Data is added and removed from PST files under normal usage. For instance, a user may move emails from their mailbox to a PST file which increases the data size. A user may also delete emails they no longer require from a PST file, reducing its data size.
Changes in the data stored in a PST do not necessarily have a direct effect on the PST file size on disk. Initially the PST file contains no data, but, PSTs created by Outlook have a file size of 265KB on disk. As you add data to a PST file, the file size expands to accommodate the addition of data. If you then remove data from the PST, the file size does not change, only the data size is reduced. When data is removed from the PST it is replaced with white space. If you then add data to the PST file, it utilizes the white space first before expanding the file size again. If you do not add further data to the PST file, the white space can contribute to a considerable storage cost as the PST file size on disk is considerably larger than the data it holds. The only way to reduce the file size of a PST and reclaim white space is to compact the PST file.
Barracuda ArchiveOne reports on the initial data size, the current data size, and the file size on disk. This gives you visibility of white space and processing progress.
PST File White Space
You can determine the amount of white space in a PST file by comparing the data size to the file size. If the initial data size is significantly smaller than the file size, this indicates the PST contains large amounts of white space.
PST file Processing Progress
If you are archiving messages, and replacing the full original email with a smaller message stub, or migrating messages from the PST file to the owner’s mailbox, then the current data size compared to the initial data size will reduce. There will be no change to the PST file size on disk at this time; the only way to reduce the file size is to compact the PST.
For example, if you are migrating messages from a PST to the owner’s mailbox:
- Once the PST file is discovered, the file size on disk, data size, and initial data size display. A PST file with 2GB size on disk, may only contain 1.8GB of data, in other words, it contains some white space. The initial data size and data size are both the same (1.8GB) as no processing has taken place.
- A migrate message policy is applied and migrates all the messages to the owner’s mailbox. The file size on disk remains at 2GB as the data has been removed from the PST file, but the PST has not been compacted. The initial data size is still 1.8GB as this value is not modified after the first time the PST is discovered; this is the starting value. The current data size however is now showing as 0KB as all data has been removed from the PST.
- If the policy is not set to delete the PST file, the PST file utilizes 2GB storage on disk even though it contains no data. If the PST is not going to be deleted, enable compaction settings to reduce storage requirements. Once compacted, the PST file size shows as 265KB, the minimum file size. The initial data size still shows as 1.8GB as this is the starting value and is not updated.