The overall performance of Exchange especially the perceived performance at the user level can be influenced quite dramatically by the speed of the disk the transaction log resides on, and here’s why: A user finished typing up an email and clicks send. The item is passed to Exchange via Outlook/OWA/POP/IMAP and is stored in memory and then written to the log. A routing decision is made and the mail is stored locally or sent out and written to the log, and a new item is written to reflect in the sent items and written to the log. All of these decisions are written to the transaction log BEFORE they are written to the Exchange database. Every transaction reflects in the log, before it is committed to the database.

Performance counters can help you understand how busy it gets here. Exchange gives you a number of Database performance objects depending on the number of databases you have. Remember that from Exchange 2000 onwards a new transaction log is started per Storage Group, not per database.  

Databases ==>Instances  “Log Threads Waiting” is a good starting point as a performance counter. If this counter is high, you have an indicator to the log being a bottleneck.  What is high and what isn’t? I would suggest taking this log counter and the other three log specific counters and monitor them for a while to create a baseline of what is normal in your environment. Do that for each of you Exchange servers as well, and you will quickly gain an understanding of how your Exchange logs are performing over different times of day and different load conditions.

Not all Exchange performance issues are easy to solve. If this is the first time you have heard of transaction logs and their implication, this article may be for you, but please don’t treat it as a cure all. Exchange relies on a number of interconnected systems, storage optimization is one classic bottleneck, but unfortunately there may be others. If you have checked out the performance counters and checked out your log configuration, you may have found that your transaction logs were sharing disk with your databases or that they were written to a slow disk. Moving your logs to a faster disk will certainly shift one classic bottleneck and hopefully get mail moving faster..