[saga-rg] Fwd (hupfeld at zib.de): Re: SAGA Strawman API Version 1.0

[Andre Merzky]

Hi, 

in a chat with John Shalf, he offered following opinion to
the topic of consistence, which he agreed to let me post to
the list.  The paper John refers to is the paper cited by
Jon McLaren (see quoting below).  SSI here means Single
System Image.

John:

   I think SAGA has no business defining a consistency
   model, but it should be able to accomodate consistency
   models that exist in the underlying implementations.

   SAGA is just the API.  We should lay out some existing
   consistency models and make sure the error handling
   supports it.  The problem with POSIX+NFS that was pointed
   out in that paper was that they could not extend the
   error codes that were available to POSIX.

   Felix is right on that account.

   However, most grid technologies are trying their best to
   create an SSI consistency model.  The existence proof is
   there (WAN-GPFS provides the same consistency model
   semantics as a local FS connection).

   So I reject any claim that a "Grid" filesystem neccessitates 
   some bizzaro consistency model that is not the same as
   Single System Image (SSI).

   I think any remote-filesystem strategy should have SSI
   as its goal ultimately.  I think it is somewhat of a
   religious battle as to whether a remote consistency 
   model must be radically different than the local one.
   Certainly, we need to deal with different kinds of
   failures (eg. an open filehandle that suddenly becomes
   unavailable).
   
   It should be discussed at GGF [...].

   But just to keep the discussion focused, the core comment
   is "SAGA has no business defining a consistency model,
   but it should be able to accomodate consistency models
   that exist in the underlying implementations." and also
   to say that "Felix is right about the programmer
   potentially having to worry about the worst-case."  
    
   [...]


Best Regards, 

  Andre.


Quoting [Jon MacLaren] (Jun 15 2005):
> 
>>> <snip>
>>> If SAGA choses to give single-system like guarantees, this must be
>>> explicititely stated. All interfaces that deal with data are unusable
>>> without a
>>> specification of consistency guarantees.
> 
> I don't believe that it is possible, or even desirable, to try to  
> make distributed systems look like they are not distributed.  For  
> example, I don't think you should provide POSIX behaviour on a  
> distributed filesystem.  If you look at AFS, it doesn't fit the POSIX  
> model.  Most people write code that ignores what the filesystem might  
> be, and assume POSIX.  How many people check the failure status on a  
> file close?  With AFS, you can get "Host not found" when you do a  
> file close.  You can wait, and try again.  If you quit, your changes  
> are lost.  (As a library writer, you can try and "squash" the errors  
> by putting a clever layer of code between the app and the filesystem  
> that know tricks like this.  The Condor people do this, I seem to  
> recall.)
> 
> The point here isn't that developers should never assume a POSIX  
> filesystem, it is that they should know what kind of filesystem they  
> are dealing with, so that they can write appropriate code.  When you  
> go distributed, there are a whole new set of error conditions that  
> can occur.  I don't think that there is anything to be gained from  
> pretending that remote objects are the same as local objects, so that  
> people's code can stay the same.  If the code doesn't know it's  
> dealing with something that is remote, rather than local, then at  
> best (i.e. if there is lots of error checking) it will fail far more  
> often.  Probably though, it won't be robust.
> 
> It might be worth looking at the following paper, which says  
> eloquently what I'm grasping for.
> 
> "A note on distributed computing" by Jim Waldo et al., available  
> from: http://research.sun.com/techrep/1994/abstract-29.html
> 
> Cheers,
> 
> Jon.



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