The mystery of the Mailbox Replication Service

One of our key aims during this upgrade has been to minimise the period of coexistence between Exchange 2007 and Exchange 2010. This is because our testing phase had revealed a number of potential areas in which we could expect user dissatisfaction, at least up until we were able to migrate their mailboxes to the new servers. These potential issues included:

  • OWA Double-authentication
    In this scenario (non IE users) are asked to logon to Exchange 2010 OWA, are redirected to Exchange 2007 to find their mailbox, at which point they’re then asked to authenticate again. This is due to ISA presenting a cookie that only IE is happy to accept.
  • Mac Mail reconfiguration
    It seems that Mac Mail only uses Autodiscover during its initial set-up, so wouldn’t be redirected to the ‘legacy’ namespace during coexistence. Mac Mail would need to be reconfigured with a new URL at the start of coexistence and then back to the original one again once the mailbox had been migrated. This configuration data is held in a PLIST file and although it’s possible to be edited, it’s stored in a binary format that also contains user-specific values (so we couldn’t easily provide a downloadable version to do the reconfiguration for our users)
  • Other EWS clients
    Our UNIX population would potentially suffer the same need to reconfigure (twice) as Mac Mail users
  • Outlook 2003
    We initially expected problems here too (due to the product not being aware of Autodiscover).

Clearly the sensible approach is to minimise the amount of time spent in coexistence and avoid these issues completely. Our Project Board recently confirmed that this was the tack we should be aiming to follow. But other decisions we’d made along the way, such as sticking to the same namespace, while great for avoiding users having to reconfigure, are not so good if you want a ‘big bang’ migration. A lengthy period of coexistence seemed inevitable.

Figures showed that we could consistently achieve throughput figures in the region of 20GB/hr when migrating between the two systems. But with 25TB to move that would still leave us with those coexistence worries for far too long. Something had to give: we either needed a rethink to avoid (or at least mitigate) the coexistence problems or we’d have to find a way to make the migration happen faster.

A bit of digging revealed that we might be able to improve things on the latter. Data transfer was being throttled back by the Mailbox Replication Service (MRS). This runs on the Client Access Servers and effectively takes the effort of moving data off the mailbox servers. That’s good news for two reasons: you get faster mailbox servers and move requests no longer lock out the console during the task, as it used to.

However transferring the moving task to the CASs means that user connections could be affected by back-end mailbox move tasks taking up too much of the system’s resources. To ensure that the CASs are still able to serve user connections during mailbox moves the default MRS settings have therefore been set to pretty conservative values.

This makes sense in a production environment: client responsiveness is usually more important than a mailbox move. But since our servers aren’t going to be handling user requests just yet we don’t need quite so much caution. I therefore did some editing…

The file which controls the Mailbox Replication Service (MRS) is called MSExchangeMailboxReplication.exe.config and (on a default installation) you’ll find it here:

C:\Program Files\Microsoft\Exchange Server\V14\Bin

Right at the end of this file is the section that we’re interested in:

MaxMoveHistoryLength = “2”
MaxActiveMovesPerSourceMDB = “5”
MaxActiveMovesPerTargetMDB = “5”
MaxActiveMovesPerSourceServer = “50”
MaxActiveMovesPerTargetServer = “5”
MaxTotalMovesPerMRS = “100”

The values which had potential to affect users on the current servers were left alone (that’s MaxActiveMovesPerSourceMDB and MaxActiveMovesPerSourceServer). These values can range from zero to 100 and 1,000 respectively.

The MaxActiveMovesPerTargetMDB value was the setting I increased, first to 25, to gauge the effect. This setting is also on a zero to one hundred scale. I then tweaked MaxActiveMovesPerTargetServer to 25. This value goes up to 1,000 so represented a pretty cautious increase, just to see what kind of load it generated. Finally the MaxTotalMovesPerMRS value can be upped too. Depending on where you read it, this value tops out at either 1000 or 1024. Since the config file itself lists its ceiling as 1024, that’s the number I’ve assumed to be right. On that basis though, Microsoft’s technet seems to be quoting the erroneous value.

The ‘Microsoft Exchange Mailbox Replication’ service must be restarted for changes to take effect and of course the edits will need to be done on all of your CASs.

To allow migrations to be tested without impacting upon service I’ve been using the ‘suspendwhenreadytocomplete’ switch on the Powershell command. Essentially this copies over the bulk of the users’ mailboxes and then suspends the job just before it commits the change to Active Directory. If an autosuspended move is cancelle,d instead of being completed, the destination server’s data gets removed on the same cycle as for deleted mailboxes. These move requests won’t get removed automatically – even the successful ones – so if you’re planning on doing subsequent moves you’ll have to get into the habit of housekeeping…

Users are none the wiser about this background copying of their mailbox: their live data has remained exactly where it was. The other great feature of this ‘move and hold’ option is that you get a chance to find which mailboxes have corrupt content – those mailboxes will report as a failed move – again without affecting anyone’s service. If you’re an Outlook user, it’s pretty similar to the process by which Outlook creates an offlline copy of your mailbox (the OST file) at your desktop.

Once all of your data has been copied across, and all the mailboxes are showing as ‘automatically suspended’, completing the move only involves committing the changes to the directory and copying over the deltas (the changed content since that initial copy operation). In theory this could be months later – although your retention period might start deleting the suspended moves after a while. But even if that happened it doesn’t stop the final move from working: the normally-brief delta-copying phase will simply become another full mailbox copy.

This final stage is the only point at which users might notice a service impact (as the final commit briefly locks the user’s mailbox). Outlook users will be told ‘An administrator has made a change which requires you to close and restart Outlook’.  OWA users will be told that their mailbox is being moved; other clients may find their program ‘gets confused’. This will therefore be the one part of the job where we need to keep our users and IT support staff well informed.

In theory this ‘move and hold’ option would allow us to migrate all 50,000 mailboxes in a much shorter coexistence window, but only if we can get the data across at a reasonable speed and if having this number of suspended moves didn’t break something. Nothing on the internet suggested that anyone had tried a ‘move and hold’ operation on the scale I was proposing…

Posted in Uncategorized | 7 Comments

7 Responses to “The mystery of the Mailbox Replication Service”

  1. webhost says:

    webhost I know this is really boring and you are skipping to the next comment, but I just wanted to throw you a big thanks – you cleared up some things for me!…

    I know this is really boring and you are skipping to the next comment, but I just wanted to throw you a big thanks – you cleared up some things for me!…

  2. Hayden says:

    Hi Matthew,

    You’ve possibly provided me a perfect solution for an unfortunate situation. How did you go with the suspended mailbox moves? Did this still allow users access to their mailboxes whilst the data was being replicated?

    I ask, as I’ve got some (insanely) large mailboxes to migrate, as high as 30Gb, with little to no available downtime. I’d ideally like to replicate the entire mail store, and then resume the suspended moves 5 or 10 at a time when the data has successfully replicated.

    When restoring the suspended move request on your largest mailboxes, how long did the process take?



    • Matthew Gaskin says:

      Hi Hayden,

      If you use the suspendwhenreadytocomplete switch there is no user impact at all during the big data copy operation. Users can happily continue using email while the data is being copied across. We used this feature for testing: the move could be cancelled (again without user impact) once we had data for how long it took to copy. Our largest mailbox was in the region of 25GB so potential downtime was a big issue for us too.

      When the copy operation is complete the mailbox reports itself as 95% copied and the move suspends. It’s only when you resume the task that the user could be affected (and then only briefly). This final step compares the copied data with the source mailbox. There’s a quick re-synch to take account of any changes since you did the copy. It’s only at this point that the source mailbox is locked (to prevent further changes and update AD). If you’re careful not to leave it too long between the copy and the final commit the user effect should be no more than a minute or so. We advised our users beforehand, and scheduled the commit task overnight, because some of our users like to leave Outlook open around the clock. We were committing between 2500 and 4000 mailboxes per night and each batch took a couple of hours. Remember that any one user would only be affected for a minute or so during that time.
      It’s a lot easier than you might think!
      Good luck


  3. Andor says:

    Hi Matthew,

    Registered some remarks under the following BLOG, and I am anxious to read /understand your lessons learned, and so on.

    You make disregard the message that my posts were missing, apparently (re)registering my latest remark did the trick and my posts reappeared.


  4. Matthew Gaskin says:

    Pleased to hear it!


  5. Good article! I found it very insightful.

  6. dude, can I hug you! 🙂