Unless you’re running an Enterprise edition of Exchange, your
Information Store (IS) limit is 16 GB. If you exceed this limit, the IS will
shut down and won’t restart.

Recovery from this unfortunate occurrence in Exchange 5.5 is
time-consuming and painful. You can’t start the IS until you compact the
database and get below 16 GB, which typically takes hours. In the meantime,
your users are without e-mail.

Exchange Server 2003 / 2000 Server Standard Editions include
a feature that mitigates the damage. You can temporarily increase the database
size limit from 16 GB to 17 GB with the addition of a registry entry. This
temporary limit increase buys you some time to mount your database and schedule
the downtime needed to reduce the size of your IS, or you can purchase
Enterprise Edition and plan an upgrade. Most importantly, your e-mail continues
to flow.

Exchange Server 2003 ships with this functionality, but
Exchange 2000 Server requires the application of a post SP3 rollup. To find out
more information about this rollup, check out Microsoft
Knowledge Base 824282.

Follow these steps to temporarily increase the database size
limit:

  1. Open
    the registry editor, and navigate to the following key: HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\MSExchangeIS\<Exchange
    Server Name>\Private-<long hexadecimal string>.
  2. From
    the Edit
    menu, select Add Value, and then type Temporary DB Size Limit Extension in the Value Name
    box.
  3. Select
    REG_DWORD
    for the Data Type, and then click OK.
  4. Enter
    a value of 1, click OK,
    and exit the registry editor.

The system will read this value when the service starts,
allowing you to mount a database that is up to 17 GB in size.

Note: Editing the
registry is risky, so make sure you have a verified backup before making any
changes.

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