Estimate Deleted Item Space Usage

Estimate Deleted Item Space Usage

Once mailbox items are deleted from a mailbox (either by emptying the Deleted Items folder or by performing a hard delete), they are moved to the deleted item or deleted mailbox cache. This

feature is nicknamed the database dumpster. Data is retained in this dumpster for a length of time specified on the mailbox database’s Limits property page or on the user’s Storage Quotas properties Once this time has passed, the deleted item or mailbox is marked as permanently deleted and the space is recovered. This occurs nightly during online maintenance. Data in the database dumpster does not count against a user’s storage quotas. However, data that has been recently moved into the dumpster may not be reflected in the user’s mailbox size for an hour or two.

There are a number of factors that contribute to how much space will be in the dumpster:

• How long users want to retain the information they receive

• Mailbox sizes

• Message delivery rate and average messages size

If you are interested in measuring how much actual deleted space there is in an existing database, there are a couple of tools for doing this. For example, to see the number or the size of deleted

items in the mailbox database, use the Windows Performance Monitor console and add the MSExchangeIS Mailbox object’s Total Size of Recoverable Items counter. For this particular

mailbox database shown in Below, the actual database size is about 4GB so the amount of deleted space is almost two percent of the total size of the database.

Viewing the amount of space used by deleted items.

You can see the same information if you search through the Application event logs for event ID

1207 from the MSExchangeIS Mailbox source. This event is generated during online

maintenance. In below you can see that for the database Mailbox Database there is

approximately 63MB of deleted items.

Viewing deleted item space through the event log.

Deleted mailbox space through the event log.

Online maintenance will also generate event ID 9535 whose source is MSExchangeIS Mailbox. This event will tell you how much space is currently consumed by deleted mailboxes. The below

shows this event. In this example, there are 26 mailboxes waiting to be deleted and they are consuming approximately 507MB of disk space in the Mailbox Database. In this case, this is

approximately 10 percent of the total size of the database. In Exchange 2003, deleted mailbox content is retained for 7 days and deleted mailboxes are

retained for 30 days by default. In Exchange 2007, the deleted mailbox content is retained for 14

days and deleted mailboxes are retained for 30 days by default.

Viewing deleted mailbox space through the event log.

Coming up with a formula that will help you to estimate how much deleted item space is in your mailbox databases is difficult because this value will vary significantly over time. You may have

to periodically remind your users to empty their Deleted Items folders or clean up old and unwanted messages. You may run the Exchange 2003 Mailbox Manager or the Exchange 2007

Messaging Records Management tools against mailboxes and clean up older content yourself. In academic environments, you might delete former students’ mailboxes at the end of the school

year and free up a lot of disk space. One recommendation I have seen from Microsoft regarding estimating deleted item space is that the total dumpster size will be equal to the total size of incoming messages times the length of the deleted item cache. The total overhead, of course, would be factored against the mailbox sizes.

Here is an example using this formula. Let’s say a typical user at an organization receives an average of 40 messages per day and the average message size is 50KB. Let’s say this user deletes

most of this incoming mail, but retains a few of these messages. Plus the user deletes a few older messages. The total deleted item cache for this user would be 2MB per day or 28MB at the

default 14-day time period. For a user with a maximum mailbox size of 350MB, this would be approximately an eight percent overhead. For larger mailboxes, the percentage of overhead is

even less. Of course, most users are not so diligent about deleting their mail as they read it and no longer need it. Users usually delete their mail after they are nagged about it by an administrator or when

they get a warning indicating that they are approaching their limit. However, in a mature messaging environment, this behavior may average out anyway. I typically recommend using 20 percent of the total allowable mailbox sizes as the maximum amount of space. This allows for some ‘wiggle’ room and it estimates space used by deleted mailboxes for a typical organization. Thus, for the sample organization with 838GB of mailbox storage, there would be an additional 168GB.