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Past Images in RAC [message #386340] Fri, 13 February 2009 19:14 Go to next message
tjay
Messages: 37
Registered: September 2008
Member
Hi
I`m liittle bit confused about the Past Images in Rac environment.

"When the same dirty block is requested by some other instance, an image of the block is created in owning instance and then the block is shifted to requesting instance. This image copy of the block is called Past Image (PI)."

What is the point of creating Past Image,I mean there is already undo blocks for consistent read...

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Re: Past Images in RAC [message #386365 is a reply to message #386340] Sat, 14 February 2009 00:30 Go to previous messageGo to next message
Michel Cadot
Messages: 68731
Registered: March 2007
Location: Saint-Maur, France, https...
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Where did you get this from? Post a reference or better a link we can read the whole section.

Cache Coherency and the Global Cache Service:
Quote:
A consistent read (CR) version of a block represents a consistent snapshot of the data at a point in time. The LMSn processes produce consistent read versions by applying rollback segment information to past images. Both current and consistent read blocks are managed by the GCS.
If one instance holds a block that it has modified and another instance requests it, then the holding instance maintains a past image (PI) of the block. In the event of failure, Oracle can reconstruct current and consistent read versions of the block by reading PIs.

Does this answer your question?

Regards
Michel

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Re: Past Images in RAC [message #386824 is a reply to message #386365] Tue, 17 February 2009 02:22 Go to previous messageGo to next message
tjay
Messages: 37
Registered: September 2008
Member
Well,
what is the diffrence between using undo blocks and Past Images?

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Re: Past Images in RAC [message #387850 is a reply to message #386824] Fri, 20 February 2009 19:25 Go to previous message
tjay
Messages: 37
Registered: September 2008
Member
the Failover parameter in the client`s tnsnames.ora file determines which listener to go.( if the first instance fails, it will go other instance)
and
Failover parameter in serverside`s tnsnames.ora file makes the application to automatically reconnect to a database, if the instance fails.

Is that correct?

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