Re: [Bug-tar] tar writes invalid mtime for times before the epoch

[Joerg Schilling]

Paul Eggert <address@hidden> wrote:

> As I vaguely recall, this extension was designed by both of us in 
> collaboration, and superseded an earlier base-256 format that GNU tar 
> still supports but does not document. I'm too lazy to consult the email 
> archives to check my memory; it's not a big deal either way.

I remember that GNU tar used a base-64 method that could store up to 60 bits 
when I designed the base-256 method. I contacted you as I did not like to 
implement something that cannot store at least 64 bits.

We had some discussions about the method I used to mark a base-256 block (which 
is using the top bit of the left most byte).

> > The background is that base-256 allows up to 95 bits + sign bit and this is
> > sufficient for all possible storage as long as you cannot manage to store 
> > part
> > of the data in a parallel universe.
> No parallel universe should be needed to exhaust the format's limits. In 
> 2001 Seth Lloyd estimated that the observable universe, if treated as a 
> computer, would contain about 10**120 bits of information if quantum 
> gravity were taken into account. This would require 396 bits to address, 
> assuming byte-addressible storage. Admittedly Lloyd's estimate is very 
> rough and could well need updating in the light of more-recent physical 
> discoveries. Still, 95 bits does not seem to be nearly enough. And even 
> if 396 bits is right just now, eventually it'll be too small as the 
> number of bits in the universe is growing.

Well, if you look at existing storage, typically less than 1% of the matter 
used in such a device is active storage matter and you should allow others to 
create their own storage instead of using up all the universe ;-)

I made a practical assumption that a storage unit should not include more than 
aprox. 5x5x5m of volume from active storage matter and this results in aprox. 
one megamol of storage matter. Whith this assumption you either don't need more 
than 95 bits or you need a parallel universe.

Jörg

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