Re: srfs - a new file system.

[Jan Hudec <bulb@ucw.cz>]

On Thu, Oct 23, 2003 at 09:50:01 +0200, Nir Tzachar wrote:
> hi there.
> 
> > That's annoying when >50% of your servers were unavailable for a period
> > of time, because all recent changes will be lost when connectivity is
> > restored.
> 
> well, if u want a _full_ self stabilizing file system, you cannot behave 
> any other way. When you have a self stabilizing algorithm, you __have__ to 
> operate under the assumption that transient errors can and will happen.
> so, a cosmic ray can hit N out of your 2N+1 hosts, and corrupt the data 
> they hold. its very slim, but you have to take these kind of errors into 
> account to prove the correctness of the algorithm.

But the vector time approach solves this too and does so a lot better.

If we return to the example with notebook. Assume there is a computer
lab with 20 computers and all have replicas of some file. Assume, that
I take a laptop, connect it to the system, replicate the file and
disconnect.  Then I work on it while disconnected and then reconnect
again.

With vector time, the system decides that the copy on all 20 computers
is ancestor to my copy and replace everything with my copy. With
majority vote, my copy loses 1:20 and is lost.

Imagine further, that my friend does the same.

Now, with vector time, the system decides, that
  * All copies in the lab are old and invalidates them.
  * Our copies conflict. It does not blindly choose one, rather it asks
    for assistance.
While with majority vote, both our copies loose 1:20 and are discarded.

-------------------------------------------------------------------------------
						 Jan 'Bulb' Hudec <bulb@ucw.cz>