Keeping track of calendar items such as meetings to ensure that a definitive version of the item is available is sometimes hard to do, especially when multiple clients or multiple users access a calendar. Exchange 2010 made architectural changes in the server to concentrate processing in a common set of business logic, a step that helped matters because it stopped items being processed differently in various places.

It is still possible for misbehaving clients to wreak havoc with calendars, as in the case of the woes of Apple’s iOS email app, However, better awareness of potential problems within mobile device vendors, new code to protect Exchange against malfunctioning clients, and the introduction of mobile clients that don’t use ActiveSync (like OWA for Devices) have all helped to reduce calendar problems.

Apart from consolidating the business logic for calendar processing, Exchange 2010 introduced calendar versioning to improve overall reliability for events like meetings. Calendar versioning means that Exchange keeps track of all the changes that occur to calendar events so that the Calendar Repair Assistant (CRA) can validate that the calendar reflects the most current and accurate state. Sounds good, which is why calendar versioning is enabled by default. Enabling the feature comes with a certain price, which Microsoft’s sizing guidelines for Exchange 2013 puts at an increase of approximately 3% in mailbox size.

If you want to disable calendar versioning for a mailbox, you can run the Set-Mailbox cmdlet as follows:

Set-Mailbox –Identity TRedmond -CalendarVersionStoreDisabled $True

After you’ve disabled calendar versioning, the Managed Folder Assistant will clear out the saved items the next time that it processes the mailbox.

So where does the Recoverable Items folder come into play for calendar versioning? Well, Exchange has to hold the changes made to calendar items somewhere if the CRA is to be able to validate matters. As it happens, the Recoverable Items folder is the place where the changes are held, if only because this location is invisible to clients like Outlook and OWA. You could argue that it would be better to record the data elsewhere, such as another hidden folder, but the Recoverable Items folder is available and it is quota-controlled, as we’ll come to later.

The normal state of affairs is to hold “stripped” versions of calendar items in the root of the Recoverable Items folder for 120 days. Stripped means that only the essential details of the items are retained. Attachments any extraneous information are removed. In essence, sufficient information such as meeting location, time, duration, and so on is kept to allow CRA to do its work, but no more.

But when information needs to be preserved, such as on a short term basis (Single Item Recovery) or longer (litigation or in-place hold), then Exchange retains full copies of changed calendar items in the Recoverable Items\Deletions folder (if the item was first soft-deleted and then changed) or Recoverable Items\Versions folder (if the item is changed in the Calendar folder). A stripped version of an item is also preserved if a hard-delete operation is performed against a calendar item in the Recoverable Items\Deletions folder. All of this is done to make sure that information is preserved in an immutable fashion. In other words, users cannot take actions to remove information when they should not.

Getting back to Recoverable Items quotas, it is possible that a mailbox that hosts a very busy mailbox might accumulate a lot of calendar versioning data in the Recoverable Items structure, especially if it is common practice to include large attachments in meeting requests, as is often used when circulating information that will be discussed during the meeting. Despite steps taken by Microsoft in Exchange 2010 SP2 RU2 and RU3 to restrict the amount of quota consumed by calendar versioning, it is conceivable that a large percentage of the recoverable items quota might be taken up by calendar items with a knock-on effect of excessive database growth and potentially problems moving mailboxes. For very active mailboxes, the space consumed by calendar versioning might even push the mailbox to up its recoverable item quota.

All of this goes to prove that the Recoverable Items folder deserves some tender loving care from Exchange administrators. While you might not need to learn the exact details of what happens under the covers until problems occur, it is worthwhile to keep an eye on the use of the Recoverable Items folder in your organization.

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