How to Speed up mailbox migration to Exchange 2016

Moving mailboxes from 2010 to Exchange 2016 or Exchange 2016 can often go very slowly, even when the network and server resources are fast and abundant! 
The Exchange Mailbox Replication Service (MRS) has extensive resource throttling enabled by default in order to prevent mailbox moves from choking out the rest of the users.  Because of this you may see mailboxes with a status of RelinquishedWlmStall and if you look at the details of the Get-MoveRequestStatistics report you will see mailboxes have a lot of time sitting idle under the TotalStalledDueToWriteThrottle counter.

Microsoft tech support suggests making changes to the “MSExchangeMailboxReplication.exe.config” file located at “C:\Program Files\Microsoft\Exchange Server\V15\Bin”.  The values to look at, along with their default settings are:

MaxActiveMovesPerSourceMDB=”20″
MaxActiveMovesPerTargetMDB=”20″
MaxActiveMovesPerSourceServer=”100″
MaxActiveMovesPerTargetServer=”100″
MaxTotalRequestsPerMRS=”100″

ExportBufferSizeKB=”512″

We typically like to set these values so that about 10 mailboxes can be moved simultaneously.  The ExportBufferSizeKB we’ve used in the past is “10240”.  The Exchange Mailbox Replication Service should be restarted after these changes.

The other suggestion Microsoft has made is to disable content indexing on the target database so that the search index scanner isn’t overwhelmed by all the new messages needing to be indexed.  You’ll want to set it back once the migration is complete.

Set-MailboxDatabase “DB1” -IndexEnabled:$False

In our experience however, these first two suggestions do NOT have tremendous impact on the overall speed.  The following two options have proven to be the most effective for us.

Use the “-priority emergency” parameter on the mailbox moves.  This will give the move the highest priority in the MRS queue.  For example:

New-MoveRequest -Identity “user@domain.com” -TargetDatabase “DB1” -Priority emergency

If the priority flag and the MRS config editing doesn’t make the moves fast enough for you, then disable MRS throttling altogether!  To do this, change the “MRS” REG_DWORD key from 1 to 0 under this registry path:

HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\services\MSExchange ResourceHealth

Then restart the Exchange Mailbox Replication service.  Now your mailboxes will move without any throttling policy.  Once the mailbox migration is complete change the value back to 1 to re-enable MRS resource monitoring.