SIRIS, ALTO & NAS: Backup skipped due to not enough free space



A backup fails with the error message:  "Backup skipped due to not enough free space. No recent error reported from ShadowSnap Agent."


  • Datto SIRIS
  • Datto ALTO


The primary storage capacity of the local Datto SIRIS device has reached maximum capacity. In order to resume local backups, you must free up space.

This may be caused by a variety of underlying conditions:

1. There is an application or process running that is conflicting with ShadowSnap and the Microsoft VSS Writers. In result to the constant changes, new base images are being taken by the device as a fail-safe measure.

2. The local Datto SIRIS is attempting to protect a Total Protected Data Set Size which exceeds 50% of its size with default retention in place.

3. The retention policy is set too low causing the device to fill up.

4. Large amounts of changes are occurring on the machine's volume(s) resulting in large recovery points.

5. The protected system or its volumes are in an unhealthy state. As a troubleshooting measure, the device has triggered a new full backup as ShadowProtect tries to avoid corruption to the backup chain.


Solution to reclaim space

1. Stop all backups on the Datto device.

  • Go to the web interface of the local device.
  • Under the Configure tab, select Device Settings.
  • Change the Concurrent Backup Setting to 0 so storage can be accessed when space is cleared.

2. Datto recommends a local retention of 5 days of intradailies, 1 week of dailies, and 2 months of weeklies stored on the SIRIS unit. Tighten up retention settings in order to remove any points no longer needed.

  • Go to the web interface of the local device.
  • Under the Protect tab, select the Configure Agent Settings of the agent.
  • Under the Basic category, click Local Backup and Retention Policy, and adjust the retention settings as needed.
  • Under Advanced, click Force Retention to apply the new retention policy to the agent's dataset. Then, return to the Protect tab and start a new backup.

3. Manually remove recovery points that are already offsite or unnecessary points no longer needed. The most space will be cleared by starting from the oldest recovery points.

  • Go to the web interface for the local device.
  • Under the Protect tab, select Manage Recovery Points for the protected machine.
  • You can remove individual recovery points, or all of the Local data for a protected machine.
Due to the inverse chain, the size displayed by the Manage Recovery Points listing does not necessarily represent the space that will be freed by deleting it.

4. If the device still cannot take a backup, contact Datto Technical Support.

Solution for Underlying Condition(s)

1. Remove recovery points that have already been replicated off-site via the Manage Recovery Points tab.

2. Adjust the retention policy outlined in the basic options section. This will decrease the number of snapshots retained locally. Datto Technical Support can enforce the retention operations on the device.

  • Retention operations are enforced on every backup.
  • Snapshots that are marked to go offsite cannot be deleted by retention.
  • If a snapshot appears to be stuck and is not being enforced by retention, contact Datto Technical Support in which they will assist with removing a stuck restore that that is preventing the snapshot from being deleted.
  • Deleting recovery points from the bottom up is the most effective way to gain additional storage space for the device.

3. When two applications are attempting to use the VSS writers, it can cause a conflict which can result in large backups. See VSS Explained to learn how to check the protected system for conflicting VSS services.

Suggested: Remove the conflicting application or process which is resulting in the conflicts with the Microsoft VSS Writers.

 4. On the local web interface, the section in the left hand column under Storage Status, Total Protected should be 50% or less of the size of your device.

 Visit this Knowledge Base article on Maintenance of Protected Systems.

Example: A S500 (500GB model) should have a Total Protected value of 250GB or less.

 5. The sizing algorithm addressed in Solution for Underlying Condition Path #2 applies only to Datto SIRIS devices with default retention values in place.  If longer retention policies are required please contact your Datto Sales Associate to size out a device for an extended retention policy.

 6. Run a chkdsk /r on the protected machine to check the status of the drives. Run Disk Defragmenter to fix possible errors that might have occurred when checking disks.

Alternatively: Crash consistent state backups can be taken by configuring the Datto device to perform backups with the STC Backup Engine as opposed to the default Application Aware VSS with STC fallback.

This enables the Storage Craft VSS Provider and not the Microsoft VSS Provider. This is not the primary suggested path as it will not provide application level support and will only provide backups in a bad state.

Proper sizing

To avoid this problem, ensure that your Datto device is properly sized and frees up used storage. See Properly Sizing a Datto Siris Device (applies to ALTO as well).

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