Re: [9fans] thoughs about venti+fossil

["Russ Cox" <rsc@swtch.com>]

> "cryptograpically strong".  the invented function may have
> the same probability of collision as sha-1, but it is not
> cryptographically strong.

let's take this discussion elsewhere, since cryptographically
strong just means "we don't know how to attack it yet"
(witness md5).  surely there is a crypto list deserving of
this non-plan 9 traffic.

> i think that venti has a different problem.  indexing by
> sha-1 hash trades time and index lookups for space.
> but disk space is cheep relative to our needs and table
> lookup and fragmentation that venti implies results
> in a lot of random i/o.  modern disks are at least 25x
> faster doing sequential i/o.

this isn't fair to venti.  yes, there is a performance cost.
but venti is interesting because it creates new functionality.
the main one is that you can build very naive systems that
don't worry about wasting disk space, because underneath
they're not.  for example, i back up 1.4TB of FFS file systems
every night by copying them to a venti server.  i don't care
how cheap disk is: if you're creating 1.4TB of disk per night
you're going to run out of disk pretty soon.  using venti,
i have about 3 years of backups stored in 1.7TB of space.
there are other interesting properties too, like the log
structure making it easy to sync venti servers against each
other even across long-distance links.

venti is for archival storage.  if you need a super fast live copy
then you want to put something in front.  (and if you want
a super fast live copy with no archival, just don't use venti.)

russ