> What you must grasp is that it is *impossible* to
> represent/re-create/look up the values of 2^65536 bits in fewer than
> 2^65536 bits--unless you concede that each checksum/hash/fingerprint
> will represent many different values of the original data--any more
than
> you can represent three bits of data with two.

that is why i have turned off all hardware and software compression on
my tape drives.  imagine trying to store more than 400GB of data onto a
single lto3 tape!  they "say" that you can store up to and even more
than 800GB, but i don't believe a word of it.  there is no way 1 nibble
of data can represent 1 byte!  once i have the time to study lzr
compression and understand it, and see whether or not it is
"data-loss-less", then i may turn compression back on.  until then,
tapes are cheap and i'll buy 2.5 times as many as i need.  :-)

thanks,
jerald

p.s.
our de-dupe vtl does the hash and then a bit by bit comparison of the
data block to ensure the data really is the same in order to eliminate
the duplicate block.  i think some of the confusion may be in not
understanding how the de-dupe process works.  once you create a hash for
a block of data, you are storing the hash AND the block of data.  you
are never having to re-create a big block a data from a smaller hash.
the backup stream of data gets re-written from a "string" of 8k blocks,
into a "string" of 160-bit pointers which point to the unique 8k blocks
of data via the hash table.  or something like that...
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