[lug] Setting up failover in Linux?

[Will]

I have never seen a long distance cluster running an application on a
shared file system such as DRBD.  Even clusters in the same room and on
shared FC storage are usually offline when their applications are updated.

I've seen data being pushed in real time but there is always some sort of
fencing mechanism to prevent corruption.  Usually a database with a
transaction log but also SANs that replicate to a swap area and only move
in the changes once all of the blocks have been transmitted.

On Tue, Apr 24, 2012 at 4:24 PM, Rob Nagler <nagler at bivio.biz> wrote:

> Hi Will,
>
> >   Clusters are run in maintenance mode during these situations so fail
> > over will not occur until both systems are back to a similar state.
>
> I can see how this would help prevent an accidental, automatic
> failover wreaking havoc.  However, I'm interested in the problem that
> the code and data are not part of the same transaction.
>
> Perhaps I can explain it this way, assume the primary and secondary
> are separated by a large enough distance that the disk blocks
> associated with the data and code upgrade take "a while" to copy.
> During that period, the primary fails.  What state is the secondary
> in?  Can it take over?
>
> Rob
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