Benefits of Offline defrag

Hi Experts

We are running Exchange 2003 SP2.

We have about 1000 users per server.

We haven't run any offline defrags since I joined the company about a year ago, and I was wondering what people's opinions were on this.

Is there anyway to tell if an Offline defrag would benefit us? And what benefits do offline defrags actually give?

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yes the event viewer creates an entry that tells you how much free space is in the database.
conventional wizdom is that it only helps if you delete a large amount of data in bulk.

if you delete a mailbox now and then it will not matter for the following reason.

you delete a user who has 50 meg of mail.
over the course of the next few weeks users save more mail.
the online defrag recovers the space inside the db.  the new mail being saved takes up this space, and the offline defrag is a very temporary gain.

You really only need to run an offline defrag if you make a major change to the database, such as moving a lot of users out of the db, or a lot of users are deleted or you run a hard repair against the db, but for normal operation it is better to leave the whitespace for the db to use when needed.

The only time an offline defrag will be of benefit is when you have archived or deleted a large amount of data from a mail store, or if you have moved a lot of data off to other mail stores. As you know, Exchange marks the data it frees up from such operations as 'whitespace', so the capacity remains allocated to the database files on disk, but is not actually being used to store any mail. It is just redundant space waiting to be used as and when it is needed by some incoming email.

If you have performed any of these operations already mentioned, you would benefit from an offline defrag since it would reduce the physical size of the mail store, enabling you to use that disk space for other things. It also gives you the chance to let Exchange re-organise the database since it can become quite fragmented over time.

Remember that running an Offline Defrag could result in downtime of several hours on larger stores, since your databases must be dismounted during the time when it occurs. Preferably, I would suggest you dismounted all databases on the disk array where the database being offline defrag'ed resides, since the constant I/O from Exchange coupled with the offline defrag's I/O could cause potential issues.


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