[Info-vax] Request description of UFS for VMS person

[Bob Eager]

On Tue, 21 Apr 2009 20:42:14 UTC, JF Mezei 
<jfmezei.spamnot at vaxination.ca> wrote:

> Bob Eager wrote:
> 
> > As you say, unlink just removes a directory entry; all directory entries
> > are 'equal', so when the last one goes, the use count in the inode drops
> > to zero. At that point, the inode is cleared and the file blocks 
> > returned to free space.
> > 
> > On MS-DOS, the first byte of the directory entry was set to a special 
> > 'deleted' value, and file blocks were marked as free. Basic, but 
> > sufficient.
> 
> Does this mean that "undelete" is not possible on Unix file systems
> because its logical equivalent to the entry in indexf is actually wiped
> out ?

Generally, yes. However, it will depend on implementation.

Once again, it's important to realise that Unix is a rather vague term 
used to apply to a whole set of systems that look vaguely similar and 
may share some common ancestry. And, within each of those, there are 
multiple kinds of file systems, each of which will have its own 
characteristics. For example, there was a file system on UnixWare that 
stored all files contiguously and had to be 'squeezed' from time to 
time. 

Even UFS is problematical. There's UFS and there's UFS2. Which is not 
quite the same as FFS. Then there are features such as soft updates and 
snapshots, all of which have been added over time.

So, saying 'Unix does this' is pretty meaningless. Nearly all of those 
systems can't legally be called Unix anyway!
> 
> With VMS and DOS, it was possible to undelete files because entries were
> just flagged as available and remained until used by another file.

Yes, although on VMS you might lose parts of the file earlier if it had 
extension headers, as that presents a bigger 'target'!
-- 
Bob Eager