Determining the size of a database store for your Microsoft Exchange Server has more to do with an enterprise's general backup and storage requirements than any kind of physical limitation of the Exchange data store itself.
"There is no technical limit," said Steve Bryant, a consultant at ProExchange, a Fayetteville, Ga., integrator that specializes in Exchange Server. "The true maximum is whatever makes a company feel comfortable."
Bryant said the standard backup-and-restore API for Exchange runs from 30 and 50 GB per hour, though the Exchange store is capable of scaling up to terabytes. Exchange administrators with more than an eight-hour service level agreement wouldn't want more than a 224 GB data store.
Continuous backup, in the form of volume shadow copy and snapshot technologies in Exchange Server 2003, let backups bypass that Exchange API to a storage area network (SAN), which allows a company to grow the Exchange store past the 224 GB, Bryant said.
But this enhanced storage capability is only available in products from vendors such as Hewlett Packard Co. and EMC Corp., which have developed the special interface using the volume shadow copy software developer kit from Microsoft. The Exchange API limitation of 32 GB per hour has been the same since the introduction of Exchange 5.5, Bryant said. Customers can expect improvements with the next version of Exchange, code-named E12.
Pointers from Microsoft would be helpful
Customers have varying message storage needs, so there is no one correct answer on how large an Exchange store should be. But customers like Fred Pretorius, a manager of systems architecture at a Boston law firm, said he would like to see more guidance from @9248 Microsoft about what is a prudent and reasonable Exchange database store size.
The law firm of Mintz, Levin, Cohn, Ferris, Glovsky and Popeo P.C. saw a 90 GB database fail last summer, and it took nearly eight hours to complete a single restore. The problem seemed unlikely -- two drives failed in quick succession. Mintz Levin had a backup plan in place, so attorneys were sending and receiving e-mail within two hours. But it took nearly two days for end users to get back into their databases, Pretorius said.
He said that as a result of that experience, the firm plans to reduce its Exchange data stores to below 30 GB per database, which will each take two to four hours to back up. Mintz Levin currently has 700 users, three mailbox servers and four databases in one storage group for each server. The firm also uses archive software to help control its store sizes.
Recommendations for database sizes
Bryant, the ProExchange consultant, suggests a rule of thumb for determining prudent database sizes. For a company that wants a maximum restore time of eight hours, he recommends a database size of 32 GB per hour, which leaves an hour for preparing the environment for recovery.
"With that number, you can divide by the number of users you need to support, which gives you a mailbox size," Bryant added. "If you need to support more, you need to break it up into additional stores or look at other backup-and-restore technologies that work around the Exchange API."
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