btrfs and o_direct i found this wiki page
https://pve.proxmox.com/wiki/Special:WhatLinksHere/Storage:_BTRFS
that mentions the folloging important information
"BTRFS will honor the O_DIRECT flag when opening files, meaning VMsshould not use cache mode none, otherwise there will be checksum errors."
this indeed is a known problem with btrfs, and not resolved.
https://bugzilla.redhat.com/show_bug.cgi?id=1914433
furthermore, when using compression...
Read more
https://pve.proxmox.com/wiki/Special:WhatLinksHere/Storage:_BTRFS
that mentions the folloging important information
"BTRFS will honor the O_DIRECT flag when opening files, meaning VMsshould not use cache mode none, otherwise there will be checksum errors."
this indeed is a known problem with btrfs, and not resolved.
https://bugzilla.redhat.com/show_bug.cgi?id=1914433
furthermore, when using compression...
Read more