Circular logging allows Microsoft Exchange to overwrite transaction log files after the data that the log files contain has been committed to the database. Circular logging is not recommended in production environments.
By enabling circular logging, you reduce drive storage space requirements. However, without a complete set of transaction log files, you cannot recover any data more recent than the last full backup.
When circular logging is disabled, every single log file goes into the transactional log database, and no limit exists as to how large that database can get. When circular logging is enabled, however, the transactional log can only grow to one megabyte (1 MB) in size. After that limit has been reached, the first log file is overwritten automatically to keep the transactional log database from growing any larger. The term “circular” arises from the fact that the set of log files starts to “rotate” once the disk space limit is reached, something like a LIFO (last-in, first-out) queue.
Circular logging is a feature of the Joint Engine Technology (JET) database used by all versions of Exchange Server that can be enabled or disabled by an administrator. When enabled, circular logging allows Exchange to overwrite transaction log files after the data contained in the log files is committed to the database. By enabling it, you can reduce storage space requirements. However, without a complete set of transaction log files, you won’t be able to recover any data more recent than the last full backup. In Exchange Server 5.5 and earlier, circular logging was enabled by default, and starting with Exchange 2000 Server and continuing to Exchange 2010, it is disabled by default.
That is a very high-level overview of circular logging. Take note that you should not enable circular logging on an Exchange database if you are doing backups as they will be inconsistent. This includes incremental backups as well. If you do however decide to enable it then it is recommended to take a full back immediately after the databases have been mounted and you have disabled it again.
Here is an example of enabling circular logging on an Exchange database
- Set-MailboxDatabase -Identity “database1” -CircularloggingEnabled:$true
The above is done from the Exchange Management Shell, you can also use the Exchange Management Console to enable it:
- To do this, once you have opened up the EMC (Exchange Management Console) then you would need to expand Organization Configuration and click on Mailbox. On the right-hand side click on the database management tab and then locate the database you want to turn this on for.
- Next right click on the database and then click on the properties option. Once the new page opens, click on the Maintenance tab and at the bottom you will see the option to enable Circular logging. Click the check box and then click on Apply and Ok.
- You will receive a warning to dismount the exchange database and mount them again for the changes to take effect. Remember, doing this will cause downtime for users as they won’t be able to connect to Exchange.
- Right click on the database again and click on “dismount database”, you will be asked if you want to proceed, click yes.
- Now right click the exchange database again and click on “mount database”.
Take a look at the drive where the logs are housed, you will see it will truncate the log files
Once it has done that, I strongly recommend that you dismount the exchange databases where this has been performed and turn off that circular logging option and mount them again and then perform a full backup of your Exchange database.
Let’s now look at a scenario where the Exchange Backups have not been taking place and the volume where the database is stored is filling up and once you hit that specific backpressure threshold then you risk the fact that the Exchange Databases will not mount.
If you are in this situation, do not just jump and enable circular logging so you can get rid of the log files to reclaim the space they have used.
Firstly, make a copy of the log and database directory, if you needed to restore the database from a backup and replay the logs then you would need them.
If you don’t have them you can end up with data loss if you have to try recover your Exchange database.
But there is more. If you are running a large environment and want to make use of circular logging in the long run, it is advised to have three copies of the data if you want to go backup-less with Exchange Native Data protection.
The next thing is, if you are running a Database availability group and you are on an Exchange version higher than SP1 for Exchange 2010, then you no longer need to dismount and mount the Exchange stores for this to take effect.
You do not need to restart the Information store service. It takes a few seconds/minutes to perform this.
The last option to look at is, if you have had a problem and enabled circular logging but the Exchange database wont mount, then would need to look at a third party Exchange Recovery tool like Stellar Repair for Exchange to repair the corrupt EDB file without the need for the log files.
This includes large .EDB files as well as multiple files simultaneously. You can even open up the corrupt .EDB file with the tool and just extract the data to another database or office 365 or even just to a .PST file to view later.
With that being said, never fear if you have a failure, Stellar Data Recovery will always come to your aid and assist with recovery.
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