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Azure File Sync cloud tiering saves space on a Windows Server volume by keeping the file and folder structure locally while moving selected file content to an Azure file share. Users can still see the files in their usual locations; opening a tiered file may require Azure File Sync to recall its content from the share. Tiering is optional and is primarily a way to manage the server’s local cache—not a separate file hierarchy.
What cloud tiering keeps on the server
Azure File Sync separates a file’s namespace from its content. The namespace includes the file and folder entries and their properties; it remains available on the server. When a file is tiered, its content is stored in the Azure file share rather than fully occupying local disk space.
Windows represents a tiered file locally with a reparse point. The file has NTFS offline and FILE_ATTRIBUTE_RECALL_ON_DATA_ACCESS attributes. This lets the file remain visible in its folder even when its full contents are not resident on the server.
Tiering does not necessarily mean that every byte of a file is either entirely local or entirely in the cloud. Depending on how an application reads a file, Azure File Sync may recall only part of its content.
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What happens when someone opens a tiered file
When a user or application requests data that is not present locally, Azure File Sync recalls the required content from the Azure file share. The request therefore depends on connectivity and recall performance, unlike a read served entirely from the local cache. Microsoft recommends at least 1 Mbps of network bandwidth for recalling tiered files; below that, a recall can time out. This is a minimum recommendation, not a guarantee of a particular response time.
More aggressive tiering can leave less content cached locally. That can mean more frequent recalls, added wait time for users and applications, and Azure transfer or transaction costs. The appropriate balance depends on how often files are accessed, how much data users typically need, and how much local storage and network capacity are available.
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How Azure File Sync decides what to tier
Cloud tiering provides two policy controls: a volume free-space policy and a date policy. The free-space policy takes precedence when both policies would apply.
| Policy | What it controls | Practical implication |
|---|---|---|
| Volume free space | Targets a percentage of the volume to keep free by tiering eligible content. | It applies to the volume, not to a particular directory or individual server endpoint. If endpoints share a volume, their settings can interact. |
| Date | Tiers files that have not been read or written for the configured number of days. | It is useful when older or less recently used files are more likely to be outside the active working set. |
Microsoft’s policy guidance recommends starting with the date policy disabled and setting the volume free-space target to 10–20%; 20% is described as a good starting point for many volumes. Treat that as a starting point rather than a universal optimum. A busy working set, frequent file changes, limited bandwidth, or strict recall-latency needs may call for a different balance. Increasing the amount of local cache can reduce recalls and related transfer costs, but uses more on-premises storage.
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Choose settings for the volume and workload
- Check the volume and endpoints. Identify which server endpoints use the volume and how much free space it has. Because the free-space policy is volume-scoped, do not treat it as a separate reserve for each endpoint.
- Start with a conservative cache target. Use Microsoft’s 10–20% free-space starting range as a guide, with 20% as a typical initial setting. Leave the date policy disabled initially unless the workload gives you a clear reason to use age-based tiering.
- Consider how people and applications access files. Frequently reopened files, high-churn data, and applications that read large amounts of content can make a smaller local cache disruptive. Less frequently used files are better candidates for being absent from the local cache.
- Check the effective policy on shared volumes. Microsoft’s monitoring guidance says that where multiple server endpoints share a volume, the highest configured free-space policy becomes the effective policy for all endpoints on that volume.
- Monitor results and adjust gradually. Review free space, cache hit rate, recall size and throughput, recall success rate, and the amount of data tiered. If recalls are more frequent or larger than users can tolerate, consider retaining more local cache, then monitor the effect.
Which files can benefit from tiering?
Tiering does not save space equally for every file. The minimum eligible file size depends on the NTFS volume’s cluster size: Microsoft documents a minimum of twice the cluster size, with an 8 KiB minimum for volumes with cluster sizes of 4 KiB or less. Cloud tiering supports volumes with cluster sizes up to 2 MiB. Very small files may therefore remain local rather than yielding meaningful space savings.
Monitor recalls and diagnose unexpected behavior
Azure portal and Azure Monitor provide cloud-tiering status and metrics. Microsoft’s monitoring guidance, last updated 2026-08-13, identifies signals that help distinguish a cache that is too small from a network or application-driven recall problem:
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- Current free space: Shows the volume’s available capacity in relation to its target.
- Cache hit rate: Indicates the bytes served locally rather than recalled from Azure. A low rate can point to a mismatch between the retained cache and the active working set.
- Recall size and recall size by application: Show how much data is being recalled and can help identify an application generating substantial recall load.
- Recall throughput and recall success rate: Help assess recall performance and whether recalls are completing successfully.
- Amount of data tiered: Shows how much content has been moved out of the local cache.
If recalls are not behaving as expected, first check the configured policy, effective volume policy, current free space, and these metrics. Microsoft’s troubleshooting documentation also identifies agent telemetry events 9005, 9006, 9009, and 9059 for investigating recall activity.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Low disk space mode and deliberate recalls
Low disk space mode
Microsoft’s overview says low disk space mode is available with Azure File Sync agent versions starting from 15.1. It can trigger more proactive tiering when free space drops below a calculated threshold: the minimum of 10% of the volume size, the configured free-space policy expressed in GiB, and 20 GiB. In this mode, recalls initiated by users are non-persistent, meaning recalled content is not kept as a lasting addition to the local cache. The documented exception is a recall initiated with Invoke-StorageSyncFileRecall. Verify that the installed agent version supports this behavior before relying on it operationally.
Recall files deliberately
For planned recalls, Microsoft documents the Invoke-StorageSyncFileRecall PowerShell cmdlet, including options for thread count and retries. Follow the cmdlet’s documentation for the parameters appropriate to the operation; the network recommendation remains at least 1 Mbps for recalls.
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