Re: [9fans] Changelogs & Patches?

["Roman V. Shaposhnik" <rvs@sun.com>]

On Tue, 2009-01-20 at 18:36 -0500, erik quanstrom wrote:
> > > > Got it. However, I'm still not fully convinced there's a definite edge
> > > > one way or the other. Don't get me wrong: I'm not trying to defend
> > > > ZFS (I don't think it needs defending, anyway) but rather I'm trying
> > > > to test my mental model of how both work.
> > >
> > > if you end up rewriting a free block in zfs, there sure is.  you
> > > can't decide which one is correct.
> >
> > You don't have to "decide". You get use generation # for that.
> >
>
> what generation number?

I'm talking about a field in each ZFS block pointer. The
field is actually called "birth txg", but I thought alluding
to VtEntry.gen would make it easier to understand what I had
in mind.

> are there other things that your argument
> depends on that you haven't mentioned yet?

Fair question. It depends on at leas cursory reading
of ZFS-on-disk specification. I felt uneasy in this
conversation precisely because I had a very vague recollection
of Venti/Fossil paper. I guess it cuts both ways:
   http://opensolaris.org/os/community/zfs/docs/ondiskformat0822.pdf

> > term% fossil/fossil -f /tmp/fossil.bin
> > fsys: dialing venti at net!$venti!venti
> > warning: connecting to venti: Connection refused
>
> well, there's your problem.  you corrupted
> the cache, not the venti store.  (you have no
> venti store in this example.)

I was specifically referring to a "normal operations"
to conjure an image of a typical setup of fossil+venti.

In such a setup a corrupted block from a fossil
partition will go undetected and could end up
being stored in venti. At that point it will become
venti "problem".

> i should have been more clear that venti does the
> checking.  there are many things that fossil doesn't
> do that it should.

Sure, but I can't really use venti  without using
fossil (again: we are talking about a typical setup
here not something like vac/vacfs), can I?

If I can NOT than fossil becomes a weak link that
can let corrupted data go undetected all the way
to a venti store.

This is quite worrisome for me. At least compared to
ZFS it is.

Thanks,
Roman.