Why do migrations of Exchange 2003 mailboxes to Exchange 2007 take incredibly long to connect to the source mailbox?

We've done a migration over the weekend of our domain controllers to Windows 2008 (from 2k3) and also upgraded Exchange from 2003 to 2007.

All went really well (for once!)  but....

 When using 2007 Exchange Management Console to migrate users (I've got over 9000 users to move) I can use the migration wizard, everything goes through fine, but when it begins to migrate it takes an absolute age to connect to the source database.  If I migrate 1 user at a time it connects almost instantly, but when migrating 100 users, it takes over 1hr to connect.  Once connected the 4 streams run off and the migration is as quick as I'd expect.

 Any idea's folks?  The mailboxes I'm migrating are small accounts (school kids mailboxes, all with a limit of 5mb) so size isnt really an issue...

Cheers.

AMB
AxemanbobAsked:
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Darksied9Commented:
MS Exchange uses MAPI threads to establish the connections to the servers.  When you try to connect hundreds of MAPI threads at one time to a server I can imagine it takes a while.  This is the very reason that MS in the 2003 version only allowed 4 simultaneous threads active in one migration wizard.  Depending on the hardware this may not be the issue.  It could also be network related.  How many domain controllers that are GCs do you have in your site (did you make any site or IP changes and forget to put the Exchange servers in the site) as it could be GC Lag which is very common as most people under estimate the requirement of Exchange on the Domain.

100 X 100 MAPI Threads (Network Contention & Slowdows) + 200 Authentications could equal slow as heck.
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Darksied9Commented:
My math was a little off there...sorry.
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NetAdminGuyCommented:
This may be a dumb comment, but our similar situation improved greatly when we asked to uses to close outlook before we did the migration.  Speed was GREATLY improved.
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AxemanbobAuthor Commented:
Hardware isnt an issue I dont think, the both servers are running 4 x quad core processors with 12GB RAM.  We've only got 1 site, and there's 2 2008 GC's, both on hih performance servers.  Network wise, the're on the same internal network, same VLAN, same IP range all running 1GB.  No site or IP changes were made over the weekend, so not that either!  Sorry!

If I try and migrate 10 user accounts the inital connection still takes over 10mins to connect, but once connected the other mailboxes migrate sucsessfully taking  between 20 and 30 seconds each.

If I try and run the mailbox migration wizard from the 2003 server (I know I shouldn't but I was just testing...) connection to the source mailbox happens imediatly, but obviously then fails...

Just to see how long it takes I've tried migrating 216 mailboxes to a new store this morning.  The first mailbox still hasnt connected and has been running for 3hrs 16mins so far.
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AxemanbobAuthor Commented:
It's a schools email server, and they're all on half term this week, so noone is logged in or using the server...
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Darksied9Commented:
Number 1, you don't have enough GCs for the environment.  If what you are saying is each EXC server has 16 procs.  Typically you should have a 4 (proc) to 1 (proc) ratio but with 9000 users the catalog is huge and the GAL has got to be pushing it with all data stored in AD.  I would probably double the GCs.  I would also look at downloading the demo of Spotlight for exchange and watching the results as there are probably some issues is network utilization.  For starters, I would:

1) Double the GCs.
2) Ensure TCP Chimney is disabled (or ensure you have the hotfix installed: http://msexchangeteam.com/archive/2007/07/18/446400.aspx )
3) Check the Memory Handles usage (if /3GB or USERVA=3030 is enabled remove them).

Go from there.
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AxemanbobAuthor Commented:
Thanks for the info Darksied9, I'll give it a go!!

Cheers

AMB
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