Re: Doubts on Data Guard and Replication

From: Howard J. Rogers <hjr_at_dizwell.com>
Date: Tue, 23 Nov 2004 17:13:20 +1100
Message-ID: <41a2d4fc$0$17541$afc38c87@news.optusnet.com.au>

DA Morgan wrote:

> Howard J. Rogers wrote:
> 

>> BlackBeltDBA wrote:
>> [snip]
>>
>>> Switching to a standby database does incur *some* downtime. It's not an
>>> automatic event. The application will probably fail and need to be
>>> restarted once the standby comes back online. If you can not afford any
>>> downtime you will probably want RAC.
>>
>>
>>
>> Could we perhaps kill this myth before it gets out of hand? There *is*
>> downtime with RAC. Blocks that require re-mastering, as well as blocks
>> which require instance recovery, will not be available until the
>> remastering and the recovery have taken place. Your select will
>> hang/pause/stall/whatever other word you'd like to use for an
>> appreciable and measurable time until those things have happened. That
>> means downtime in the sense of "Where's My Database Gone?!?"

> 
> 
> Which we measure in fractions of a second. Looks like a network glitch
> not a "Where's my database gone"?

That's the usual facile nonsense based on a sweeping generalisation we've come to expect from you of late. An instance recovery could well be done in fractions of a second, but don't imagine for one minute that there won't be trade-offs (in performance, mostly) elsewhere in the system when you arrange for it to be so. Or that to eliminate that performance hit, one may well have to invest in some pretty serious kit. Most people will therefore strike a balance between acceptable performance and recovery time, and the result for most people most of the time will therefore be multi-second time failover.

If memory serves, even Larry's famous TAF demonstration paused noticeably before the select resumed.

But: whether its a few seconds or sub-seconds: it ain't nothin'.

> BTW: TAF works with DataGuard too ... not just with RAC.

I realise that. Where in my post did I state or imply otherwise?

Since TAF was invented in 8i, and RAC didn't make an appearance qua RAC until 9i, it's rather obvious that the two are not joined at the hip.

HJR Received on Tue Nov 23 2004 - 00:13:20 CST