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Ghost Cleanup


Ghost Cleanup

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Stewart "Arturius" Campbell
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Good question, thanks Wayne
Had to dust off the old plates for this one...

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okbangas (1/15/2013)
But, there has not been any scan between the marking of ghost records and Ghost Cleanup Task, hence it has nothing to do.


I think that's a scan that's carried out by the ghost cleanup task itself, not a random scan that's done as part of a SELECT statement. May be wrong on that, but that's how I read it!
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Good question Wayne. Thanks!



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paul.knibbs (1/15/2013)
okbangas (1/15/2013)
But, there has not been any scan between the marking of ghost records and Ghost Cleanup Task, hence it has nothing to do.


I think that's a scan that's carried out by the ghost cleanup task itself, not a random scan that's done as part of a SELECT statement. May be wrong on that, but that's how I read it!


+1

Great question Wayne, thanks.

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okbangas (1/15/2013)
As far as I understand it, there is something missing from the explanation. If we assume that only what's written in the scenario has happened, the scenario is as follows:

* Table is created
* Records are inserted
* Records are deleted -> Marked as ghost
* Ghost Cleanup Task is running (every 5 seconds).

But, there has not been any scan between the marking of ghost records and Ghost Cleanup Task, hence it has nothing to do.

I agree with you that in this scenario the scan was not done, so nothing was done, but that is not relevant for the explanation: the page will never be completely deleted by the Ghost Cleanup Task, even if it would have been scanned before. As far as I can see that was the important lesson to be learned here.
WayneS
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okbangas (1/15/2013)
As far as I understand it, there is something missing from the explanation. If we assume that only what's written in the scenario has happened, the scenario is as follows:

* Table is created
* Records are inserted
* Records are deleted -> Marked as ghost
* Ghost Cleanup Task is running (every 5 seconds).

But, there has not been any scan between the marking of ghost records and Ghost Cleanup Task, hence it has nothing to do.

The ghost cleanup task doesn't just start up when it's told to – it starts up in the background every 5 seconds and looks for ghost records to cleanup. Remember that it won't be told to go cleanup a specific page by a delete operation – it's a subsequent scan that does it, if a scan happens.
http://www.sqlskills.com/blogs/paul/inside-the-storage-engine-ghost-cleanup-in-depth/


You're absolutely correct. However, the question states:
If you delete the first 12 records, how many data pages will the table have after the Ghost Cleanup process has been run against this table?

It's not going to run against this table until something scans this table, but once it does, it will leave that page with only one record in it.

Wayne
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Hi,
Interesting question!

The expected one data page will be listed after rebuilding the index on the table.

alter index PK_Test on dbo.Test rebuild
dbcc ind ('TestDB','Test',1)

Thanks,
IgorMi

Igor Micev,
My blog: www.igormicev.com
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IgorMi (1/15/2013)
Hi,
Interesting question!

The expected one data page will be listed after rebuilding the index on the table.

alter index PK_Test on dbo.Test rebuild
dbcc ind ('TestDB','Test',1)

Thanks,
IgorMi


Absolutely correct (and this was pointed out in the demonstration example on my blog at http://blog.waynesheffield.com/wayne/archive/2012/12/ghost-cleanup-process/)

Wayne
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Sean Lange
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Great question Wayne. I knew the page would not be deallocated but I didn't know all of the details. The article was very interesting and informative read.

I wonder if Celko will start bashing Paul Randal for calling them records instead of rows?? :-D

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Sean Lange (1/15/2013)
Great question Wayne. I knew the page would not be deallocated but I didn't know all of the details. The article was very interesting and informative read.

I wonder if Celko will start bashing Paul Randal for calling them records instead of rows?? :-D


Hehe



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