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I am going to start a migration from and Exchange 2003 cluster to a new Exchange 2010 SP2 with 14 mailbox databases of a supposed maximum size of about 400 GB each. Each database will be divided into 2 LUNS, data and transaction logs. LUN size takes into account to leave about 20% of free space to avoid monitoring alerts. As mailboxes will be distributed randomly during migration, how can I be sure that 400 GB database size limit will not be exceeded? And after migration, as mailboxes will grow time by time, how to make Exchange respect database size limits?

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Why are the mailboxes being distributed randomly? Why the 400GB limit (that's not MS's recommendation)? –  Chris S Aug 2 '12 at 14:40
    
Mailboxes are distributed randomly by default because of Automatic Mailbox Provisioning feature. MS recommendation about database size is about 200 GB if DAG is not used, otherwiese 2 TB. We use DAG and we think that 400 GB would be a good size to manage in case of failure. –  Danilo Brambilla Aug 2 '12 at 14:55
    
200GB was the recommended max for 2007; 2TB is for 2010. I'm not sure what you mean by "manage in case of failure" as the DAG provides high availability. Usually when doing a migration of this size the mailboxes are done in groups and assigned to specific database (or group of DBs), though this is certainly not necessary, it's just common practice. –  Chris S Aug 2 '12 at 15:24
    
I have a psuedo-random method for assigning mailboxes. Large users mailboxes (say over 10GB) end up in their own database. Everyone else gets put in the small database at time of mailbox creation. This has loads of downfalls, but with only 200 users, it is manageable. I try to keep my databases under 50GB though as moving them around if needed gets to be quite a chore when they get larger than that. And rebuild/defrag times are larger as well. Simple version - keep the databses small and make more of them. –  MikeAWood Aug 2 '12 at 23:36
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2 Answers

up vote 2 down vote accepted

The only way to limit the size of a database is to add/change a registry key. However, this has the highly undesirable effect of dismounting the database when the size limit is reached. Manual distribution is the common and most elegant solution to the problem you pose.

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I was supposed to have some mailbox automatic distribution method in a so advanced mail system, something like: set a maximum database size and then leave exchange to create new mailboxes on databases with more space available or automatically move mailboxes to free databases when database size is exceeded. But it seems that manual intervention is required :-( As I can see someone uses custom powershell script to balance mailbox between databases as this functionality is missing out-of-the-box –  Danilo Brambilla Aug 3 '12 at 8:13
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If you have a hard limit (SAN limits, restore limits) of 400GB then I would just have more DB's and when DB's grow to over 300GB then create another DB and move some mailboxes. Setting a hard limit only ads risk IMO.

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There's always a hard limit. Without a registry key specifying the limit, it's 1TB. –  Chris S Aug 3 '12 at 12:41
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