Subject: Re: IDEA: NFS mirroring
To: Grey Wolf <>
From: Matt Ragan <>
List: current-users
Date: 12/06/1996 15:03:01

Grey Wolf wrote:
>This is one of those "what if" things.
>You know how if your home directory is on a central server, if the
>central server goes down, you can't log in _any_where because the NFS
>server is not responding.
>Well, if it were possible to do NFS mirroring and dynamic rebinding,
>all that would happen is that the mount would glitch and bind to the
>mirror instead.  Same thing if the server suddenly went down.
>It would be a flip-flop relationship; if the master goes down, it comes
>back as the slave.
>That's a general idea -- what do you guys think?

Auspex does something like this with their ServerGuard software using
Ethernet multicasting.  Basically, for filesystems you want to mirror,
you use a secondary IP address to mount the filesystem from that the
server(s) respond(s) to using an Ethernet multicast address.  Obviously,
this requires that the master and slave servers be on the same Ethernet
segment (or bridged or whatever), and if you are using Ethernet switches
you have to make sure that your switch supports Ethernet multicasting 
properly.  When the master goes down, the slave detects the lack of 
responses, and takes over.

I haven't looked at this in depth at all, so I don't know what kind of
master-slave synchronization is used, or what happens when the master
comes back up (whether it becomes a slave, or reverts to master status).

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