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Re: Multiplexing Datafiles?

From: Rob Jolliffe <rjolliffe_at_my-deja.com>
Date: 2000/08/13
Message-ID: <8n7caa$ror$1@nnrp1.deja.com>

Thanks,

This is useful.

-Rob

In article <3fzl5.936$x16.61541_at_monger.newsread.com>,   "Van Messner" <vmessner_at_bestweb.net> wrote:
> There are three issues involved. Backup, Redundancy for purposes of
 load
> sharing, and Redundancy for purposes of failover and disaster
 protection.
> Howard is right about backup.
>
> Oracle offers three choices for redundancy: a standby database,
 parallel
> server, or advanced replication.
>
> A standby database does not lend itself to either load balancing
 or true
> failover. It does lend itself to disaster protection since the
 primary and
> standby can be physically separate. The standby is configured like
 the
> primary. As Oracle archives its online redo logs they are sent to the
> standby which applies them. Thus the standby is always behind the
 primary
> and almost never has all the changes which have been made on the
 primary.
> Anything in the primary's online redo logs that have not yet been
 archived,
> hasn't even been sent to the standby, let alone applied.
> When the primary fails, as much information as possible is applied
 to
> the standby, which is then brought online. This takes time and some
 data
> may be lost. In addition the standby server, which is as expensive as
 the
> primary server, does no work - it just waits for the primary to fail.
 And
> until you repair or replace the primary, the standby has no standby.
> Of course all the information that flows from primary to standby
 moves
> across the network. If the machines are close enough to use some
> non-network means of communication, then they may be vulnerable to
 some
> other form of failure - lightning, intentional damage, etc.
>
> Parallel server creates multiple instances that run against a
 single set
> of datafiles. This implies a single set of disks, which removes some
 of the
> redundancy that's useful for disaster protection. Also, the instances
 run
> on what Oracle calls "a cluster or massively parallel hardware
 platform".
> As with a standby database, a parallel system is vulnerable to some
 type of
> site failure - lightning, intentional damage, etc.
> While the processors share the load of doing whatever can be done
 in
> memory, all instances are reading and writing to a common set of
 disks.
> Failover is achieved from the fact that instances can be added and
> removed from the parallel server environment. If an instance fails,
 the
> other instances keep running and working. If loads on the system
 increase,
> new instances can be added.
>
> The third method is advanced replication. In this method two or
 more
> identical servers are configured in the same way with each containing
> identical databases and each having its own set of disk drives. All
 servers
> can be used to do meaningful work, so load balancing is a real
 possibility.
> Changes made to the database on any server are replicated to the
 others -
> either immediately or with some programmed delay. If a server is
 lost, the
> others can continue to process requests. And the servers can by
 physically
> separate, although that implies a lot of update traffic over the
 network.
> This method sounds promising, but I could list several pages of
> potential pitfalls in its actual application. So you'll have to go to
 the
> Oracle advanced replication manual for the gory details.
>
> Van
>
> "Howard J. Rogers" <howardjr_at_www.com> wrote in message
> news:3995df0c$1_at_news.iprimus.com.au...
> >
> > "Rob Jolliffe" <rjolliffe_at_my-deja.com> wrote in message
> > news:8n4295$mqp$1_at_nnrp1.deja.com...
> > > Hi,
> > >
> > > I'm just beginning to learn Oracle, and I was reviewing some
 Email
> > > regarding Raid5 vs Raid0 performance. It seems Raid 0 is
 significantly
> > > better for performance - but you lose the redundancy of Raid 5 (or
 even
> > > Raid 1).
> > >
> > > I was trying to also determine if in a data tablespace it is
 possible to
> > > multiplex (mirror) datafiles the same way you can with a Redo log
 group.
> > > I was totally unable to determine if you are or are not.
> > >
> >
> > Absolutely not. Never has been, and there never will be -because it
 is
> > entirely pointless. The reason for duplexing your data files is,
> > presumably, to avoid complete disaster in the event that one of them
 dies,
> > gets deleted, corrupted etc. That is precisely what the Redo Logs
 are
 there
> > for. Provided you've kept all your redo since the time of the last
 backup,
> > you can reconstruct any given data file up to the precise time of
 failure.
> >
> > So if you're mirroring your Redo, you have provided all the
 protection
 your
> > data files ever need.
> >
> > (Conceivably, you can use external, hardware mirroring of data files
 to
> > enable recovery to take place *faster* -you just break the mirror
 and use
> > the good copy. No need to wait for all that Redo to be re-played.
 But
> > that's a mean-time-to-recovery issue. It's certainly not
 necessary).
> >
> > > Can anyone tell me? I know you can stripe tablespaces - but can
 you
> > > mirror them? Perhaps this requires creating two tablespaces and
 somehow
> > > matching them?
> > >
> >
> > Forget it. There's no such concept in Oracle, because the Redo does
 all
 the
> > protection work we need.
> >
> > Regards
> > HJR
> >
> >
> >
> > > Thanks very much
> > >
> > > -Rob
> > >
> > >
> > > =============
> > > From the desk of Rob Jolliffe
> > >
> > >
> > > Sent via Deja.com http://www.deja.com/
> > > Before you buy.
> >
> >
>
>

--
=============
From the desk of Rob Jolliffe


Sent via Deja.com http://www.deja.com/
Before you buy.
Received on Sun Aug 13 2000 - 00:00:00 CDT

Original text of this message

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