Personal Storage Table - Overview

Overview

In Microsoft Exchange Server, the messages, the calendar, and other data items are delivered to and stored on the server. Microsoft Outlook stores these items in a personal-storage-table (PST) or off-line-storage-table (OST) files that are located on the local computer. Most commonly, the PST files are used to store archived items and the files to maintain off-line availability of the items.

The size of these files no longer counts against the size of the mailbox used; by moving files from a server mailbox to PST files, users can free storage space on their mailservers. To use the .pst files from another location the user needs to be able to access the files directly over a network from his mail client. While it is possible to open and use a PST file from over a network, this is unsupported, and Microsoft advises against it, as PST files are prone to corruption when used in this manner.

Both the PST and .ost files use a fixed-block-based allocation scheme; the file is enlarged by a fixed amount of bytes, and the file internally maintains information about the allocated and non-allocated blocks. So, when data files like email messages are added to a PST file, its file size is automatically adjusted by the mail client (if necessary). When mail is deleted from a PST file, the size of the PST file will stay the same, marking the space as unallocated so that it will hold future data items.

To reduce the size of PST files, the user needs to compact them.