Randy
Circular logging is either enabled or disabled. There isn’t an in between. They work like this.
When an event happens, rather than exchange having to find the correct place in the database file to update, it instead writes the event to the end of the transaction log. It does this because it’s much quicker to write an event to the end of a transaction log. When multiple events are happening each second they would soon start to back up if it was written to the correct place in the database file each time.
What then happens is that when there is a lull in the activity of the server, the system attendant will come along and will commit the events in the transaction logs to the correct place in the database files.
With circular logging disabled, the log files will continue to build up until something deletes\flushes them. Exchange aware backup programs (backup exec, arcserve, built in windows backup) will flush committed logs when they back the database files up. Alternatively you could delete them yourself as long as the exchange services are stopped.
The idea behind disabling the circular logging is that in the event of a database corruption, you would be able to restore the database from your last full backup, then when the exchange services start up they would replay the logs into the database – meaning you get to restore right up to the point of corruption, without even losing an email.
The idea behind enabling circular logging is for servers without too much disk space to play with or where backups are not being done\are not an issue. With cl enabled, exchange will work with only 5 log files. When they are full exchange will then go back to the first log file and overwrite it. When that is full it will overwrite number 2 and so on, always circulating the 5 files.
When exchange is shut down cleanly, any remaining transactions to be written to the database are committed. So when the services shutdown cleanly, all the events in transaction logs are in the database files.
My advice would be if you are short of disk space and have a problem with the backups
• Enable circular logging
• Stop the exchange services daily and copy the contents of <program files>exchsvr>MDBDATA to another machine on your network (another disk if part of a raid array would be ok. Ideally on a machine in a different building) – size will be an issue when copying. Remember you can schedule this overnight
• Fix your backup problems ASAP
• Disable circular logging
• Use an exchange aware backup program that will flush the logs
Hope this helps
David
