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Exchange 2010 Transaction Logs

Posted on 2011-02-18
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Last Modified: 2012-06-27
http://msdn.microsoft.com/en-us/library/aa579166(v=exchg.140).aspx
I am trying to understand Exchange 2010 transaction logs as explained On the link above.
I can't understand some terminology such as:
write-ahead transaction logs
checkpoint files
the in-memory database  Versus the on-disk database
write-ahead logging.
page-based writes and checkpoints.

I also want to confirm whether it s true that neither the incremental nor the differential back up of exchange clears up the log files and checkpoint files except for the Full back.

Thanks
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Question by:jskfan
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mass2612 earned 500 total points
ID: 34924664
Hi,

Ok the transaction log can be complicated however some consider it more important than the database file so its good to have a good understanding of it. Write ahead transaction logs and write ahead logging are pretty much the same thing.

One of the first things to remember with databases including Exchange is that data is not modified on the disk. The data is written to memory and modified and then the modified data needs to be committed (written back to the disk). Many databases including Exchange write the modified data to a transaction log first for resiliency and recoverability. Therefore when Exchange updates data and the data needs to then be written back to the database the entry is first written to the transaction log before then being written finally to the database. When you look inside a transaction log you see a list of entries such as begin transaction, insert, update, delete, end transaction.

The checkpoint file keeps track of the relation between the log files and the database.

In memory database or the DB cache is basically pages from the database that are currently being stored in memory. If the data in the in memory page is different to the data in the database on the disk then the page is considered dirty. This just means that the data has been changed and is different than what's on the disk. If the page in memory has not been changed then the page is clean. When an application needs to modify data in a database it must read the data into memory and then change the data in memory. Finally that changed data must be written to disk. The on disk database is the data in the EDB files on the disk.

A page is the smallest unit of data in Exchange and in Exchange 2010 its 32kb.

A full and an incremental backup purges the transaction logs however a differential backup does not purge the transaction logs.

Incremental backups
http://msdn.microsoft.com/en-us/library/dd877022%28v=exchg.140%29.aspx

Differential backups
http://msdn.microsoft.com/en-us/library/aa579399%28v=exchg.140%29.aspx


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by:jskfan
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Thanks for your explanation
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