Exchange Database Size

Look for event id 1211 an application log, this will tell you how much free space is available in the edb file. The edb file never gets smaller untill you make an offine defrag for it.

the size difference could be due to deleted items in the dumpster.

Single instance storage can also make the mailbox size not match with the database size.  One message sent to multiple users is stored only one time in the database.  Each mailbox just has a pointer to that message, and the message is not deleted from the database until it has been deleted from all of the mailboxes.
check http://support.microsoft.com/default.aspx?scid=kb;en-us;175481 to know more about single instance storage

Haven't had this question for a while.

The reason there is the discrepancy is because ESM does not show you the true size of the mailbox. It only shows you part of the size.

"When you view the space that a mailbox uses in Exchange System Manager, the amount only includes the space that is used by the Priv.edb file. The amount does not include the space that the Priv.stm file uses. "

Source: http://support.microsoft.com/kb/828070/

Therefore it is normal for the sum of the mailboxes not to equal the size of the database.

An offline defrag was a waste of time.

So to answer your question, you should be doing nothing, and have no concerns.

-M

Thanks for all the answers but I'm still not clear:

ehabsalem:  there are no error 1211 in my application logs

rakeshmiglani: we have deleted item retention set to 2 days
 - I"m not exactly sure how single instance restore would factor in here, can you explain further?

Mestha:
- I've seen that before and maybe I'm misunderstanding, but this would mean that my situation is unusual.  When I look at the mailbox sizes in ESM they add up to about 8GB, and according to the quote above, this should reflect the size of the Priv.edb file.  However, my Priv.edb is 15GB on its own. The Priv.stm file is an additional 2.5GB.
   Am I misunderstanding something?

Thanks for getting back to me Mestha.

So I checked the events you mentioned and after a sampling I see that its reporting anywhere from 10-50mb of free space.

I went back all the way to early 2008 and those seem to be pretty consistent all the way back.

Am I correct in understanding that the values in this event would represent how much space I would expect to gain with an offline defrag?  
This was kind of what got us down this road, after an offline defrag of the DB, we didn't really gain any space at all, so I thought there might be  a problem with exchange.

So as far as your experience goes, there is nothing to worry about if the priv file is double what is shown in ESM.  And not gaining any space from an offline defrag is because we didn't really have any white space as indicated by the 1221 events?

We have already changed the max allowable exchange size to prevent it from going offline at 16GB.

If you are only seeing 10 - 50mb in event ID 1221 then an offline defrag is a waste of time. That is the maximum space you would gain.
So a total store size that is double what you see when you add up the list of mailboxes, while a little high, wouldn't be unusual.

-M

Thanks for your help!