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We recently started using Microsoft's Hosted Exchange offering with Outlook 2010, and are very happy with it. So far we have been able to easily migrate calendars and contacts from old PST files into Exchange without any problems.

The only data still in PST files is archived email. I'm concerned about the bandwidth implications of having users upload all of their archived messages. We have a 30-40 GB of stored messages for all users combined, with some users having PST files that are several GB in size. We have three sites with the following bandwidth specs:

  • Main office (25 users) is 5 Mbps / 5 Mbps
  • First branch (10 users) is 5 Mbps / 512 Kbps
  • Second branch (5 users) is 512 Kbps / 512 Kbps

If all the users start copying their messages into their Exchange mailboxes at once, will this kill our inter-site performance? Is Outlook/Exchange smart enough to reduce bandwidth use when the network connection is saturated, or do I need to ensure that QoS is configured before starting this?

Looking further down the road, what happens when a user with a 5 GB mailbox moves to a new computer? Does the entire 5 GB need to be re-downloaded, or can the OST from the previous computer be used to accelerate the process?

Any insight would be appreciated.

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+25

To start: yes it is going to completely kill your inter-site performance. Outlook/Exchange is not smart enough to limit the traffic, you will need to configure QOS if you plan to do this during business hours. As far as re-downloading the mailbox when a user gets a new PC: this is also something that will have to happen with cached Exchange mode. When you configure Outlook's connection to your Exchange server it has to create a new local .OST file (local cached mailbox). A quick search of Google shows there are ways to create the .OST file, stop the download, and replace the new .OST with an old .OST file of the same name. However I have never tried this and cannot condone it. I recently consolidated users in my company onto one domain and had to deal with the re-download of the local .OST file (very Frustrating).

A side note: Although Outlook 2003/2007 allows up to 20GB .PST(http://support.microsoft.com/kb/830336/en-us), I HIGHLY recommend you keep them no higher than 1.5 Gigs anything over that will cause problems. The problems are a mixture too, sometimes outlook will fail to start, sometime it’ll stop sending and receiving, sometimes it will randomly delete mail, other times the entire file can get corrupted. It took me a while to narrow down what was wrong with some of my users Outlook, after talking to another Sysadmin who experienced the same problems I found out it was over-sized PST's. I just want to save you some headaches a year or two down the line.

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+1 Thanks for confirming my suspicions. Also, good point about oversized PST files. I think might already be running into problems related to that. –  Nic Nov 6 '10 at 23:01
2  
Building large .OST files is the biggest failing of the whole "hosted Exchange" scenario. Microsoft needs to give this scenario some attention. –  Evan Anderson Nov 8 '10 at 15:40

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