PST files can have data added and removed from them under normal usage. For instance, a user may move emails from their mailbox to a PST file which will increase the data size. A user may also delete emails they no longer require from a PST file which reduces its data size.
Changes in the data stored in a PST do not necessarily have a direct effect on the PST file’s size on disk. Initially the PST file contains no data when first created. However, PSTs created by Outlook will have a file size of 265KB on disk even though they don’t yet contain any data. As you add data to a PST file, it will expand the file size to accommodate the addition of data. If however you then remove data from the PST, the file size will not change; only the data size is reduced. The PST file now contains white space. If you add more data to the PST file again it will utilize this white space first, before expanding the file size again. If you don’t add any further data to the PST file, then this 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. For more information, refer to the Microsoft Office article Reduce the size of Outlook Data Files (.pst and .ost).
When performing PST processing using ArchiveOne Enterprise, Express or PST Enterprise you will notice that these products all report on the initial data size, the current data size and the file size on disk. This gives you visibility of:
- the 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.
- the progress in processing a PST file.
- 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 would be to compact the PST).
For example, if you are using PST Enterprise to migrate messages from a PST to the owner’s mailbox you would expect to see:
- Once the PST file is discovered, the file size on disk, data size and initial data size are reported.
- A PST file with 2GB size on disk, may only contain 1.8GB data (ie. 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 is still 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 will not be 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 would utilize 2GB storage on disk even though it contains no data. If the PST is not going to be deleted, then the compaction settings should be enabled to reduce storage requirements.
- Once compacted, the PST file size will show as 265KB (the minimum file size) and the current data size will be 0K. The initial data size will still show 1.8GB as this is the starting value and will not be updated.