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Re: Is RAC really HA on Linux

From: Stephen Evans <evans036_at_mc.duke.edu>
Date: Wed, 29 Sep 2004 20:49:58 -0400
Message-ID: <OFD7B29456.DA935131-ON85256F1F.0002EC48-85256F1F.000491E2@notes.duke.edu>


carel-jan,
good post.

there is a good chance that any unplanned outage (especially involving failover to stndby) will take at least 1 hour (ie by the time database support is called, entries in ldap & onames servers changed; current active redos obtained from old primary etc). This is after the oncall dba has diagnosed the problem & decided we should fail o ver.

our users inform us that patient care (we are a hospital) becomes seriously affected once systems have been down for more then 30 mins.

even planned outages require a tremendous amount of time on our part to get concensus from all the users as to when is the best time. and of course we always have do this kind of maint around the 4:00am timeframe

in short, we are quite motivated to providing increased up time where we can shutdown an instance without any discernable disruption of service

so when i talk about HA, i mean increasing the likelyhood that an outage of a machine (or one of our data centers) will not cause the application to fail.

ofcourse there is no such thing as true HA (especially with oracle in that rolling upgrades are not supported). which was really the question of my original post.

are folks finding that RAC on linux is significantly improving their ability to provide continuous availability of applications to end users

thanks,

steve

Carel-Jan Engel <cjpengel.dbalert_at_xs4all.nl> 09/13/2004 03:06 PM  

        To:     evans036_at_mc.duke.edu
        cc:     Niall Litchfield <niall.litchfield_at_gmail.com>, "oracle-l_at_freelists.org" 
<oracle-l_at_freelists.org>
        Subject:        Re: Is RAC really HA on Linux


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_at_gmail.com> 09/13/2004 10:27 AM
Please respond to Niall Litchfield  

        To:     evans036_at_mc.duke.edu
        cc:     oracle-l_at_freelists.org
        Subject:        Re: Is RAC really HA on Linux


Comments inline
On Mon, 13 Sep 2004 10:11:41 -0400, Stephen Evans <evans036_at_mc.duke.edu> 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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Received on Wed Sep 29 2004 - 19:45:39 CDT

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