Can you clarify how this is an example of NIH? They're switching the default from btrfs to ext4, not writing their own filesystem. Also, it's necessary if they

Can you clarify how this is an example of NIH? They're switching the default from btrfs to ext4, not writing their own filesystem. Also, it's necessary if they want to switch to overlayfs as the Docker backend, because btrfs lacks whiteout support which causes problems with overlayfs. Switching to overlayfs for Docker makes a lot of sense, given that you get the dedupe benefits of AUFS with lower overhead than devicemapper/btrfs and it's now part of the mainline kernel tree.