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From: allbery@ncoast.UUCP (Brandon Allbery)
Newsgroups: comp.unix.wizards
Subject: Re: Backups on Live Systems
Message-ID: <2766@ncoast.UUCP>
Date: Thu, 2-Jul-87 20:55:31 EDT
Article-I.D.: ncoast.2766
Posted: Thu Jul  2 20:55:31 1987
Date-Received: Sat, 4-Jul-87 18:02:50 EDT
References: <132@dvm.UUCP> <725@aramis.rutgers.edu> <20070@sun.uucp> <1589@oliveb.UUCP> <3114@cit-vax.Caltech.Edu>
Reply-To: allbery@ncoast.UUCP (Brandon Allbery)
Followup-To: comp.unix.wizards
Organization: Cleveland Public Access UN*X, Cleveland, Oh
Lines: 59

As quoted from <3114@cit-vax.Caltech.Edu> by mangler@cit-vax.Caltech.Edu (System Mangler):
+---------------
| To add this to Unix, you'd need two new facilities:  a relative of
| creat() to create a file with zero links, yielding a file descriptor;
| and an "flink" call to make a link to a file given a file descriptor.
| [N.B. flink() should unlink the target if necessary, like rename()].
| Perhaps nicer would be to build flink() into close(), making close()
| take a filename, and if you give NULL as the filename, you get the
| old behavior.  Nicer still would be to have the modified creat()
| stash away the filename and pointer to the directory inode for later
| use by the close routine, but with BSD long filenames, it might be
| difficult to find a place for it.
+---------------

Why?  One easy and compatible way to do it is as follows:

Store a version number for a file in the inode.  (Yes, inode-based versions,
not path-based ones.  This is necessary given the UNIX way of doing things.)
A good place would be byte 39 of the block array, given that the blocks only
fill bytes 0-38.  On the other hand, 255 generations may be too few.  The
inode must also contain the inode number of the previous version, so that
previous versions can be retrieved.

When a file is creat()'ed, an empty, unnamed (link count = 0) is allocated;
data is written to this file.  When it is close()'ed,, either directly or by
exit(), the inode is updated, then the inode and the original are swapped
within the i-list for the file system.  Thus, the inode number never changes
between versions of a file (otherwise, it has to change every directory entry
the file has).

To specify a version of a file, you use an extension of the current naming
scheme; append a slash and the version number to the file name.  When namei()
gets to a file and there is a slash and a valid number afterward, it can
follow the chain back to the correct inode.

Programs aborting via kill() (and possibly by exit(n) for n != 0 or n < 0 or
etc.) would have the new inodes deallocated, leaving the previous version
intact.

Programs which don't know about version numbers then get the version seman-
tics for free.  Also, you can implement GENERATION-RETENTION-COUNT as yet
another variable in the u block, with a system call to get or set it.

This means that directories can't have versions.  Then again, there isn't
much reason for them to have them anyway.

Any comments?

++Brandon
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