What type of compression is achieved?
After archiving a 50GB Exchange store, only 2GB of data remained in Exchange, with the archive store containing 30GB of compressed data. There are two figures that Outlook gives you when you ask for your mailbox size – ‘Local Data’ and ‘Server Data’. The Local Data tab refers to the size taken up in the offline copy of your mailbox on your local machine (stored in a .OST file), whereas the Server Data tab shows that actually stored in Exchange. For larger messages, these two figures are pretty similar, but if you have a lot of very small messages, the inefficiencies in the .OST file format start to show – a two line message will still consume about 7KB in the .OST file, but maybe only 1KB in Exchange. If you’ve had lots of messages archived, the discrepancy between the two figures will start to get larger, since the archived stubs are generally very small indeed.
