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EV will increase Outlook OST size ???

Created: 15 Feb 2013 • Updated: 01 Mar 2013 | 8 comments
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Hi All,

    We are using EV 9.0 and currently we are facing some issues with outlook.

All our Exchange users enable with EV Archive and all the users are using outlook 2010 SP1 with caching mode.  I verified users OST file and it’s compare to exchange mailbox size OST size is double.

Find the below sample mail box size details.

EV Archive enable mailbox with short cut.

 Exchange mailbox size: 500 MB

 Client OST file size       : 1.3 GB

Without Archive mailbox

Exchange mailbox size: 500 MB

Client OST file size       : 700 MB

I want to make sure EV increasing the OST files size or not? 

if anyone have idea please share with me.

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JesusWept3's picture

Why not try a test?

1. Export out your archive to a PST file, note how big the PST file is
2. Create a new test mailbox in Exchange
3. Import the PST file in to the exchange mailbox, note down how big the exchange mailbox is
4. Create a new outlook profile and set it to cached mode and let it download the items
5. Note how big the OST, and how big it is compared to the original PST and the exchange mailbox
6. Enable the test mailbox for archiving
7. Perform some run nows , turning the items in to shortcuts
8. Afterwards, check the size of the archive through usage.asp
9. Check the size of the exchange mailbox in the EMC
10. Check the size of the OST
11. After all tests have been run, delete the archive and test mailbox

My best guess is that you are seeing white space in the OST file
So if you have say a 1GB mailbox and you download it to an OST, you then archive 500MB of items
it shrinks down the mailbox as it converts the items to shortcuts, but that space is not being reclaimed in the OST file itself's picture


 Thanks for your reply, Actually i did the above mention test.

I shrieked  the 1.3 GB OST file and after shrink its Reduced to 1 GB. even still OST file is double compare to Exchange Server actual mailbox.

now i don't have any idea how to check. and which way to troubleshoot.

if anyone have idea please share with me.

Rob.Wilcox's picture

Data in a PST or OST will usually be TWICE the size of the Exchange mailbox.  In the past there used to be Microsoft Support articles about it, it's because each message has the body as text, and as HTML.

Hope that helps,

JesusWept3's picture

Very good point
Also I don't know whether it has the same thing with page sizes
Ie a page size is 32k, shortcut is 4k, you have 28k of wasted space that can't be reclaimed etc's picture

Thanks for your inputs, 

Still I have one test mail box and that mailbox OST file is not that much bigger compare to EV archive enable mailbox.

Hi Rob, if you can you share with me MS article number. Even your point is not valid for this situation.

below mailbox OS is not duble.

Without Archive mailbox

Exchange mailbox size: 500 MB

Client OST file size       : 700 MB

Rob.Wilcox's picture

Okay, I'm not sure you want to go this route, but here goes...

What is meant in your initial comments about 'users with an EV archive' ?  Do they have any items archived? Do they have Vault Cache enabled? Is the Vault Cache trawler enabled or disabled (if it's enabled, then a few minutes after Outlook starts it will begin trawling Outlook for potential items to add to Vault Cache.. this trawling will be touching the OST file if you're in cached mode).

Next up..  there is no mention of the types of data that make up this 500 Mb of mailbox space.  Is it a single 500 Mb message, with the entire size made up of just the message body? Is it random messages, with random attachments, is 500 1 Mb messages?

What Operating Systems has this been tested on?

And File Systems, eg NTFS, FAT32 ...

Some information which might help: << has links in it to the Microsoft information.'s picture

Hi Rob,

    Thanks for your reply, All the users with windows 7 ENT + NTFS file system and Outlook 2010 SP 1.

Every user’s mailbox I can see the Archive shortcuts and enable the EV cache with 2 GB limit.

Average users mailbox quota is 1 GB and Archive policy configured if mailbox reached to 75 % then start to archive the message items.

I have one Exchange user that user Exchange mailbox size 6 GB with EV shortcut items and OST file size is 13 GB. After shrieked the OST file and it’s reduce size to 12 GB.

This user complaining for outlook, most of the time outlook stuck and not responding. I opened support ticket with MS team and they asked me to check with Symantec.

Rob.Wilcox's picture

I think you can clearly see from the links that I've given that this isn't something 'unique' to the way that Enterprise Vault is involved.  Particularly this from MSDN:


When deploying Cached Exchange Mode for Outlook, be aware that users' OST files can increase in size by 50 percent to 80 percent over the size of the mailbox reported in Exchange Server. The format Outlook uses to store data locally for Cached Exchange Mode is less efficient than the server data file format, resulting in more disk space used when mailboxes are downloaded to provide a local copy for Cached Exchange Mode.


The reason that I asked about whether or not Vault Cache is involved is because of the testing ...  I imagine that when you take a non-EV user, with a 500 Mb mailbox, what you're doing is opening Outlook and letting it build a new OST file.  Once built, you're closing Outlook and looking at the size of the OST file?

In other words you're not doing any activity on the OST file.

When Vault Cache is enabled it will trawl through the OST file.  This will cause the OST file to grow... in part because the indexes which Outlook maintains for the OST file will be built and amended.

I'm not really sure what problem you're trying to discover, or fix here..  it's the nature of the beast unfortunately.

If you took that 500 Mb mailbox, on a non-EV enabled user, and let that user use that OST for a period of a few weeks, you'll see that the OST file is going to be 'much' larger than the initial build size.

Hope that helps,