Common rule is:
- RAC is protection against short failures (server reboot, you need to
insert new card, RAM error,
you need to patch Oracle, etc);

- DataGuard is protection against long failures (disk array failure, power
failure in the building - if you have remote location,
DBA made error and removed important tablespace, etc etc).

So, with RAC, you wil not see short (5 - 30 minutes) downtimes which you
usually have to maintain system, or when system panic, reboots and so on.

With DataGuard you wil not see long (6 hours) failures which you can see if
database is damaged, disk ssytem die, DBA make mistake during upgrade and so
on (I did not see it for a long, but such events happen time to time).

No one system with shared _something_ (disks for example) and with 100%
syncronized state (RAC) can protect you for a long-turm failures. Any disk
system can fail, or important data can be damaged easily, or software bug
can kill RAC cluster (very likely event, btw).


----- Original Message -----
From: "Rami Aubourg-Kaires" <rami.aubourg@(protected)>
To: <suse-oracle@(protected)>
Sent: Tuesday, April 11, 2006 1:22 AM
Subject: Re: [suse-oracle] Server and Service Level Failover


> C'est Pierre wrote:
>
> > I am currently entitled to evaluate which solutions can support an
> > active/passive ("Cold Failover") system based on SuSE Enterprise Linux
> > 9 running Oracle 10G.
> > On another SuSE maillinglist, I was told about Oracle RAC and I talked
> > to our local Oracle DBA. He told me that RAC serves the purpose when
> > there's 2 oracle instances, each on a different server, both sharing
> > the same storage. That's not our purpose. We want that if the active
> > node (server/instance) dies for some reason (network outage, hardware
> > failure, etc) the other server takes the lead, launches an oracle
> > instance and mounts the storage.
> >
> > We are going to use the hardware on the following list to accomplish
this:
> >
> > 2x HP BL25p, with 4xOpteron 200 series and 8gb ram. Fibre channel
adapter
> > 1x HP StorageWorks EVA 3000 for shared storage.
>
> Hi,
>
> Well, Polyserve does that with NAS. But you should have two NAS'es. The
> material you described is perfect for two-node RAC with shared storage.
> On the other hand, if you've done some mountaneering or caving, you
> should ask yourself: "What happens if the shared storage goes down? Do I
> fall and die?"
>
> The cheapest and least fault-prone solution is DataGuard: two servers on
> your LAN in MAXIMUM PROTECTION or MAXIMUM AVAILABILITY mode, eventually
> a third remote one if you've got a 10 Meg WAN connection in MAXIMUM
> PERFORMANCE mode to ensure backup in case of nuclear war.
> With DataGuard, you will need only one Oracle licence, unless you use
> the standby server for reporting purposes over a few days a year.
>
> The downside of DataGuard is that you will need to issue three or four
> commands if your main server goes down. No automatic startup.
>
> You might also have a two-node RAC configuration with shared storage as
> you described, plus one DataGuard standby server to start when both of
> your RAC servers panic simultaneously because a mouse was running on the
> server room's floor.
>
> Basically, your purpose is to ensure maximum data protection and minimum
> downtime. Not increasing downtime because of a technology you don't
> master, or loosing your data because you put all of your data in one
place.
>
> Rami, who's seen seven disks all hosed up in a shared storage bay.
>
>
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>


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