Clearing the Transaction Log

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Clearing the Transaction Log

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Steve Cullen

Posted Tuesday, June 01, 2010 12:38 PM

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Great question. This is what the QOD is all about. A great explanation of the answer as well.

Converting oxygen into carbon dioxide, since 1955.

Post #930927

UMG Developer

Posted Tuesday, June 01, 2010 8:23 PM

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Thanks for the great question! I am learning so much more from these questions and their explanations.

Post #931062

Daniel Bowlin

Posted Wednesday, June 02, 2010 11:18 AM

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Got it wrong. Angry

Thanks for a good explanation. Smile

Post #931531

WayneS

Posted Wednesday, June 02, 2010 11:47 AM

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I got lucky... I got it right. Partly from discounting some, and the rest by what didn't make sense.
I also agree... excellent explaination of the answer.

Wayne
Microsoft Certified Master: SQL Server 2008

If you can't explain to another person how the code that you're copying from the internet works, then DON'T USE IT on a production system! After all, you will be the one supporting it!

Links: For better assistance in answering your questions, How to ask a question, Performance Problems, Common date/time routines,
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Post #931553

Oleg Netchaev

Posted Wednesday, June 02, 2010 2:11 PM

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This is a great question, thank you Paul. It would have been very difficult to answer it correctly, but fortunately for me, incorrect options were clearly incorrect, and therefore it was not too difficult to not answer the question incorrectly:

B. VLFs are physically deleted? No, VLF is, well, VLF, a fragment
C. VLFs are overwritten with zeroes? No, in Windows nothing is overwritten with zeroes as there is no need to do that. Even formatting the drive does not actually overwrite data with zeroes.

With above out of the way, All of the above and Answers A and C have to go too.

This leaves only A and D to seriously consider, and while this is true that the only way to shrink the file is to first have it inactive VLFs (Status = 0 when running dbcc loginfo()) bunched in its tail, the log file shrinks when dbcc shrinkfile is called.

Thus, answer A is the only answer which is not incorrect.

Oleg

Post #931669

Paul White

Posted Wednesday, June 02, 2010 3:05 PM

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Oleg Netchaev (6/2/2010)

C. VLFs are overwritten with zeroes? No, in Windows nothing is overwritten with zeroes as there is no need to do that.

Hey Oleg,

Log files are initialised with zeroes (there's some complexity around that - see Paul's blog and Jonathan Kehayias' entries:
http://sqlblog.com/blogs/jonathan_kehayias/archive/2010/05/13/does-the-tempdb-log-file-get-zero-initialized-at-startup.aspx

Windows uses zero-initialised memory pages too (see Demand Zero Pages).

Paul

Paul White
SQL Server MVP
SQLblog.com
@SQL_Kiwi

Post #931704

Oleg Netchaev

Posted Wednesday, June 02, 2010 4:06 PM

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Paul White NZ (6/2/2010)

Oleg Netchaev (6/2/2010)

C. VLFs are overwritten with zeroes? No, in Windows nothing is overwritten with zeroes as there is no need to do that.

Thank you Paul, it is good to know. Usually, Windows does not need to write any zeroes (when you delete files or format drive, just the records from allocation table are deleted and this is good enough), but initializing a log file with zeroes does make a lot of sense.

Since the question was about clearing existing log file, I think that my assumption about not overwriting existing inactive fragments with zeroes is still OK. If not then I guess I got lucky while dismissing options which I perceived as incorrect. In any case, I got my 2 points out of the question posted by Paul Randall, and to get those is never a trivial task.

Oleg

Post #931737

ziangij

Posted Wednesday, June 02, 2010 9:25 PM

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thanks Paul Smile

of all the QOTD, i find your questions the most difficult... Smile

Post #931811

Tush

Posted Thursday, June 03, 2010 3:45 AM

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I "Lost" 2 points Angry

but "gained" a very good knowledge Smile

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Most Judgement Comes From Bad Experience "

Post #931951

jrandolph

Posted Sunday, June 06, 2010 10:05 AM

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Another good question which made reading the relevant tech pages to answer it interesting and well worth the time.

Post #933244

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