Re: [9fans] Subject: Venti, redundancy, and hardware failure.

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

> occurred to me that, although I know how to mirror my arenas, I'm not
> clear on how to recover from hardware failure.  That is, suppose sdD0
> and sdD1 are dedicated to arenas, they are mirrored, and venti.conf
> has something like 'arenas /dev/fs/arenas'.  Now suppose sdD0 goes
> belly up.  Do I change '/dev/fs/arenas' to '/dev/sdD1/arenas', in
> venti.conf and fossil conf?

Only if you've changed your mind and
no longer think mirroring is a good idea.

What you want to do is re-establish the
mirror.  Assuming that sdD1 is still working,
you replace sdD0, run dd (or pump(1)) to
initialize sdD0 with the contents of sdD1.
Then you keep using /dev/fs/arenas.

> Maybe rerun fmtindex?  I recall reading
> about setting up venti and recovering fossil from venti, but I don't
> remember seeing anything about this.

Let's assume you need to run off sdD1 alone
for a while until you get your sdD0 replacement.
The easiest thing to do is change your mirror
config so that /dev/fs/arenas is now a mirror of
one disk (if that's not allowed, it should be), just sdD1.
Then nothing else has to change, since the data
is still available as /dev/fs/arenas.

But suppose you are asking "I changed the disk
device name in venti.conf.  What else do I
need to do for that to take effect?"
The answer is nothing: fmtindex does write
down info about the disks, but it names them
using the names you provided to fmtisect
and fmtarenas (things like index0 and arenas0.1).
The only place that a full path name is given
is in venti.conf.  If you move the device to a
different name (but have the same data), only
venti.conf needs to change.

Russ