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consistent-read of a block
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Message Score Author Date
Dear Experts , I do not understand following on...... Jill Salalila Sep 18, 2018, 14:08
Because it is a <b>consistent-read</b> request. ...... Michel Cadot Sep 18, 2018, 14:30
Hi Mike, I do not understand. What do you mean...... Jill Salalila Sep 18, 2018, 14:41
So the question is not "Why LMS needs to rolls b...... Score: 100 PtsScore: 100 PtsScore: 100 PtsScore: 100 PtsScore: 100 Pts Michel Cadot Sep 18, 2018, 14:52
Hi Michal, Anywhere explained this concept ? ...... Jill Salalila Sep 18, 2018, 16:01
This is correct, what don't you understand or wh...... Michel Cadot Sep 18, 2018, 16:12
Let me explain with examples First Data block...... Jill Salalila Sep 18, 2018, 16:43
1) If instance 1 did not modified the block or i...... Score: 100 PtsScore: 100 PtsScore: 100 PtsScore: 100 PtsScore: 100 Pts Michel Cadot Sep 18, 2018, 17:08
Hi Mike, I got partially ! But still something...... Jill Salalila Sep 18, 2018, 18:46
<i>f there are no undo segments, What is the ne...... Michel Cadot Sep 18, 2018, 19:19
Hi Mike, It was typo error. I thought to wri...... Jill Salalila Sep 18, 2018, 20:59
Back to your questions. UPDATE requires <b>cu...... Score: 100 PtsScore: 100 PtsScore: 100 PtsScore: 100 PtsScore: 100 Pts Michel Cadot Sep 19, 2018, 09:54

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Subject: consistent-read of a block
Author: Jill Salalila, Philippines
Date: Sep 18, 2018, 14:08, 301 days ago
Os info: RHEL 6.7
Oracle info: 11.2.0.4
Message: Dear Experts ,

I do not understand following one ..
Please clarify.

LMS Background Process in RAC

If there is a consistent-read request, the LMS process rolls back the block , makes a Consistent-Read image of the block and then ship this block across the HSI (High Speed Interconnect) to the process requesting from a remote node.


Why LMS needs to rolls back the blcok ?

Thanks
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Subject: Re: consistent-read of a block
Author: Michel Cadot, France
Date: Sep 18, 2018, 14:30, 301 days ago
Message:
Because it is a consistent-read request.

Regards
Michel
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Subject: Re: consistent-read of a block
Author: Jill Salalila, Philippines
Date: Sep 18, 2018, 14:41, 301 days ago
Message: Hi Mike,

I do not understand.
What do you mean exactly ?

<p> What is consistent read ? </P>
A block which is updated or not updated.
Please elaborate little more ..

Thanks
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Subject: Re: consistent-read of a block
Author: Michel Cadot, France
Date: Sep 18, 2018, 14:52, 301 days ago
Score:   Score: 100 PtsScore: 100 PtsScore: 100 PtsScore: 100 PtsScore: 100 Pts
Message:
So the question is not "Why LMS needs to rolls back the blcok" but "What is consistent read ?".

A consistent read of a block is a pseudo-read of the block as it was at some point in the time.
In the opposite a current read of a block is an actual read (in memory or disk) of the block as it is now.

Regards
Michel
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Subject: Re: consistent-read of a block
Author: Jill Salalila, Philippines
Date: Sep 18, 2018, 16:01, 300 days ago
Message: Hi Michal,

Anywhere explained this concept ?

I have lot of confusions..

Someone has written ..

For example if a node requests consistent-read of a block, The LMS process makes a Consistent-Read image of the block from another node with the help of undo segments
and then transports the blocks through the network to the node who requested it.

REF_LINK : http://practical-tech.blogspot.com/2012/03/rac-background-processes.html

Thanks

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Subject: Re: consistent-read of a block
Author: Michel Cadot, France
Date: Sep 18, 2018, 16:12, 300 days ago
Message:
This is correct, what don't you understand or what part do you understand in it?
It is hard to explain you without knowing what you know and understand and what you don't.

Regards
Michel
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Subject: Re: consistent-read of a block
Author: Jill Salalila, Philippines
Date: Sep 18, 2018, 16:43, 300 days ago
Message: Let me explain with examples


First Data block goes to 1st Instance, that same block is requested by Instance 2

1) In this case, Why LMS goes to undo segment ?

Consistent read means - Indicates SELECT Query

Thanks mike for your replies
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Subject: Re: consistent-read of a block
Author: Michel Cadot, France
Date: Sep 18, 2018, 17:08, 300 days ago
Score:   Score: 100 PtsScore: 100 PtsScore: 100 PtsScore: 100 PtsScore: 100 Pts
Message:
1) If instance 1 did not modified the block or if instance 2 requires the block after instance 1 has committed, undo segments are not needed.

The sentence "LMS goes to undo segment" is a general one, as it is often the case that the block is no more in the state required by the other instance.

You will agree with the following sentence:
to run with my car I have to I have to fill it.
Won't you?
But this is not true if it is already full. OK?

Consistent read means SELECT or INSERT SELECT or WHERE clause or SERIALIZABLE transaction... There are many cases when consistent reads are used.

Regards
Michel
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Subject: Re: consistent-read of a block
Author: Jill Salalila, Philippines
Date: Sep 18, 2018, 18:46, 300 days ago
Message: Hi Mike,

I got partially !
But still something i cant understand !

If there are no undo segments,
What is the need to look undo segments by LMS ?

Consider this way too ..

If block is modified by Instance 1 and not committed but same block is required by Instance 2 for further transaction !

On node2, user tries to update the same row !
In this case, already there is a lock on that row which was made from Instance 1.

Now please elaborate as per my scenario

1) I have still confusion on 'Consistent read block' ?
2) Why oracle needs 'Consistent read block' ?
3) I there something special ?

Thanks for kind patience ..


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Subject: Re: consistent-read of a block
Author: Michel Cadot, France
Date: Sep 18, 2018, 19:19, 300 days ago
Message:
f there are no undo segments,
What is the need to look undo segments by LMS ?


There is no need and it does not do it.

1) What is the relation with your new scenario?
2) For many reasons, some I listed in my previous posts; still don't see the relation between this question and the new scenario (which is not clear "required" vs "update" 2 things or the same one?)
3) I don't understand this sentence: "have you something special?", I don't know as I never met you; please write it again in another way

Regards
Michel
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Subject: Re: consistent-read of a block
Author: Jill Salalila, Philippines
Date: Sep 18, 2018, 20:59, 300 days ago
Message: Hi Mike,

It was typo error.

I thought to write following below

IS there some thing special ?

but unfortunately written following below ...

"I there something special?" ===> Just Ignore it !

I never met you !
Thanks Mike ..

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Subject: Re: consistent-read of a block
Author: Michel Cadot, France
Date: Sep 19, 2018, 09:54, 300 days ago
Score:   Score: 100 PtsScore: 100 PtsScore: 100 PtsScore: 100 PtsScore: 100 Pts
Message:
Back to your questions.

UPDATE requires current read to modify the block.
There can be only one session/instance holding a block in current mode when there can be multiple sessions/instances holding a block in consistent mode (which can be different versions of the same block).

Regards
Michel
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