OST Sizing Guidance Changes

EDIT 3/24/2009: Added a link to WinSAT documentation based on reader feedback.

Hopefully you have seen the news regarding the release of the Outlook 2007 SP1 February 2009 cumulative update. If you haven’t, then you can read http://support.microsoft.com/?id=968009 for all the detail, but basically we have made significant improvements in terms of Outlook’s performance and responsiveness:

  • General Responsiveness – the Outlook 2007 SP1 February 2009 cumulative update reduces I/O disk usage and UI response time.
  • Startup – the Outlook 2007 SP1 February 2009 cumulative update removes long operations from initial startup (please note that there is a one-time data structure upgrade cost after the first install that takes a relatively linear amount of time based on the size of your PSTs and OST).
  • Shutdown – the Outlook 2007 SP1 February 2009 cumulative update makes Outlook exit predictably despite pending activities (for more information, please see http://msdn.microsoft.com/en-us/library/dd239276.aspx).
  • Folder/View Switch – the Outlook 2007 SP1 February 2009 cumulative update improves view rendering and folder switching.

Note: All of these fixes will be included in Office 2007 SP2 (and as a bit of historical trivia, all of these fixes were originally intended for SP2, but based on their quality and the rigorous testing that has been done with them, the Outlook team made the decision to backport the SP2 work into this cumulative update so that you, our customers, can take advantage of the changes sooner rather than later).

In the past, from an Exchange perspective, we made a statement with regards to the Outlook 2007 Cached Exchange Mode experience (notably in the Mailbox Server Storage Design TechNet article) along the lines of 1 GB mailboxes with around 1 GB RAM requires at least 5400 RPM hard disks, and 2 GB mailboxes with 1-2 GB of RAM requires at least a 7200 RPM hard disk. We also recommended for high performance on mailboxes larger than 2 GB, that you reduce your mailbox size or use Online Mode. This guidance is no longer applicable with the latest product update.

With the release of Outlook 2007 SP1 February 2009 cumulative update, we are updating our Mailbox Storage Sizing Guidance to reflect the improved performance and responsiveness when utilizing Cached Exchange Mode with respect to mailbox/OST sizes:

  • Up to 5 gigabytes (GB): This size should provide a good user experience on most hardware.
  • Between 5 GB and 10 GB: This size is typically hardware dependent. Therefore, if you have a fast hard disk and much RAM, your experience will be better. However, slower hard drives, such as drives that are typically found on portable computers or early generation solid state drives (SSDs), experience some application pauses when the drives respond.
  • More than 10 GB: This size is where short pauses begin to occur on most hardware.
  • Very large, such as 25 GB or larger: This size increases the frequency of the short pauses, especially while you are downloading new e-mail. Alternatively, you can use Send/Receive groups to manually sync your mail.

As a result of this guidance change, you may have a few questions. We’ve tried to anticipate those questions here and have provided answers. In addition, for more information on troubleshooting Outlook performance issues, please see http://support.microsoft.com/kb/940226.

Question: What if I am using a first generation solid state drive (SSD) or a slow rotational hard disk drive?

If you are running Outlook 2007 on a computer that has a first generation solid state drive (SSD) or a slow rotational hard disk drive (HDD), you may experience frequent pauses when you perform typical operations in Outlook (especially if you have not installed the February 2009 cumulative update).

Storing and retrieving data in an Outlook data file involves a high number of small noncontiguous reads and writes. A good metric to use when gauging data file performance on specific hardware is the disk drive’s noncontiguous write performance. The Windows performance team has provided a tool named WinSAT (see below for download and KB 940226 as well as WinSAT documentation for execution instructions) that can be used to benchmark specific disk drives. As you can see in the table below, the estimated experience for Outlook 2007 SP1 with the February 2009 cumulative update is better.

The WinSAT tool is part of the Windows Vista Upgrade Advisor, and is included in Windows Vista. For non Windows Vista machines, you can download the Windows Vista Upgrade Advisor here:


The following table shows an estimated Outlook experience based on the score returned by the WinSAT tool (again, see KB 940226 as well as WinSAT documentation for execution instructions):

WinSAT score

Drive characteristics

SP1 before Feb update

SP1 after Feb update and later versions

Less than 15 MB/s

Generation 1 and Generation 2 SSDs (early 2008)


Reasonable, responsive, limited hangs and pauses

Between 15 MB/s and 30 MB/s

Most Generation 2 SSDs (after April 2008); many 5400 rpm laptop rotational drives

Many hangs and pauses throughout the day

Reasonable, responsive, limited hangs and pauses

Between 30 MB/s and 40 MB/s

Some high-end Generation 2 SSDs (after September 2008); many 7200 rpm rotational drives

Reasonable, responsive, limited hangs and pauses

Quick, responsive, only occasional hangs

Greater than 40 MB/s

Generation 3 SSDs, both MLC and SLC technology (November 2008); many 10000 rpm rotational drives

Quick, responsive, only occasional hangs

Quick, responsive, rarely hangs

Question: What if I have many items in a single folder?

If you have a large number of items in any single folder, you may experience performance issues during certain operations in Outlook while using Cached Exchange mode or using a Personal Folders file. These performance issues are especially noticeable when you switch into and out of that folder:

  • When you use Outlook 2007 with the Performance Update, or you use Outlook 2007 SP1 versions before the February 2009 cumulative update, generally, when 10,000 or more items are in a single folder, these issues can occur.
  • When you use Outlook 2007 SP1 with the February 2009 cumulative update or higher and have more than 50,000 items in a single folder, views other than Arranged By: Date can be slower.

We recommend that you move several items in these larger folders to separate folders in the same store or to an archive store, and use Arrange By: Date when running Service Pack 1 (SP1) with the February 2009 cumulative update for folders that have a large number of items.

If you are not using Cached Exchange Mode, then please review the following TechNet article Understanding the Performance Impact of High Item Counts and Restricted Views.

If you have any questions, please let us know. The Mailbox Server Storage Design article will be updated in a future TechNet refresh and account for these updated recommendations.

Ross Smith IV

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Comments (34)
  1. jay says:

    I am confused… Is this a cumulative update or a hotfix (or is there such a thing as a combo of the two?) Was this released via Windows Update/WSUS?  If so I can find no trace of it and I had to submit a hotfix request to download it from the link at the top of the post.

    Will this update ever be pushed out or will it be a part of the larger Office 2007 sp2?

  2. T Man says:

    The instructions at the link provided for using WinSAT are incomplete.  You must go to http://technet.microsoft.com/en-us/library/cc742157.aspx to get usage instructions.  I’m guessing here, but the proper command to run is "winsat disk -ran -read -drive c" to test the random read performance of the C: drive.  But, that is not specified in the chart.

  3. Brian says:

    Jay – I believe they are just doing this release so you can get it (the Outlook improvements coming in SP2) early if you want.  It will be released as part of Office 2007 SP2.  I don’t think they are going to publish it for Windows Update except via SP2.

  4. pesospesos says:

    Do you guys recommend this for a terminal server environment where we only use ol2007 in online mode?

  5. Mark says:

    Update seems to break Out of Office and free/busy functionality when connected over a VPN. The VPN does not have split-tunnelling enabled, but I can still successfully ping Autodiscover.mydomain.com. I’m not also getting prompted for authentication continuously (until I hit cancel).

    Was excited about the performance benifits, but may have to roll-back (if possible) if I can’t get these other features working again.

  6. Doug says:

    Am I missing something or this this cumulative update actually two seperate hotfixes – 961752 and 967688?

  7. Jay,

    This is a cumulative update for Outlook 2007 and does require you to download it via our hotfix infrastructure.  This update (like all cumulative updates from Office) will not be available on Windows Update.  And yes, all of this will be included in Office 2007 SP2.


  8. Doug – KB 967688 addresses a specific issue (as defined in the KB article).  KB 961752 specifically addresses the overall responsiveness of Outlook.


  9. Mark,

    I would open a support case so we can track and help resolve this issue for you.


  10. Michael Dragone says:

    Briefly – you do NOT want to use an early generation SSD drive. I used one once in a ThinkPad T61 that was just fine with a 7,200 rpm drive but absymal with one of the early SSDs.

  11. Mike Crowley says:

    is this x86 only?  i didnt see otherwise on the download page.

  12. Exchange says:

    Mike – are you referring to Outlook fixes? If that is so – the answer is yes. Office 2007 shipped only in 32 bit variety… it runs on top of 64 bit OS but it is still an 32 bit app.

  13. bink says:

    This is great news!  I have been spending time searching for a Microsoft Outlook alternative due to the poor performance of the client, so I am very happy to see that this is being actively addressed!  Also, while I used to be a heavy cache mode user, I am using IMAP more these days and hope you guys revisit supporting this protocol as a first class citizen.

  14. tom says:

    One small problem with that update: when switching between folders in Outlook 2007, the reading pane divider appears for a moment and then disappears. I have disabled the reading pane in all views. Before the update the reading pane divider did not appear, even for a moment.

  15. TheAngryPenguin says:

    Many, many thanks and praise for releasing these bits before SP2!  The update yielded a noticeable improvement in the overall experience of OL2K7 running in an XP SP3 VM in Fusion.  I second Bink’s comment on IMAP support (is that /the/ Bink?!?).  FWIW, I have not been able to reproduce Tom’s disappearing divider issue.

  16. Lowell says:

    I am also confused.  The main article refers to two hotfix articles.  One says "To download the updates for Outlook, see the following Microsoft Knowledge Base article: 961752 ".   The other says "For full Outlook functionality, you can install the Cumulative Update package that is associated with the following Microsoft Knowledge Base article:


    The two hotfixes list different dlls and have different issues lists.

    What gives?  Is it one, both?  what?

  17. EdH says:

    I am with others on this page. This looks like two download packages – both have a link to the hotfix site.

    961752 points you to file officekb961752fullfilex86glb

    967688 points you to file officekb967688fullfilex86enus

    Which one do I need for Office 2007 on Vista 32bit?

  18. Exchange says:

    For those that are confused about two fixes for Outlook – I asked an engineer in Outlook to help with this (thanks, Greg!) and here is what he said:

    Both updates were released on the same day. The one with 99% of the fixes is 961752. There is only one fix in 967688 and the article outlines that fix. So, if you don’t need the fix in 967688 then all you really need to do is install 961752.

  19. EdH says:

    Thanks! That makes it clear.

  20. Michel says:

    Great! Huge performance improvement with the Outlook hotfix (and I think the final version of Xobni (1.7, released today) helps speeding things up as well..)

  21. Lowell says:

    Thank you very much for the very clear explanation!  We can’t wait to get this applied.

    A comment for the authors of the page http://support.microsoft.com/?id=968009 : The language in the first few paragraphs of that article needs to be changed.

    It would be very simple to change the wording describing the second hotfix to say something like "This hotfix, which is also included in SP2, addresses <another issue> with Outlook and is provided separately.  If you are not experiencing the specific issue, it is not necessary to apply it."  What we get is "For full Outlook functionality…."  which is at best misleading.

    Thanks again for the quick response!

  22. Darren says:


    I have installed the hotfix and the performance has been *significantly* improved.

  23. Kristoff says:

    This update is amazing… Tested in a Company with No email Quotas…

    Performance on 10GB mailbox + 11.000 items /view + .ost + Vista indexing… Now people are able to work, even on SSD Disks.

  24. John says:

    The fix is great. Now, if only I had an easy way to roll it out to my 500 or so users…

  25. Chris says:


    How can I get help on this fix. After installing I’m seeing problems using the Instant Search Pane and searching All Items.

    The result set of the the search only returns items in my personal folders and not my Mailbox.

    Some results from my personal folders will be returned and then the stats us reads "Searching…" and nothing more is returned.

    My indexing status reads "0 items remaining…" and I’ve reset indexing by rebuilding my Instant Search catalog.

    Need help.



  26. We have discovered one incompatibility with the Hotfix. It breaks Symantec DLO (Desktop and Laptop Option) for backup exec. On all of the machines we have installed this hotfix on, the DLO option will crash, here is the information out of the event viewer.

    Event Type: Error

    Event Source: Application Error

    Event Category: None

    Event ID: 1000

    Date: 4/13/2009

    Time: 9:13:31 AM

    User: N/A

    Computer: DHILL-VM-XP


    Faulting application dloclientu.exe, version 3.1.338.6101, faulting module unknown, version, fault address 0x52202920.

    For more information, see Help and Support Center at http://go.microsoft.com/fwlink/events.asp.


    0000: 41 70 70 6c 69 63 61 74   Applicat

    0008: 69 6f 6e 20 46 61 69 6c   ion Fail

    0010: 75 72 65 20 20 64 6c 6f   ure  dlo

    0018: 63 6c 69 65 6e 74 75 2e   clientu.

    0020: 65 78 65 20 33 2e 31 2e   exe 3.1.

    0028: 33 33 38 2e 36 31 30 31   338.6101

    0030: 20 69 6e 20 75 6e 6b 6e    in unkn

    0038: 6f 77 6e 20 30 2e 30 2e   own 0.0.

    0040: 30 2e 30 20 61 74 20 6f   0.0 at o

    0048: 66 66 73 65 74 20 35 32   ffset 52

    0050: 32 30 32 39 32 30 0d 0a   202920..

    Does this issue have a resolution?



  27. What about Deploying in a AD domain?  As this isn’t available on WSUS and it isn’t a MSI so we cannot deploy via AD.

    What are our other options?

  28. Jeff25 says:

    Just got off the phone with Symantec.  Office 2007 SP2 caused DLO to crash whenever Outlook was launched.  We were able to remove items from startup that lead to the issue.  So far we have determined that the Windows Sidebar and Windows Mobility both cause the crashes.  If you remove these from startup that issue goes away.  The crashes do not occur on Windows 7.

    The Symantec tech said he was sending the info up to the engineers.

  29. Daniel says:

    Thanks for the info on the DLO-issue Jeff. We have this problem as well on XP clients and DLO 12.5.

  30. Mike V says:

    I installed the Outlook Patch kb961752 and noticed this happen on Outlook startup.  I then deployed Office SP2 and it is occurring on all of the clients that have DLO 12.5 (latest updates as well) and Outlook 2007 (Standard and Professional).

  31. Mike V says:

    Ok…I called Symantec and here is the official response.


  32. KMAC says:

    We had the same issues here and had to remove SP2 form our users computers. Has there been any progress on a fix yet?

  33. Ice says:


    we have the same issue here like Chris:

    After installing I’m seeing problems using the Instant Search Pane and searching All Items.

    Some results from my personal folders will be returned and then the stats us reads "Searching…" and nothing more is returned.

    My indexing status reads "0 items remaining…" and I’ve reset indexing by rebuilding my Instant Search catalog.

    It’s getting even more weird here – we have roundabout 30 machines experiencing problems with the search – and minutes later it’s working fine. Rebuilding the index seems to help for a few hours – depends on the day :-(

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