Some of the folks attending Exchange Connections raised the question of why Exchange 2013 mailboxes are so much larger than their legacy equivalents. The situation is addressed in the Exchange 2013 release notes, which say:
Mailbox size increase when migrating from previous Exchange versions When you move a mailbox from a previous version of Exchange to Exchange 2013, the mailbox size reported may increase 30 percent to 40 percent. Disk space used by the mailbox database has not increased, only the attribution of space used by each mailbox has increased. The increase in mailbox size is due to the inclusion of all item properties into quota calculations, providing a more accurate computation of space consumed by items within their mailbox. This increase may cause some users to exceed their mailbox size quotas when their mailbox is moved to Exchange 2013.
However, the text in the release notes is pretty nebulous on why such an increase might have happened in Exchange 2013. I’ve covered this issue before, but here’s a more considered thought on what might have happened.
Up to Exchange 2010, the schema used for the Information Store favored large tables where data for every mailbox in a database were held. This worked well for the small databases found on early versions of Exchange but less well as time went by and mailboxes and databases swelled. The developers therefore revamped the schema for Exchange 2010 to use an approach focused on smaller tables that are mailbox-specific.
Every database has a certain amount of overhead that is used for internal purposes. Exchange is no different. The schema change in Exchange 2010 enabled a true identification of all of the information associated with a mailbox but nothing was done in that release to change the way that mailbox storage was reported.
Time went by and Exchange 2013 development progressed. A decision was made to use the information now available to report mailbox sizes more accurately by including everything associated with a mailbox. Unfortunately this comes with the unpleasant side-effect of an apparent increase in mailbox size that can impact users by causing them to exceed their quota.
Think of a user who has a 900MB mailbox on Exchange 2010 and a quota of 1GB. Their mailbox is now moved to Exchange 2013, but the move fails because Exchange 2013 considers that the real size of the mailbox is 900MB + 30%, or 1,170MB, comfortably over the 1GB quota. On the other hand, the Mailbox Replication Service (MRS) will be able to move a 750MB mailbox that has a 1GB quota because the newly accurate reporting size on Exchange 2013 is 750MB + 30% or 975MB. The user might not be delighted to find that their spare quota has suddenly diminished, but at least the mailbox move succeeds.
MRS cannot dynamically increase quota size for a mailbox move. The quota is what it is and can only be adjusted by an administrator. This means that anyone planning a migration to Exchange 2013 needs to review and adjust mailbox quotas to ensure that mailbox moves will succeed. It’s as simple as that.
A failed mailbox move isn't a disaster. MRS checkpoints as it processes a mailbox and is able to resume a move from the last checkpoint so if you blow a quota, you simply increase the quota and resume the move request.
Let me try this analogy to explain what’s happening. In the U.S., consumers are accustomed to seeing a price that does not include sales tax. Thus, when you pay at the register, the price on the receipt is always different to the price shown on the article. This is the situation with legacy versions of Exchange – mailbox sizes are reported without the sales tax.
On the other hand, European consumers must (by law) see the full price on articles for sale. That price includes all applicable taxes. This is what is now happening with Exchange 2013 as the Information Store is now charging mailboxes the full sales tax up front. Or rather, the full database tax is applied to mailboxes to report the real cost of storing mailboxes in the database.
You might consider this to be an unreasonable change but I don’t think so. Sure, the change requires some action on the part of administrators as they prepare to move users to Exchange 2013, but the simple act of running a PowerShell script to increase every quota to some large arbitrary figure is surely not outside anyone’s capability. It’s quite safe to do this because users won’t fill those quotas. And later on, when the migration is over, you can either decide to leave the large quotas alone or reduce them to some other figure. The choice is yours.
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