From mboxrd@z Thu Jan  1 00:00:00 1970
Date: Tue, 20 Jan 2009 14:30:33 -0800
From: "Roman V. Shaposhnik" <rvs@sun.com>
In-reply-to: <30fef936405d9216f98c10a5c32b42ea@quanstro.net>
To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>
Message-id: <1232490633.11686.26.camel@goose.sun.com>
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References: <30fef936405d9216f98c10a5c32b42ea@quanstro.net>
Subject: Re: [9fans] Changelogs & Patches?
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On Tue, 2009-01-20 at 09:19 -0500, erik quanstrom wrote:
> > > in the case of zfs, my claim is that since zfs can reuse blocks, two
> > > vdev backups, each with corruption or missing data in different places
> > > are pretty well useless.
> >
> >
> > 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.

> > P.S. Oh, and in case of ZFS a damaged vdev will be detected (and
> > possibly re-silvered) under normal working conditions, while
> > fossil might not even notice a corruption.
>
> not true.  one of many score checks:
>
> srv/lump.c:103: 				seterr(EStrange, "lookuplump returned bad score %V not %V", u->score, score);

I don't buy this argument for a simple reason: here's a very
easy example that proves my point:

term% fossil/fossil -f /tmp/fossil.bin
fsys: dialing venti at net!$venti!venti
warning: connecting to venti: Connection refused
term% mount /srv/fossil /n/f
term% cd /n/f/test
term% echo 'this  is innocent text' > text.txt
term% cat text.txt
this  is innocent text
term% dd -if /dev/cons -of /tmp/fossil.bin -bs 1 -count 8 -oseek 278528 -trunc 0
this WAS
8+0 records in
8+0 records out

term% rm /srv/fossil /srv/fscons
term% fossil/fossil -f /tmp/fossil.bin
fsys: dialing venti at net!$venti!venti
warning: connecting to venti: Connection refused
create /active/adm: file already exists
create /active/adm adm sys d775: create /active/adm: file already exists
create /active/adm/users: file already exists
create /active/adm/users adm sys 664: create /active/adm/users: file already exists
        nuser 5 len 84
term% mount /srv/fossil /n/f2
term% cat /n/f2/test/text.txt
this WAS innocent text
term%

Of course, with ZFS, the above corruption would be always
noticed and sometimes (depending on your vdev setup)
even silently fixed.

Thanks,
Roman.