Look, I simply highlighted a major user of btrfs. Sorry if you have some complex emotions about them. But for some reason, I doubt you'd say the same thing when someone mentions Netflix using FreeBSD.

>(a) You do not know how or where Facebook use BTRFS

Their engineering team has posted a few of their use cases.

>(c) Facebook probably employ the guy who invented BTRFS and an army of kernel developers on top of that .... how much in-house support do you have for BTRFS ?

Uh, about as much as any other file system? Those changes and improvements are upstreamed to the kernel anyway. It's not like Facebook has some sort of special version of btrfs they are using.

>As far as I am concerned, the fact that they STILL have not fixed RAID5 in BTRFS says everythng you need to know.

As far as I know, the issue with RAID5 in btrfs is highly complex and it would take quite a bit of dedicated effort to make it work. I suppose it's a architectural shortcoming of btrfs. But then again, it's RAID5, a/k/a something only shoestring hobbyists really care about. Hence why no one is bothering to make it work in btrfs.

At the end of the day, btrfs is perfectly fine for home users and workstations. ZFS beats it out on servers, that's fine. Traditional filesystems are not the end-all be-all of storage anymore. No one has made a better ZFS because the industry has moved on to things Ceph, vSAN, AzureHCI, etc.