Large mailbox migrations are slow because of network latency

Applies to: Exchange OnlineExchange Server 2016 Enterprise EditionExchange Server 2016 Standard Edition More


Hybrid mailbox migration for one or more large mailboxes is slow because of network latency. Analysis of the move request shows that the SourceSideDuration value is greater than the expected range of 60 to 80 percent.


For Microsoft Exchange Server 2013 with Service Pack 1 and later versions, you can increase the export buffer size. To do this, add the ExportBufferSizeOverrideKB parameter to the MRSConfiguration section of the MSExchangeMailboxReplication.exe.config file. This setting reduces the number of migration calls and reduces the time that's spent in network latency. The recommended maximum value for this setting is 7,500, and the minimum value is 512. However, note that if you change the value of the ExportBufferSizeOverrideKB parameter, you should also change the DataImportTimeout value.

The following example shows the ExportBufferSizeOverrideKB setting after it's been added to the configuration file:
OldItemAge = "366.00:00:00"

You must also set the DataImportTimeout value in the MRSProxyConfiguration section of the MSExchangeMailboxReplication.exe.config file to correspond to the minimum upload rate that you can sustain during the migration. For example:
7.5 MB/(60 seconds) = 131,072 bytes/second (for reference, 1,024 KB/s = 1 Megabit/s)
This means that you can never go lower than this rate. You can increase the DataImportTimeout value a bit more than the computed minimum. The following example shows the DataImportTimeout setting in the configuration file. In this example, it's set to 5 minutes. You can safely increase the value to 20 minutes.
<MRSProxyConfiguration MaxMRSConnections="100"
DataImportTimeout="00:05:00" />

Warning This solution disables cross-forest downgrade moves between Exchange 2013 SP1 and later versions to Exchange 2010 servers.

To learn more about how to analyze mailbox migration performance, see Mailbox Migration Performance Analysis.