SIRIS, ALTO, and NAS: Understanding the retention process



This article explains the function of the retention feature on Datto appliances.


  • Datto SIRIS
  • Datto ALTO
  • Datto NAS
  • Datto Cloud
  • Private Cloud


Understanding Datto's Inverse Chain backup technology is key to understanding retention settings on a Datto appliance.

Figure 1: Datto's Inverse Chain

The Inverse Chain Technology engine is sparse, efficient, reliable, and virtualizes in an instant. Each backup creates a recovery point on the Datto device. Incremental backups contain only the data that is unique to that snapshot's point in time.  In other words, the data that has changed since the last incremental backup. For most backup engines, this is where the innovation ends. If part of a recovery point becomes lost due to data corruption, all of the recovery points after that point are useless, because they depend on a broken piece of the chain. The Datto Inverse Chain does not have this limitation. If one recovery point is lost, the recovery points after it refer to data that is elsewhere in the chain, and can rebuild themselves in an instant.

Local Retention

The local retention process runs at the end of every backup and nightly as part of Datto scheduled operations. You can also force retention to run the process in between backups. When retention runs, specific recovery points get deleted from the chain. Think of it as data pruning.

Since each backup point only contains the data that is unique to itself, this may not remove as much data as you might think, because the device moves much of the information to other recovery points rather than deleting it.

Retention only deletes recovery points not flagged for offsite replication. Any points queued for replication which have not synchronized with the cloud will remain. Agents with a backlog of points pending replication might appear to use more space on the Datto appliance than normal until their offsite synchronization is current.

For Paused and Archived agents, retention will continue to run by default, according to a nightly job schedule, so you should configure your custom retention settings accordingly

Example Retention Process

This example explains how local retention works using customized settings, as shown in Figure 2.

Available retention settings can vary depending on your billing policy. Each policy has different features, and the choice of a policy depends on your circumstances.

Figure 2: Customized Local Backup Retention Settings

Figure 3 shows how the retention process prunes recovery points.

Figure 3: Example of the local retention process (click to enlarge)

Intra-Daily to Dailies

Pruning of intra-daily backups takes all the snapshots from one day and leaves one point from that day in the chain for recovery purposes. Which point remains depends on when retention runs, but it is usually either the first point or the last point of the day. In the above example, intra-daily backups will have pruning run on them after seven days. Up to the seven-day mark, all backups for all days remain available. After seven days, only one backup from the pruned period remains available; this point is the last backup of each day.

Dailies to Weekly

After one week (in this example), the daily recovery points created in the previous step prune down to one weekly recovery point; this point is the last backup of each week.

Weeklies to Monthly

The default setting is to keep weekly recovery points for one month. After one month, all recovery points except for the last backup of the month get pruned.

Keep Local Backups

This setting schedules when to delete all local recovery points. The default is to keep the monthly recovery points for three months.

Cloud Retention

Except where noted, for this article, the term 'Cloud' applies to both the Datto Cloud and Private Cloud storage.

Cloud Retention (sometimes called 'Offsite Retention') works much in the same way as local retention. When the retention runs, the default action for a protected system with no changes to its offsite sync or retention policies is to preserve consistent daily points for the last backup of the day, both locally and in the cloud.

There are some important things to note, however:

  • Offsite (Datto Cloud) retention runs on a regular schedule: every day at 2:00 AM EST.
  • The offsite (Datto Cloud) retention process deletes 300 recovery points at a time. Therefore, it may take some time to consolidate very large data sets.

Keep in mind that, with the default settings on a Datto device, only one recovery point per day replicates to the cloud. Therefore, there is no Intra-Daily to Daily retention, and no points are marked for being deleted in retention. The result is that the only setting that affects the size of offsite data points is the Keep backups in the cloud for... option.

If you have purchased Infinite Cloud Retention, but you have specific protected systems which must retain data only for a set period, you can still customize the retention settings for your device. However, you can only do so within the limits of the plan and at the level of granularity which offsite retention permits. If you have purchased Time-Based Retention, you will not be able to modify its cloud retention settings. For more information about these plans, see the BCDR Terms of Use: Pruning of Incremental Backups for Infinite Cloud or Time-Based Retention Schedules document on the Datto Partner Portal, or contact your Datto Sales Executive.

Figure 4: Datto Cloud Backup Retention, default

Figure 5: Example offsite retention process (click to enlarge)

Unless you specify custom retention values, offsite recovery points are subject to immediate and automatic pruning based on a pre-defined schedule of intra-dailies to dailies after seven days, dailies to weeklies after two weeks, and weeklies to monthlies after six weeks. Monthly backups prune on a rolling basis, oldest first, after one year. 

Resist the temptation to set your Offsite Retention policy to the same settings as your Local Retention. For this to have any effect, you would have to send every recovery point to the cloud, which is unnecessary and not recommended.

For more details about offsite retention and cloud billing plans, see our BCDR Terms of Use or contact your Datto Sales Executive.

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