But those interfaces [to allocate bytes/pages before other bytes/pages, allocate bytes/pages in the middle of bytes/pages, to prune bytes/pages before other bytes/pages] to manipulate said parts are non-portable between *NIX implementations, usually limited to certain filesystems with specific implementation-specific minimum byte sizes.
So like how the OS pretends filesystems are in a tape, the filesystem pretends that the files are in a tape.
And tape splicer is optional with a not-standard interface (logical volume management, fallocate, etc).
Then don't do that part, allocate at the end and only rewrite existing blocks rather than deleting them. It's like how just because you can't easily sbrk(2) new memory before at the beginning of a heap doesn't mean you can't have multiple threads despite that being the resource you're contending.
So like how the OS pretends filesystems are in a tape, the filesystem pretends that the files are in a tape.
And tape splicer is optional with a not-standard interface (logical volume management, fallocate, etc).