Nobody likes to tie up resources with a long-running backup. So how do you shrink your backup window? There are many approaches, but in SAN environments, sometimes the best solution is to back up directly from the shared storage rather than from the servers themselves. These so called "off-host" backups have its own challenges, however, so you need to weigh your options carefully.
Worst-case backup scenarios usually involve a server that maintains a huge amount of data -- or a server that squirrels away data in hundreds of thousands of tiny little files. In either case, backing up over the network using traditional backup agents can result in incredibly long backup windows -- often the better part of a day for a single server. The usual culprits include poor network performance, poor source disk performance, poor backup device performance, and/or the overhead of cataloging the files being backed up.
With direct-attached storage, you have few options to shrink your backup window. If you have so much data to move that a single gigabit Ethernet link chokes -- or, more often, if the performance of the source server simply isn't up to snuff -- good luck.
In SAN-attached environments, you can opt for off-host backups, where you present the source server's storage volumes on the SAN directly to the backup host. The backup host can then draw the data from the SAN without loading the source server at all. Better, if you have enough spare SAN performance headroom, you can freely back up throughout the day without causing problems, as the source server chugs along happily unaware that a backup is taking place.
Off-host backups vary depending on whether virtualization is in place. In non-virtualized environments, the backup software generally contacts the source server and asks it to create a snapshot of the disks you want to back up. The source server then uses software provided by the SAN manufacturer to request that a snapshot be created on the SAN. The backup server mounts the snapshot and performs the backup from that. Without these steps, the backup wouldn't be consistent, because it would be performed from a volume that was actively changing rather than a point-in-time copy.