Thursday, October 22, 2009

Is Brick-level Necessary for Exchange?

By:Rik Hoffelder

The need for brick-level backup of Exchange mailboxes is often a topic of discussion with customers. This is of particular concern to those with large message stores, short backup windows, and demanding users. In my opinion Exchange provides enough out of the box functionality that brick-level backups are really unnecessary in all but the most crucial of mailboxes. You know the mailbox that belongs to the person who signs your paycheck! In all seriousness I feel brick-level backup is mostly a waste of tape and/or disk.


Let's first take into consideration the deleted items retention configuration on mailbox stores; a feature that has been with Exchange through many generations. This feature marks a deleted message as hidden to the database thus places it in a "dumpster." By deleted message I mean a message that has been purged from the mailbox, not left in the Deleted Items folder. The length of time the message remains in the dumpster is determined by the deleted items retention configuration on the message store. A value greater than zero will keep the message hidden for that many days, after which time the database page(s) are marked for reuse in effect purging the message from the database. However during the time the message is in the dumpster it is recoverable.
Many of you may already be familiar with the Recover Deleted Items function under Outlook's Tools menu. By default this menu option is grayed out unless you have selected the Deleted Items folder. Microsoft has a little known knowledge base article that documents a registry setting allowing the Deleted Items Recovery option to be available to all folders, not just Deleted Items. This setting not only allows recovery from all folders within the mailbox but all public folders as well. The public folder store also has a deleted items retention configuration. (Additional information on this registry setting can be found in How can I recover items that I have "hard deleted" in Outlook? )

Deleted Item Recovery using this method not only has the benefit of being able to recover any message from any folder; you can also recover messages that have been hard deleted when someone uses SHIFT+DELETE. As a result you can recover messages that have been delivered after the last backup, eliminating the data loss a SHIFT+DELETE would present using brick-level restore. Additionally all recovery is done online, you never need to mount a tape so it literally takes seconds to locate and restore messages. But wait there's more; you can configure this for all end users so they can recover their own messages never having to wait hours, even days for recovery. Do you think that can reduce admin overhead and help desk calls for those little uh-ohs?

Yes there are a few gotchas, what did you expect for out-of-the-box? For instance, each mailbox or public folder store is limited to 4 GBs of dumpster space within the database. For large message stores this will reduce the amount of time you can keep deleted items before reaching the limit. If you exceed the 4 GB limit messages are purged on a First In - First Out (FIFO) basis. It has been my experience that a 100GB mailbox store under normal utilization can support up to 14 days worth deleted items comfortably. This obviously varies by environment; fortunately Microsoft took some the guess work out. Exchange 5.5 Server or later include performance counters under the Information Store object that show current dumpster utilization.

So how do I handle a case where the message is past the recovery period? This is where the Recovery Storage Group comes into use. First introduced in Exchange Server 2003 the RSG allows you to perform an online restore of mailbox databases to a separate storage group and mailbox store instance, then merge the data with the on-line mailbox. Again this is not a perfect solution as the active mailbox must reside on the same server that the backup was taken from. In other words if I moved the mailbox to a different store since the backup I wish to restore was taken, I need to move the mailbox back in order to merge the data. This is very well detailed in Microsoft's knowledge base article How to use Recovery Storage Groups in Exchange Server 2003. The other issue is that the original user account must exist in Active Directory, if this has been deleted you cannot use a RSG. However you can follow the steps I outline in my blog posting How to Build an Exchange 2003 Disaster Recovery Server.


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