The Preservation Hold Library Is a Storage Trap. It's Also Replaceable.
In a SharePoint Online tenant with active Microsoft Purview retention policies, the Preservation Hold Library silently accumulates copies of every deleted or modified document — preserved in expensive SharePoint pool storage rather than disposed of. For enterprises running retention at scale, this is the single largest unexplained driver of SharePoint storage growth.
The good news for IT and compliance teams is that the Preservation Hold Library is not the only way to satisfy "preserve deleted content for compliance" requirements. SmiKar's Squirrel includes a feature called Recycle Bin Capture that delivers the same operational outcome — deleted content preserved, recoverable, auditable — but archives the preserved content to the customer's own Azure Blob Storage instead of expensive SharePoint pool storage.
This post explains how the replacement works, when it is appropriate, and where the Preservation Hold Library still has a legitimate role.
Why the Preservation Hold Library Grows
When a Microsoft Purview retention policy or retention label is active on a SharePoint site, the SharePoint platform itself enforces the preservation requirement by intercepting deletion and modification actions. The user appears to delete the file. The platform actually moves a copy of the original into a hidden Preservation Hold Library on the same site, where it remains for the duration of the retention period.
The architectural consequence is that every modified or deleted document subject to retention generates a copy in the Preservation Hold Library, and the copy counts against the tenant's SharePoint pool quota at SharePoint pool storage rates. Per Microsoft's retention documentation:
"When a user changes an item that's subject to retention from a retention policy or a retention label that marks items as a record, or deletes any item subject to retention, the original content is copied to the Preservation Hold library."
For a tenant with thousands of users and millions of documents under active retention, the Preservation Hold Library can grow to multi-terabyte size silently. It is hidden from normal SharePoint navigation. It is invisible to standard users. And it sits at the same $0.20/GB/month SharePoint overage rate as any other SharePoint pool storage above the included tenant entitlement.
This is the operational pattern that drives most of the "why is our SharePoint storage growing so fast" investigations across regulated industries.
The Squirrel Recycle Bin Capture Architecture
Squirrel's Recycle Bin Capture intercepts the preservation requirement at a different point in the SharePoint lifecycle — not by hooking into the platform-level retention enforcement, but by monitoring the SharePoint recycle bin across the tenant.
The mechanism:
- Continuous recycle bin monitoring across every site in the tenant
- Configurable retention window — typically around seven days — during which deleted content stays in the SharePoint recycle bin as normal
- Automatic archive to customer-owned Azure Blob Storage at the end of the retention window
- A dedicated SharePoint site (commonly named "Recycle Bin Capture") that holds a lightweight stub for every archived item, preserving the file name, original location, deletion metadata, and a recovery link
The operational outcome is that deleted content remains preserved, discoverable, and recoverable for the configured retention period, but the storage burden sits in customer-owned Azure Blob Storage instead of the SharePoint pool. End users searching for accidentally deleted content can find it via the Recycle Bin Capture site. Audit and compliance teams can review what has been preserved with full chain-of-custody.
The full product detail is on the Squirrel Recycle Bin Capture page.
How the Costs Compare
The economic difference is the headline outcome. The same compliance requirement — preserve deleted content for the retention period — produces dramatically different storage costs depending on which mechanism handles the preservation.
| Component | Preservation Hold Library | Squirrel Recycle Bin Capture |
|---|---|---|
| Where preserved content lives | SharePoint pool storage (the same expensive tier as active document libraries) | Customer-owned Azure Blob Storage (Hot, Cool, Cold, or Archive tier - customer chooses) |
| Counts against SharePoint tenant pool? | Yes - silently inflates pool consumption | No - lives entirely outside the SharePoint pool |
| Per-TB-per-month cost above tenant entitlement | $0.20/GB at Microsoft's published SharePoint overage rate | Azure Blob storage at the customer's chosen tier, typically a fraction of the SharePoint overage rate |
| Visible to SharePoint admins? | Hidden system library, not surfaced in normal navigation | Dedicated SharePoint site with stubs - fully visible and auditable |
| Configurable retention window | Tied to the retention policy or label configuration | Configurable per tenant (typically ~7 days before archive triggers) |
For an enterprise with 100 TB of accumulated Preservation Hold Library content, the recurring SharePoint pool cost is in the hundreds of thousands of dollars per year at Microsoft's published rates. The same content at Azure Blob Storage Cool tier in the customer's own tenant is a fraction of that figure, and Azure Blob Archive tier is dramatically cheaper still.
Documented customer outcome
At one enterprise SmiKar customer, the Preservation Hold Library had silently accumulated over 100 TB of preserved content over years of active retention policy enforcement - all sitting in SharePoint pool storage and counting toward the tenant entitlement. After turning off the operational retention policies driving the PHL growth and switching to Squirrel's Recycle Bin Capture for the same compliance outcome, the customer's tenant storage was reduced by an additional 100 TB on top of the active-content archiving Squirrel was already performing. The PHL content moved into customer-owned Azure Blob Storage at Azure tier rates rather than SharePoint pool rates, and the compliance position around deleted-content preservation was unchanged.
This is the magnitude of the Preservation Hold Library storage trap that most enterprises do not realise they have until they audit it.
The full economic picture is covered in how the Preservation Hold Library inflates SharePoint storage costs.
How the Replacement Works in Practice
The pattern is straightforward. The customer turns off the retention policies that are driving Preservation Hold Library growth, and Squirrel's Recycle Bin Capture handles the deletion-preservation requirement instead.
The compliance posture is materially equivalent:
- Deleted content is preserved for the configured period
- Deleted content is recoverable through the dedicated Recycle Bin Capture SharePoint site
- Deleted content is auditable with full chain-of-custody
- The preservation is automatic, not user-controlled
The economic posture is dramatically better:
- Storage lives in customer-owned Azure Blob Storage at Azure tier rates
- The customer chooses the storage tier (Hot, Cool, Cold, Archive)
- No silent SharePoint pool inflation
- No dependency on Microsoft's SharePoint overage billing
For the broader compliance design context of which Microsoft Purview construct applies to which scenario, see retention policies vs retention labels in SharePoint and sensitivity labels vs retention labels in SharePoint.
How the Recycle Bin Capture Pattern Interacts with Microsoft Copilot
A consideration that affects 2026 deployment decisions: SharePoint content remains discoverable by Microsoft Copilot only while it is indexed in the SharePoint search index. Content in the standard SharePoint recycle bin is excluded from the index. Content in the Preservation Hold Library is excluded from the index. Content archived via Squirrel Recycle Bin Capture is also out of the SharePoint search index by default - it lives in Azure Blob Storage and only the stubs in the Recycle Bin Capture site remain in SharePoint.
This is a different consideration from active Squirrel archiving with Nutshell AI, where archived files retain Microsoft Copilot grounding via embedded summaries in the stub. Recycle Bin Capture is the deletion-preservation pattern, not the active-document-archiving pattern.
For full Microsoft Copilot grounding behaviour on archived content, see Microsoft Copilot and archived SharePoint content.
Operational Considerations Before Switching
Before adopting the Recycle Bin Capture pattern in place of the Preservation Hold Library, four operational checks are worth running.
1. Audit the current Preservation Hold Library size
Most enterprises do not know how much of their SharePoint pool consumption is Preservation Hold Library content. The SharePoint Storage Explorer free Windows tool surfaces this in one view. The size of the existing Preservation Hold Library determines the magnitude of the cost benefit.
2. Confirm which retention configurations can be turned off
A SharePoint tenant typically has multiple Microsoft Purview retention policies, retention labels, and possibly legal holds in effect simultaneously. The Recycle Bin Capture pattern replaces retention-driven Preservation Hold Library growth, not legal-hold-driven preservation. The compliance team needs to identify which retention configurations are operational (and can be replaced) versus statutory (which should remain).
3. Design the recycle-bin retention window
Squirrel's Recycle Bin Capture is configurable for how long content stays in the SharePoint recycle bin before being archived. Seven days is typical, but customers can extend this for scenarios where immediate end-user recovery within SharePoint is preferred. Longer windows mean more content sits in the recycle bin (which itself counts against the SharePoint pool); shorter windows move content into archive faster.
4. Plan the existing PHL drain
Existing Preservation Hold Library content does not automatically migrate when retention policies are turned off and Recycle Bin Capture is adopted. Squirrel can archive existing Preservation Hold Library content into Azure Blob Storage as part of the initial deployment - this is where the largest one-time storage reduction typically comes from.
Frequently Asked Questions
Q: Is the compliance posture equivalent?
A: Yes. Both mechanisms preserve deleted content for the configured period, both make it recoverable, both produce an audit trail. The difference is where the preserved content physically lives - the Preservation Hold Library uses expensive SharePoint pool storage, Recycle Bin Capture uses customer-owned Azure Blob Storage at the customer's chosen tier.
Q: Can users still recover their own accidentally deleted content?
A: Yes. The Recycle Bin Capture site in SharePoint contains a lightweight stub for every archived item, with original file name, original location, and a recovery action. Authorised users can search this site and trigger restoration of any archived item through the familiar SharePoint interface.
Q: What happens to existing Preservation Hold Library content when we switch?
A: It does not automatically migrate when retention policies are turned off. Squirrel can archive existing Preservation Hold Library content into Azure Blob Storage as part of the initial deployment, which is typically where the largest one-time storage cost reduction comes from.
Q: How does this affect Microsoft Copilot's ability to find archived content?
A: Recycle Bin Capture archives deleted content - by definition this content is not active SharePoint content and would not have been part of the SharePoint search index even before deletion. Microsoft Copilot grounding on active SharePoint content is unaffected. For Copilot grounding on actively-archived (not deleted) SharePoint content via Nutshell AI, see the dedicated documentation on that separate Squirrel capability.
Q: What is the typical configuration window?
A: Most deployments configure a seven-day window during which deleted content stays in the SharePoint recycle bin as normal, after which Squirrel automatically archives it to Azure Blob Storage. The window is tenant-configurable.
Audit Your Tenant Before You Change Anything
Whichever direction the compliance and IT teams decide to go, knowing the current Preservation Hold Library size and SharePoint pool consumption first makes the design decision defensible. SharePoint Storage Explorer is a free Windows tool that surfaces site, library, and file-type consumption across the tenant in one view - including the size of the Preservation Hold Library content that is invisible to standard SharePoint navigation.
For the broader context on how Squirrel approaches SharePoint archiving across both active content and deleted content, see the SharePoint archiving guide and the enterprise buyer's checklist. For the customer case studies documenting the operational pattern at petabyte scale, see the Squirrel customer case studies.
