What Is Tiering in Azure File Sync

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One of the benefits of deploying Azure File Sync (AFS) is the ability to cloud-tier your code file server storage –it’s the primary reason, in my opinion. You can enable file sync in a granular manner: for each server endpoint in each sync group. For example, if I had a sync group for a share called Marketing with server endpoints synchronizing this share between three different file servers, I could enable a different tiering policy on each server.

When you enable tiering, the mechanism should not change how users see or use the data, or how permissions are applied – and that’s exactly how AFS does things. When a file is tiered to the cloud, the cold file is removed from the file server, and is replaced with a pointer or reparse point, with the same name and permissions, that points to the cloud copy of the file. From the users’ point of view, little has changed:

  • The file is in the same place
  • The filename has not changed
  • The permissions are the same as before
  • The icon is dimmed
  • The file now has an O (offline) attribute

Note: the tiering is handled by a filter driver called StorageSync.sys.


As I mentioned before, you can configure tiering on each server endpoint that you want to enable tiering on.
The settings are simple:

  • Cloud Tiering: Enabled or Disabled (default).
  • Volume Free Space (%): How much space should tiering try to free up on the volume that the folder is on.

Enabling cloud-tiering in an Azure File Sync server endpoint [Image Credit: Aidan Finn]
Enabling cloud-tiering in an Azure File Sync server endpoint [Image Credit: Aidan Finn]
Imagine that you have three server endpoints on D: of a file server, synchronizing with AFS, and you want to enable tiering:

  • D:\Shares\Accounting: 10% free
  • D:\Shares\Marketing: 20% free
  • D:\Shares\Sales: 30% free

When the sync agent on this file server receives the above policies, it will not implement three policies. It will take the largest amount and apply it across the three folders. The coldest files from all three endpoints will be tiered until 30% of drive space is free on the drive – if it is possible to free up 30% of drive space by tiering these folders.

Requirements of Tiering

There are some requirements for tiering:

  • You cannot enable cloud tiering with a server endpoint on a system drive.
  • Files must be at least 64 KiB in size.
  • The anti-virus on the file server should understand the “O” attribute to avoid downloading cloud-tiered files during a scheduled scan.
  • It is recommended that you use Azure Backup to protect the cloud-tiered (and all synchronized) files instead of on-premises backup, also to avoid unwanted downloads.
  • Data deduplication must not be enabled on the server endpoints volume.