Single Instance Storage (SIS) removed in Exchange 2010
Long a staple of the Exchange world, single-instance storage was ideal for the enterprise when storage costs were sky-high. These days, with storage costs so low that the amount of data being racked up multiples on a regular basis, Microsoft has traded storage space for overall storage performance.
In Exchange 2007, when a user sent a message with an attachment to 1,000 users, only a single copy of that attachment was stored in the Exchange database; pointers were then used to point individual mailboxes to those shared attachments. With Exchange 2007, Microsoft took the first steps toward the eventual elimination of single-instance storage by not “single-instancing” message bodies. Prior to Exchange 2007, everything — messages and attachments — was single-instanced.
Over the years, the capacity of new hard drives has gone through the roof, whereas overall disk performance has accelerated at a comparative snail’s pace. Exchange 2010’s capacity vs. performance choice falls squarely on the performance side of the equation with significant improvements in overall I/O being the result. Another result is that Exchange 2010 supports a broader array of storage devices, including less expensive SATA disks. However, this also means that each time a user sends out a message with a 10 MB attachment, you can truly multiply that message size by the recipient count as the overall impact to the database size. To help counter this issue, Exchange databases are now compressed, at least to a point; messages headers and bodies are compressed but attachments are not.
Make sure you plan for adequate storage and understand that you now pay for each and every byte or each and every message and attachment. Exchange 2010’s support for SATA disks should help keep costs in line.
Technet: understanding single instance storage open
MsExchange Team: single instance storage in Exchange 2007 open
MsExchange Team: single instance storage in Exchange 2010 open