Posted by: kurtsh | June 17, 2012

INFO: Why the heck is my Outlook .OST so big?

No, seriously.  Why’s my Outlook 2010 offline store file so massive?  I have 2.1GB in my mailbox – including calendar, email, contacts, yadayadayada… and the .OST file is a monstrous 7.5GB

7.5GB – 2.1GB = 5.4GB. Why do I have an extra 5.4GB in my .OST?  That’s the question I asked last night.  So I searched the Internet as you would expect and came up with basically nothing.  The typical, “you have whitespace in your .OST and need to compress it manually” is really the only thing I read – which I of course already did.

One thing I did know however was that, in the corporate world, we open other people’s Exchange calendars to check their availability for meetings & conference calls.  And we do this a LOT. 

Some might ask, “why don’t you just use the data picker (free/busy)”, but to really get a sense of what people are doing, you kinda have to have their calendar open fully & visually.  For example:  Someone might be blocked at 1PM-2PM… but if the appointment is an hour away, they’re going to need time to drive to the location before and after and won’t be able to be present for meeting.  This is feasible in the Outlook meeting date picker however it’s less clear & easy to miss.

Well, it turns out that by default, when you open a person’s calendar, it syncs the calendar to your .OST.  It stores itself as a ‘checkbox’ on the left hand column of the calendar view and will remain there, continuing to sync in the background.  How much it syncs, I don’t know, but what I DO know is that if you open up 50 people’s calendars over the span of say 6 months, you may have 50 people listed on the left column of the calendar… and all these calendars are being cached & taking up space in your .OST.

The answer is to right mouse click each calendar and REMOVE them… then compress the .OST again and watch the file shrink dramatically. 

In my case, I deleted all the other people’s calendars in my list that I’d looked at once but never really looked at again… and easily cleared out about 2.1GB of space in my .OST resulting in a more reasonable 5.4GB file size.  I’m not kidding.

Keep in mind that even if you only keep the calendars of the people you frequently to look up, these calendars will be cached.  This can be a huge amount of storage.  Also if you have been delegated access to another mailbox – say an executive’s inbox/contacts/calendar/tasks – and at one point you access each of these ‘shared folders’, you’ll be caching ALL of that email, contacts, calendar & tasks data into your .OST by default. (This was a change made as of Outlook 2010)

To halt this behavior: (And get the 3rd party’s inbox information only when you’re online & not offline)

  1. Go to FILE – ACCOUNT SETTINGS –ACCOUNT SETTINGS.  The Account Settings dialog box will appear.
  2. Click the “Data Files” tab and DOUBLE-click your .OST file for cached content.  A “Microsoft Exchange” dialog box will appear.
  3. Click the “Advanced” tab.  You will see a checkbox group called “Cached Exchange Mode Settings”.
  4. Uncheck “Download shared folders”.  Click OK button. You’ll return to the Account Settings dialog box. (In the photo to the right, I’ve already unchecked the box.  Normally by default, it’s checked.)
  5. Click Close button.

By unselecting this check box, you effectively stop syncing the data from the other people’s mailboxes that you once viewed.  For example, if you ever open anyone else’s calendar, it won’t cache it to your machine.  If you are delegated access to another mailbox – like a shared mailbox that multiple people have access to – the mail won’t sync to your PC and you’ll only be able to view it while online.

This reduced my .OST’s size by 3GB to a reasonable 2.7GB.  I still haven’t figured out the remaining 600MB difference which could be anything from RSS feed content, to Windows Live Messenger contacts to anything else that may not be counted by my Exchange Mailbox folder size total, but I’m sort of satisfied with the above modifications (and the 4.9GB file size reduction) I made so I’m not going to spend any more time on this.


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