One of the common messaging system migrations is from Novell GroupWise to Microsoft Exchange 2010 and 2013. There are many differences between GroupWise and recent versions of Exchange and one of the more challenging issues involved in planning this migration path is deciding how much storage to allocate to Exchange.
Single Instance Storage (SIS) is a system’s ability to keep one copy of content that multiple users share. It is a means to eliminate data duplication and increase efficiency. In the case of email systems that support SIS, this would mean that a single copy of a message is held within its database whilst individual mailboxes or accounts access the content through a reference pointer. While disk space reduction is a prime goal of SIS, it also greatly enhances delivery efficiency of messages sent to large distribution lists.