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PSTs Should Never Be in Cloud Storage
Updated: July 14, 2021
On May 17, Microsoft published message center notification MC256835 to advise tenants about the introduction for what they call a “PST version retention policy.” This has nothing to do with retention labels or retention policies. Instead, it’s about controlling PST storage in SharePoint Online by limiting the number of versions kept for PST files stored in SharePoint Online and OneDrive for Business document libraries.
Versioning in SharePoint Online
Versioning is a SharePoint feature. In a nutshell, as users make changes to files in document libraries, they create versions of the files. In some cases, such as when editing Office documents using Autosave, a single edit session might generate twenty or thirty versions, depending on the number of changes made. The number of versions kept in a document library is defined in library settings (Figure 1) in a range of 300 to 50,000.
SharePoint keeps multiple versions of files to ensure that the user can go back to a previous version. To do this, select a document and then Version history. You can then select a version to restore (Figure 2).
Both SharePoint Online and OneDrive for Business also support options to restore a library to a point in time over the previous 30 days. Without versions, it would not be possible to do this.
Why PSTs End up in SharePoint and OneDrive
Versioning is good, so what’s the problem with PSTs? Before addressing that question, we should ask about why PST storage in SharePoint Online or OneDrive for Business comes about. A PST (Personal Storage Table) is for email storage. It is a container to allow users to store messages they wish to keep. People might have moved PSTs from network file shares into SharePoint, but it’s a bad idea to use PSTs in SharePoint.
- The PST file format is not intended for concurrent shared access. These are personal files. If a problem happens with a PST file stored in SharePoint, it might lead to data loss.
- Even though they are in SharePoint, the messages stored in PSTs are inaccessible for eDiscovery.
- Over the years, Microsoft consistently advised against the use of shared PSTs on network file shares because of the potential for corruption.
You might think the problem of concurrent access to a shared file is addressed by using the OneDrive sync client to have a local copy of PSTs synchronized with the master copy in SharePoint. But as pointed out in this post by a Microsoft support engineer, the way Outlook locks PST files for exclusive access creates many problems for the sync client (Figure 3). Basically, the sync client is frustrated by the lock taken out by Outlook and can’t process the PST.
People who replace local workstation storage with OneDrive for Business for well-known folders like Documents might end up with PSTs in OneDrive. To avoid problems, they should move these files out of a synchronized location.
The Impact of PST Storage in SharePoint Online
The problem now being addressed by Microsoft is that holding multiple PST versions can consume a huge amount of SharePoint storage quota. Remember, a PST is a container rather than an individual file, and if it’s in active use, Microsoft says this generates “multiple versions which leads to storage being quickly consumed.”
Because of the generous quotas available to OneDrive for Business users, consuming storage is less of an issue for OneDrive for Business than it is for SharePoint Online. Microsoft makes 1 TB plus 10 GB per licensed user available for the organization and charges extra if more storage is needed. Using retention labels and retention policies to ensure files cannot be removed from SharePoint can already consume large amounts of storage, so adding PSTs to the mix is like pouring fuel on a raging fire.
Microsoft’s solution is to retain no more than 30 days’ worth of PST versions. This is enough to ensure that the Restore library feature works, even when PSTs are in a library. While the best answer is not to allow users to store PSTs in SharePoint Online or OneDrive for Business, restricting versions for PSTs is an acceptable method to restrain storage demand. Organizations can block users from synchronizing PSTs by including the file type in the blocked files list defined in the Settings section of the SharePoint Online admin center (Figure 4). Given the impact this could have on users, it’s a good idea to communicate about the block before its implementation.
Microsoft Implements the New Policy
Starting June 28, organizations can use the Set-SPOTenant cmdlet from the SharePoint Online PowerShell module to control the new policy: By default, the policy will be on, meaning the permanent deletion of PST versions once they reach 30 days old. If you don’t want to restrict PST versions, you can opt-out from the policy by running:
Set-SPOTenant -DisableOutlookPSTVersionTrimming $True
The new switch for the Set-SPOTenant cmdlet is available in the 16.0.21411.12000 release of the SharePoint Online management shell (released on July 12). You can download the module from the PowerShell Gallery.
The opt-out command must be run by August 13, so organizations have roughly six weeks to decide to opt-out. The policy becomes effective on August 16 and running the command to opt-out afterwards will have no effect. The big caveat is that the opt-out applies only to existing libraries. Any new library created after August 13 will apply the 30-day retention for PST versions.
The Badness of PSTs
I’ve been trying to persuade organizations to stop using PSTs for years. They’re a 25-year-old answer to the problem of small server mailboxes which existed then and doesn’t now. PSTs are insecure, compromise the ability of organizations to search for information and apply compliance policies, and prone to failure. There is nothing to recommend their continued use and even less to think that it’s a good idea to store PSTs in SharePoint Online and OneDrive for Business. If you’re still unconvinced, listen to this on-demand webinar Why PSTs are Such a Bad Idea in the Cloud, where I try my very best to explain why.
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