[lug] [Slightly OT] File Management?

[Zan Lynx]

Paul E Condon wrote:

> On a slightly different level, I am skeptical of the very idea of
> "point in time snapshots". At a level of detail that we can think
> about, and may actually be coming into reality, there is no single
> definition of time that is valid everywhere. This is a well known
> result of the Special Theory of Relativity. More significantly,
> because of transmission delays, it is not possible to know of a
> commit of a transaction until some time after it has actually
> occurred. This can lead to the system letting other transactions
> to commit that would have not been allowed had the first commit
> been known about promptly. This is not a problem of a mistake
> being made because of an inadequate implementation, but because of
> an inadequate understanding of the nature of time. Talk of a
> "point in time" is evidense of an inadequate understanding of
> the nature of time. 
> 

Computer systems go to a lot of effort to share the definition of 
"time".  In the case of a filesystem with point in time snapshots, think 
of it in terms of transactions.

Pick a point in time for the snapshot.  Every file transaction before 
this point in time is completed.  Every transaction after this point in 
time has not yet started.

This may mean that the "point" in time is actually rather wide, at least 
as wide as any of the transactions that were in progress at that point. 
  But it does not mean there is any possibility of being confused as to 
what has happened and what has not yet happened.

-- 
Zan Lynx
zlynx at acm.org

"Knowledge is Power.  Power Corrupts.  Study Hard.  Be Evil."