HP published an interesting white paper about Backup and recovery best practices for Microsoft Exchange Server 2007. Here’s a small excerpt from the TOC:

• Testing objectives
  • NTBackup with Exchange ESE streaming backup API
  • Backing up using 4 concurrent streams to 4 disks
  • Backing up using 4 concurrent streams to 4 tape drives
  • Backing up using 16 concurrent streams to 16 disks
  • Backing up using 16 concurrent streams to 4 disks
  • Backing up using 32 concurrent streams to 32 disks
  • Backing up using 10 concurrent streams to 10 tape drives
  • Testing summary
• Exchange 2007 backups and VSS
  • Exchange 2007 continuous replication and backups
    • Incremental backups
    • Passive database backups
    • Off-host database backups
  • VSS and streaming backups

And here are some of the conclusions:

This paper presented results and best practices derived from online streaming backup and recovery testing, and reported VSS backup testing observations. Both online and VSS backups can have a valid place in Exchange 2007 backup design.
The specific recommendations resulting from this work demonstrate that early planning can have a large impact on the ability to perform efficient, timely, and effective backup and restorations.
Planning considerations:

• Server workload is important to monitor. If the workload is low, consider reconfiguring your backup jobs for more concurrency, leading to better performance.
• Larger database sizes might not be better. Mailbox size needs to be manageable for faster restoration and decreased workload.
• Leverage storage groups to maximize concurrency, configuring storage groups on dedicated LUNs.

Findings:

• Based on four storage groups (the limit for Exchange 2003), no performance improvement was observed for disk-to-disk backup between Exchange 2003 and Exchange 2007. Disk-to-tape showed an approximate 15% performance improvement.
• By taking advantage of the new Exchange 2007 storage group limits, users can increase concurrent streams