Re: Is RAC really HA on Linux
- From: Carel-Jan Engel <cjpengel.dbalert@xxxxxxxxx>
- To: evans036@xxxxxxxxxxx
- Date: Mon, 13 Sep 2004 21:06:09 +0200
Steve,
Using a SAN-based replication (i.e. copying all your changes on the SAN
synchronously to a remote DR site) might cost you too much bandwidth.
Further, considering whether RAC or any other solution is HA or not is
not an objective discussion. I'd rather take the requirements of the
company as a starting point. What do you consider as HA? What is the
Maximum time allowed for an unplanned outage? And what for a planned
outage, and how many of them are allowed?
The maximum availability scenario combines RAC and Data Guard (from the
point of view of Oracle). One of them might be enough for your
situation. That should be investigated first, one shouldn't jump into
technology as a panacee.
I've several customers running Data Guard alone as their HA solution.
Compared with the SAN replication they save quite some bandwith: In
stead of copying changed blocks (or even tracks, depending on the HW
brand), you're copying redo information. No database blocks, nor copied
archives need to go over your WAN/LAN, just the change vectors. You get
the ablilty to set delays in remote redo-applying (mind the info got
sent, just waits to get applied, giving you protection for logical
errors keeping the zero-data loss intact) and the ability to open the
standby R/O (again, redo keeps getting sent). When an outage of a couple
of minutes is affordable, DG might be more helpful than RAC. It will
cost you less most of the times.
Bottom line: figure out the requirements, investigate the disasters
you'd like to be protected from, what donwtime is allowed with each type
of disaster, budget, and possible solutions, before jumping into a
particular solution that might only cover a part of the requirements and
might exceed the budget without providing optimal protection.
Best regards,
Carel-Jan Engel
===
If you think education is expensive, try ignorance. (Derek Bok)
===
On Mon, 2004-09-13 at 20:28, Stephen Evans wrote:
> niall,
> agreed with the single database bit. We still plan on having a stand-by
> that can provide us with zero data loss (but definitely not HA).
>
> it is interesting that you perceive RAC as not addressing HA (but only
> scalability). and of course the db is a single point of failure in a RAC
> config (unless you mitigate that with some kind of SAN based continuous
> copy with auto failover to that too).
>
> so do folks generally consider (not withstanding the db as a single point
> of failure) that RAC is NOT considered high availability? I think i'm
> inclined to agree with Niall's viewpoint if we cannot do rolling upgrades
> within the cluster. From memory, oracle RAC can only withstand rolling
> upgrades if the patch is designated as such (and patchsets are NOT).
>
> does anyone know if future versions of oracle RAC will support rolling
> upgrades/patchsets?
>
> hope i'm not rambling too much.
>
> steve
>
>
>
>
>
>
> Niall Litchfield <niall.litchfield@xxxxxxxxx>
> 09/13/2004 10:27 AM
> Please respond to Niall Litchfield
>
>
> To: evans036@xxxxxxxxxxx
> cc: oracle-l@xxxxxxxxxxxxx
> Subject: Re: Is RAC really HA on Linux
>
>
> Comments inline
> On Mon, 13 Sep 2004 10:11:41 -0400, Stephen Evans <evans036@xxxxxxxxxxx>
> wrote:
> > i hope i addressed this right - its my first post.
>
> looks like it!
>
> > i am looking at RAC to provide an HA environment on Linux (most likely
> > Redhat AS3)
>
> I think I'd view RAC as a scalability solution for Oracle rather than
> an HA solution. You still only have the one database with RAC, what
> happens if that DB suffers a failure?
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