[Info-vax] Hard links on VMS ODS5 disks

[Johnny Billquist]

On 2023-07-12 15:34, Chris Townley wrote:
> I am just about to install V9.2-1 and intrigued about hard links.
> 
> When I installed E9.2-1 it asked me if I wanted to enable hard links, 
> with a warning to read the docs, but I wasn't much wiser afterwards.
> 
> I get he impression that it is much cleaner with them enables, but what 
> are the downside risks?
> 
> I have used soft links on *nix for many years, but only for linking to a 
> directory, which you cannot do with hard links
> 
> Any thoughts?

I always have a bit of a hard time understanding the topic. One person I 
talked to was of the opinion that ODS "hard links" were not hard links 
because they didn't include reference counting.

But essentially, if we talk about hard links under Unix, it's just 
additional directory entries pointing to the same inode.
The equivalent of the inode number under Unix is in ODS the file id. And 
you can have multiple directory entries giving the same file id, which 
is exactly the same as the thing in Unix.

The one difference being that in Unix, a file is removed when the link 
count goes to zero. And there is no way for a program to reference a 
file through the inode number itself.

While in ODS, the file id, and file, is more unaware of what directory 
entries might exist that refers to it, and a file can be deleted without 
the directory entry being deleted. And also, programs can open files by 
ID, completely bypassing the directory lookup. Which is also why ODS 
fild ids have a generation number, so that stale file ids don't give you 
some other file you didn't expect.

But I would still already call this hard links. I wonder if "enabling 
hardlinks" in ODS5 is then adding the reference counting, and not 
deleting files until there are no references to them. Or is there 
something else/more hidden in here?

   Johnny