I set up some replication tasks, to replicate datasets from one TrueNAS (‘source’) to another (‘backup’). I ran them once, and all files in the source datasets were copied to backup datasets (previously created, on the second TrueNAS), as I expected. No snapshots were involved. Some time later, I …

[image] Divad: No snapshots were involved. This is not possible: Replication is based on snapshots. No snapshot = no replication. It’s possible that TrueNAS automatically took a snapshot to complete the task, and you failed to notice. Sending updates will require further snapshots, preferab…

Replication only replicates snapshots, not files. It works on the ZFS block level. So you need to create a new snapshot (which will contain all files present at the time you do that) and replicate the snapshot. To work on the file level there’s rsync, but ZFS replication is faster and more reliable…

[image] etorix: Sending updates will require further snapshots I’ve just looked, and there is a pre-existing snapshot for that dataset. I assume that, if I looked, I would find pre-existing snapshots for all of the other datasets that I was able to replicate. I did actually create a (new) s…

[image] Divad: there are no snapshots on that TN There are, but you’re not looking in the right place. You have to either specifically mount the snapshot or browse the hidden snapshot directory. A snapshot is not a dataset or a folder, it is the state of a dataset at a point in time. Think b…

I did find some snapshots on the ‘backup’ TN. Not under top-level menu ‘Data Protection’, but under ‘Datasets’/Data Protection/View Snapshots. [image] etorix: A snapshot is not a dataset or a folder, it is the state of a dataset at a point in time. I understand the sense / idea of this, but …

[image] Divad: Presumably, the snapshot allows full reconstruction of the data, from just that one snapshot? Or are previous snapshots also required? I think I need to read some more. Just from that one snapshot. Each snapshot contains all data present at the time the snapshot was taken. Bu…

Snapshots defy math and logic. "THEY DON'T MAKE SENSE!" Why ZFS “snapshots” don’t make sense I’m comforted by this; at least it’s not just me. I’ll read it.

Here are the settings for one of the replications: [Rep1] [Rep2] [Rep3] Regarding the error in red, the dataset definitely does exist. I think I read somewhere that this might be a bug?

Actually they do make sense if you are experienced with Unix and the concept of hard links. A hard link is a second (third, fourth, …) directory entry with a name pointing to a certain inode. The inode contains all metadata of a file except the name. You can have 10 hard links pointing to the same …

Replication - how does it really work?

TrueNAS General

pmh March 25, 2026, 10:37am 7

Just from that one snapshot.

Each snapshot contains all data present at the time the snapshot was taken.

But if you have more than one snapshot all data that is shared between them is kept only once. Hence the space savings and the “differential” replication etc.

Read this and you shall be enlightened

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Replication - how does it really work?

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