Storage groups are used for physically breaking your databases up into smaller management groups. This is done for several reasons. Chief among them are so you have more numerous but smaller databases, a logical organization of mailboxes, or faster Exchange backups and restores. The Exchange Server can run one simultaneous backup for each storage group. So if you have 10 databases spread across two storage groups, you can have two backups running in parallel; if you have 10 databases spread across five storage groups, you can have five backups running in parallel.

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