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Snapshots, Changes and Retention

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Joined: 3 years ago
Posts: 10
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Pure 2.0.8 installed on a QNAP backing up multiple VMs from an esxi server.

Question about retention. According to the info in the app

The number of snapshots to be kept, indirectly determines the retention time. If, for example, 7 snapshots are kept and a daily backup is created, 7-day old data can be restored. This default is applied to each VM when first configured.

We have indeed got the number set to 7.

What I am trying to understand is how this all works in practise.

Leaving aside backup Windows and all things being equal, am I correct in thinking:

On day 1, a full backup of each VM is taken.

On days 2 - 8 an incremental backup is done.

On day 9, a new full backup is taken and the cycle starts again.

Is it safe to assume that the first backup is not deleted until after the second full backup is successfully created?

Any further info on the mechanics would be gratefully accepted.

We're just trying to make sure that we do not have any potential vulnerabilities.

Thanks and Regards


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Marijan Kozic
Member Admin
Joined: 4 years ago
Posts: 95

Hi Nigel,

The way that backups work in Pure is that after a first full backup, there is no longer any need for further full backups. All following backups are incremental, relying on VMware's Changed Block Tracking and saving only those parts of the virtual disk that have changed since the last backup. Optional backup Verification in Pure protects against unlikely CBT failures by comparing the backup data with the live VM state.

Unchanged disk areas that were not part of a backup are shared from the previous backups. This means that when a backup is removed, it is possible that some of the data will remain as it is still needed for the subsequent backups. This means that each backup is independent and can be freely removed without impacting others.

Before each backup, Pure will check if there are any snapshots that are past their retention time and will remove them before creating a new backup. Removing an expired snapshot will remove this point-in-time references, preventing you to restore to that particular point but will only ever delete the backup data uniquely referenced by this particular snapshot.

Active Member
Joined: 3 years ago
Posts: 10
Topic starter  


Useful information to know. Thanks again for your help.