Database limits in Exchange 5.5

Will my database stop working when it hits 16GB?

The short answer is, it depends on which version of Exchange you have. Both Standard and Enterprise limit the size a single *.edb file can grow to. This limit is 16GB for Standard, and 16TB for Enterprise (1TB = 1024GB).

That means for Enterprise, you're effectively unlimited (although your backup and restore capabilities will impose a practical limit on your store size long before you reach 16TB).


How do I tell which version of Exchange is installed?

Check your Application event log - if you've got Enterprise, event 1217 from the MSExchangeIS Private source will say "Information store with unlimited capacity enabled". (Obviously it's lying, see above.)


What do I do if my Standard store hits 16GB and stops?

Microsoft's article http://support.microsoft.com/default.aspx?scid=kb;en-us;185457 gives you all the info you need about trying to recover the store - although prevention is always better than cure.


How do I prevent my server from hitting the limit?

There are various options:
[ul]
[li]upgrade to Enterprise version - you should be able to install over the top of your existing server (but sourcing and licensing the software could be an issue...)[/li]

[li]add another Standard server to the same site, and split your mailbox population between the servers - moving mailboxes between servers in the same site is easy enough, although if you have lots of shared mail items between the servers you might not get much space back[/li]

[li]implement strict mailbox limits/quotas to prevent the store from exceeding the limit in the future.[/li]
[/ul]

If you are very close to the 16GB limit, you need to act quickly. You can help by asking your users to delete as much old mail as possible, and by removing the Deleted Item Retention setting if you have one (this setting delays deleted mail from being purged from the store for a number of days). Or ask your users to archive old mail to PST files, but this can bring its own challenges.


I've deleted lots of email from the store, but this has freed up much less space that I think it should have - what has gone wrong?

In a word, nothing. Because Exchange uses Single Instance Storage within a database, it only stores one copy of each mail item regardless of how many people have a link to this mail item in their mailbox. If you were to send an email to 10 other mailboxes on the server, one copy of the message would be stored, with 11 pointers to it (1 from your Sent Items folder, 10 to the recipients' Inboxes. Even if 9 of the 10 recipients delete the message and you also delete it from your Sent Items, Exchange will still store it, because one user still has a link to it. Only when all pointers to the mail item have been deleted will the store finally flag it for deletion. (Watch out for some clients that don't empty their Wastebasket when they log out, so a Deleted Item could still sit there for many weeks.)

Even after the item has finally been deleted from every mailbox, the store will hang on to the item for as long as the Deleted Item Retention setting is set for. Only after this expires does the next database maintenance run finally mark the space in the database as reusable.


Can I temporarily increase my store size?

Not in Exchange 5.5. Exchange 2000 SP3 onwards allows you to temporarily increase the size of the store by 1GB to allow you enough headroom to restart the service, so you can reduce the contents (http://support.microsoft.com/?id=813051 has the details). Alternatively, Exchange 2003 SP2 will increase the Standard store limit to 75GB. Maybe you can use these facts as leverage to help convince Management it is time to move on from Exchange 5.5.


Will my PRIV.EDB ever shrink in size?

No. Not automatically, anyway - even if you manage to delete information from the database, it just leaves empty space ("white space") in the file. The white space will be used in the future when extra storage space is required. When all the white space is used up, the IS service automatically increases the size of the PRIV.EDB file if it's currently less than the maximum (16GB for standard , 16TB for enterprise), or stops the IS service if the PRIV.EDB is already at maximum size.


So can I ever get the PRIV.EDB file to shrink in size?

Yes, but only by doing an offline defrag, using the ESEUTIL utility. But this involves service downtime, and after you've finished compacting the PRIV.EDB to free up all the white space inside the file, the first thing the IS service will do when you restart it and new mail arrives is to extend the size of the file again so that it has somewhere to store the new mail. So you really don't need to bother with an offline defrag unless you've created a significant chunk of white space (say because of a bulk mailbox move/migration) and you're desperate for the disk space back.


How much free (white) space has my database got in it?

Check your overnight Application event log for 1221 events (one per store) that tell you how much white space you have left in your db - white space will always be used before the priv.edb file is physically increased. This is give you an indication of how full the database is, and whether it will try and extend the physical file size in the near future.