[OpenAFS] Re: IP address changes

[Andrew Deason]

On Wed, 30 Jan 2013 20:30:23 +0000
pg@afs.list.sabi.co.UK (Peter Grandi) wrote:

> > Did a guide or something tell you to run 'vos changeaddr'? See the
> > CAUTIONS section of
> > <http://docs.openafs.org/Reference/1/vos_changeaddr.html>.
> 
> I have been wondering for a while about changing the addresses
> of the AFS db servers, in part because this previous message:

'vos changeaddr' deals with fileserver addresses, not database servers.
Since fileservers automatically update their IP in the relevant places,
that's why you almost always don't need the 'vos changeaddr' command.

>   http://permalink.gmane.org/gmane.comp.file-systems.openafs.general/33296
> 
>     «Moving the database server with the lowest IP address
>     requires some care; moving database servers generally
>     requires updating clients;»
> 
> Is that still current? What is the extra care for the elected master?
> I have some guesses, but would rather know.

Moving the elected sync site means that you will lose quorum for a
minute or two, which means you can't write to the database. ("The
database" meaning, the VLDB, the protection database, or anything else
that is managed by a dbserver process.) If you bring it back up and it
is still the lowest IP machine, it is likely it will be elected sync
site again. If you have not updated the client CellServDB to include
that IP, that client will not be able to write to the database.

If you just move a non-sync-site, by comparison, a client will just not
be able to contact that site. But it will still be able to read and
write to the database as it fails over to the other sites.

Note that a "client" in terms of ubik means not only kernel /afs
clients, vos, pts, etc, but also fileservers. They are clients of the
vldb and ptdb, since they write their IPs to the vldb, and use the ptdb
to calculate group membership of accessing users.

> BTW I keep adding hints/pecularities of OpenAFS configuration to
> this section:
> 
>   http://www.sabi.co.uk/Notes/linuxFS.html#fsHintsAFS

We have a wiki, just by the way <http://wiki.openafs.org/>, which was
recently / still is getting reorganized. If you think of anything that
would be useful there, it is of course welcome.

> And these about elections:
> 
>   https://lists.openafs.org/pipermail/openafs-info/2008-June/029469.html
>   http://www.mail-archive.com/openafs-info@openafs.org/msg29311.html

If you want to know more about ubik in much more detail, to the level of
distributed systems theory and stuff like that, there are a few papers
on it floating around back when it was designed, I think. Try looking
for "Ubik: Replicated Servers Made Easy" and "Quorum Completion" by
Michael Leon Kazar. The system we use is still very similar to what was
described then. Or just read the source :)

-- 
Andrew Deason
adeason@sinenomine.net