SIRIS, ALTO, and NAS: Troubleshooting low disk space in a storage array



This article gives guidance on how to solve issues caused by not having enough space in the storage array of your Datto device. Here are some examples of errors you might see:

  • "Local storage has less than (amount) available" alert
  • Device alert saying that the device is protecting more than 50% of its storage capacity
  • "Device filesystem is full" alert
  • "Disk space critically low" alert
  • "Backup skipped due to not enough free space" error


  • Datto SIRIS
  • Datto ALTO
  • Datto NAS


The solution to the problem depends on the underlying cause. Here are the possible causes for a full device:

  • The storage array is too small for the amount of data you need to protect.
  • The retention settings are too loose.
  • The retention policy cannot be enforced because the device is not synchronizing to the Datto Cloud in a timely manner.
  • The retention policy cannot be enforced because there are active recovery points preventing the deletion of data from the local device.
  • A software conflict is causing large backups.

The next sections will go over these causes and possible solutions for each.

Storage array is too small

Retention settings are too loose

Retention settings are set to a blank custom schedule

Active Restores

Device is behind on offsite synchronization

Software issue is causing large backups

Storage array is too small

To determine if your device is too small for the amount of data you need to protect, check out the article Properly Sizing a SIRIS or ALTO. If your device is indeed too small, you can try to tighten your retention settings. A better long term solution is a storage array upgrade. To determine if an upgrade is possible for your device, contact your sales representative.

Retention settings are too loose

Ensure that retention is set and is not too loose. For a full understanding of the retention process, see the article Understanding the Retention Process.

To check your retention settings, do the following:

  • Open the Remote Web interface for the device.
  • Click on the Protect tab, and you will see a list of protected machines.
  • In the list, check the Protected Storage field for each protected machine.
  • For the protected machines that are using a lot of Protected Storage, click the Configure Agent Settings link.

Figure 1: Protected machine listing

  • Click the Backup Retention link on the side of the window.
  • Compare your retention settings to that of the default settings.

Figure 2: Default retention settings

If your retention settings are too loose, adjust them for more frequent consolidation of recovery points and deletion of data. Then, you can Force Retention. If this fails to have any impact, and your device is sized properly, check to make sure that nothing is blocking the retention process. Things that can prevent data from being deleted through retention are Active Restores and the device being behind on Offsite Synchronization.

Retention settings are set to a blank custom schedule

If your retention is not running, check your Offsite Synchronization Interval schedule in the Remote Web interface:

  • Open the Remote Web interface for the device.
  • Click on the Protect tab to see all the different agents.
  • Under the agent that is having the issue, select Configure Agent Settings.
  • Under Offsite Synchronization Interval, check to see if it is on a custom sync schedule.
  • If so, ensure that the schedule is filled in with times and days in order for the offsite data to sync 

Figure 3: Default Offsite Synchronization Schedule

Active Restores

Since each active restore uses data from throughout the Inverse Chain of backups, an active restore might prevent the deletion of data through retention. To check if you have any active restores, do the following:

  • Open the Remote Web interface for the device.
  • Click on the Restore tab.
  • If you see an active recovery as shown in Figure 3, investigate whether you can remove it. Keep in mind that removing an active recovery will delete data.

Figure 4: Active Recovery

Device is behind on offsite synchronization

By default, one recovery point per day is flagged to be synchronized with the Datto Cloud. Once this recovery point is flagged, it cannot be deleted by the retention process until the Cloud synchronization is successful. If this synchronization is not working, then the point cannot be deleted, and therefore the retention process stalls. To check if this is the case on your device, follow these steps:

  • Open up the Remote Web interface for the device.
  • Click on the Protect tab. Look for protected machines with large data sets.
  • Click on Configure Agent Settings for these machines.
  • Click on Offsite Synchronization Interval. Make sure that Replicate is set to every day

Figure 5: Offsite Synchronization Interval

If these settings were not correct, change them and click Apply to All. This will change the setting for all protected machines.

Click on the Synchronize tab. The Offsite Job Status chart shows whether your Offsite Replication backlog is growing.

Figure 6: Synchronization is behind (example)

If synchronization is behind for all or most of your protected systems, you should refer to the article Off-Site Synchronization Configuration to configure your offsite synchronization settings. Also, see the RoundTrip Overview article to determine if you need a RoundTrip to get caught up on synchronization.

A software issue is causing large backups

Certain Windows applications can cause large block-level changes on the protected system. This can cause backups to be unusually large. To investigate this possibility further, see the article Explaining the Inverse Chain and Investigating Large Recovery Points.

If you need any further assistance please don't hesitate to create a ticket or call in to support

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