Circular (logging) Logic?

Exchange logs have been a thorn in the side of server administrators forever.  Log files contain information about every transaction on the server, and can fill up a hard drive quite quickly. This can lead to the entire Exchange system grinding to a screaming halt, with no recourse but to copy the logs to another drive, reassign that drive letter with the original log disk drive letter and restart. 

Through the years of Exchange development from 5.5 through the current 2007 systems, log files have remained a critical - if somewhat annoying - part of the overall Exchange solution set.  While the size of the individual log file has changed from 5mb to 1mb in 2007, the actual amount of data stored in the logs has stayed pretty constant (so in 2007 you have more log file taking up the same space).

Recently, indications through third-party sources have intimated that Circular Logging (CL) may be making a comeback to help deal with these log files, but at a price.  CL is the process of having Exchange begin deleting old logs as it creates new ones - based on parameters set by the administrator.  For example, Exchange might keep 100 logs, then each time it creates a new log - starting at log 101 - it deletes the oldest log on the server.  This takes care of the disk space issues, but does have an impact.

Without a full set of logs corresponding to the non-committed data, Exchange will be unable to recover from unexpected faults.  While the need to leverage these logs is rare, the lack of the correct logs in an emergency can be devastating.  When a database has been unexpectedly dismounted (such as due to an unexpected reboot), Exchange will compare the last committed transaction in the database to the transactions stored in the log files.  By doing so, Exchange can play forward any missing/incomplete transactions by using the log data to reconstruct them. With CL turned on, there is a chance that one or more of the logs required to replay the transactions will have been deleted to make room for new logs, causing the process to fail, and causing data loss.

Now, there are a few ways to help manage this problem.  Backup solutions like Data Protection Manager from Microsoft, and 3rd Party tools from various vendors, can remove transaction logs that have been properly committed to the backup media (disk, tape, etc).  This way if you have to perform a recovery operation, you can restore all required logs from the backup media and be safe. There are also many 3rd Party Exchange management tools that can store copies of the log files to allow for advanced maintenance and recovery.  However, this should not be taken as a free-pass to turn on CL.

As a sudden rush of data into the Exchange system could create a large number of logs, it would be very possible that CL would kick in and delete logs that have not yet been moved to the backup media.  That would leave you in an unrecoverable position if those logs were to be required for restoration of services. 

Circular Logging is a feature turned off by default in versions of Exchange from 2000 to present.  Given that there are tools to help mitigate log impact on disk space, and that there are very real dangers in using CL overall, it is a feature that should probably be best left turned off going forward.