Office 365 migration projects with significant mailbox data will likely encounter data migration speeds of less than 500 MB/hr.   This is massively less than on-premise migration speeds of around 10GB/hr, and can affect the migration strategy, and length of any coexistence period.

Office 365 Migration Speed

Any mailbox migration speeds will be limited by your connection speed to the internet.  However, more importantly, Microsoft impose their own limits with regards to concurrent powershell sessions – this limits you to a small number of sessions – usually 9.    If you are deploying multiple migration servers, then you can only scale up to a certain level, before you hit this limit.

Some migration software offer clever tricks to use multiple O365 admin accounts to trick the O365 service, but the concurrent powershell session number remains a hard limit.  The actual parameter is:  PowerShellMaxTenantConcurrency.  See this good Quest Support article on the topic.

Increase Office 365 Migration Speed

All is not lost though, as you can log a support ticket with Microsoft and ask for the PowerShellMaxTenantConcurrency value to be increased to a much larger value – eg) 50.   As long as you are migrating over a 1,000 mailboxes to their service they should be happy to make this change for you.  But you have to negotiate the Microsoft Support service to get to the right team – often via an escalation by your Microsoft Account representative.

I understand why Microsoft set this limit, as it is a shared service, and they do not want one tenant to impact the service for other tenants.  But it is useful to know that the PowerShellMaxTenantConcurrency limit can be increased temporarily for your migration.  This allows you to deploy multiple migration servers to spread the mailbox migrations over.

For example, 4 Quest Migration Servers, each with 4 threads, will give you 16 concurrent migration sessions at 500 MB/hr, which equates to 8GB/hr, which is close to the sort of speed of an on-premise migration.     This advice also applies to any GroupWise to Office 365 migration project.

Office 365 Migration Consulting

If you need consulting for your email migration project, then please get in touch with us here at office365migrate.com .  We have completed multiple Office 365 migration projects successfully, and have gained valuable experience in avoiding the pitfalls that can be encountered.