LUN Design and Hardware VSS

This posting is written by Michel de Rooij, thanks for posting it here as well…

I had a question why you need to design seperate LUNs for Exchange database and log files when using a hardware based Volume Shadow Copy Service (VSS) backup solution, as mentioned in this TechNet article: Understanding Exchange 2010 LUN architecture

To deploy a LUN architecture that only uses a single LUN per database, you must have a database availability group (DAG) that has two or more copies, and not be using a hardware-based Volume Shadow Copy Service (VSS) solution.”

The reason for this requirement is that hardware VSS solutions operate at the hardware level, i.e. the complete LUN. Therefor, if you put the Exchange database and log files on a single LUN, it will always create a snapshot of the whole LUN. This restricts your recovery options, since you can by definition only restore that complete LUN, overwriting log files created after taking the snapshot. So, changes (log files) made after the snapshot are lost and you have no point-in-time recovery options.

For example, with the database and log files on a single LUN, suppose you create a full backup on Saturday 6:00. Then, disaster strikes on Monday. By definition, you can now only restore the database and log files as they were on Saturday 6:00; log files which were created after Saturday 6:00 are lost.

With the database and log files on separate LUNs, you can restore the database LUN, which leaves the LUN with the log files intact. Then, after restoring the database, you can start replaying log files.

So, keep this in mind when planning your Exchange LUNs in conjunction with the backup solution to be used. Note that the Mailbox Role Calculator supports this decision by letting you specify Hardware or Software VSS Backup/Restore as the Backup Methodology to be used.

If you’re interested in more background information on how VSS works, I suggest you check out this TechNet article: Volume Shadow Copy Service

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