Interesting feature, I had no idea. I just verified this with gcc and indeed the return register is always set to 0 before returning unless otherwise specified.

int main(void)
{
    int foo = 10;
}

push   %rbp
mov    %rsp,%rbp
movl   $0xa,-0x4(%rbp) # Move 10 to stack variable
mov    $0x0,%eax       # Return 0
pop    %rbp
ret

int main(void)
{
    int foo = 10;
    return foo;
}

push   %rbp
mov    %rsp,%rbp
movl   $0xa,-0x4(%rbp) # Move 10 to stack variable
mov    -0x4(%rbp),%eax # Return foo
pop    %rbp
ret

That’s literally what I’m saying; It’s fine as long as there wasn’t any unwritten data in the cache when the machine crashes/suddenly loses power. RAID controllers have a battery backed write cache for this reason, because traditional RAID5/6 has the same issue.

BTRFS RAID5/6 is fine as long you don’t run into a scenario where your machine crashes and there was still unwritten data in the cache. Also write performance sucks and scrubbing takes an eternity.