Backup working (?) but now system is full and I can't access the data

I must have done something wrong. I recently posted about backing up my TrueNAS Core system to an external hard drive using periodic snapshots and replication, and that seems to be working now. However, while it is filling up my external drive, it also seems to be filling up my NAS system. It now says it’s at 100% and while I can access the GUI and the system is running, I can’t get in via SMB and I can’t play anything from the Plex server, although it also is still running. Note that a few days ago it was at 94% despite deleting a bunch of items, and I haven’t added anything to it since then. The jump from 94% to 100% wasn’t from any data that I added.

Are my snapshots getting added to the primary dataset and then not deleted when moved to the external drive? It seems like maybe that’s happening. I go to look at snapshots in the GUI, and there are a bunch that are listed under the primary data pool name, as well as a bunch under the backup pool name. Can I just delete all the primary pool ones? Is there a way to make it not do this? Or am I way off about all of this?

Thanks…

I think you might be confusing ZFS operations with traditional file-based operations.

If you’re going to replicate snapshots from a source to a destination (and continue to do so incrementally over time), you cannot simply “delete” the source’s snapshots because “they now exist on the destination”. You need to always keep a common base snapshot between the source and destination.

Only the snapshots older than the common base snapshot (which exists on both sides) can be safely deleted.


I don’t understand what that means? Pool’s can get “filled”, not the “NAS system”. Are you referring to your primary pool also hitting 100% used capacity?


When you delete data on a dataset’s live filesystem, it still remains within any snapshots that reference them. (Even though they “don’t exist” from the perspective of the live filesystem.)

As for the used space climbing, new data is obviously being written to the filesystem. There’s no other way it can just climb for no apparent reason.

For all anyone knows, you might have Plex creating encodes/transcodes that remain in a temporary location somewhere in the media storage path.

Is there a way to determine the common base snapshot so I can delete the older ones?

Maybe a more appropriate question: Is there a way I can do incremental backups that don’t significantly increase the data stored in the primary pool? I feel like it’s been growing exponentially (that may be hyperbole, but still) since I turned on the replication tasks. For what it’s worth, it’s set up to retain snapshots for 4 weeks, and do take a new one nightly. I am not (manually) adding large amounts of data on a regular basis, so I assumed most of the nightly snapshots would be fairly small.

To clarify: Yes, I meant 100% of the primary data pool. Also, yes, something is definitely getting added…I just haven’t been doing it manually (and haven’t really been using Plex much either over the last week). FWIW, I have 6 3TB drives, so about 12TB of useable space. I had some previous backup thing running and backing up my desktop to the NAS, and there were a couple TB in there, which I deleted, but the size of the pool never seemed to change much. I’m guessing that’s because, as you pointed out, it’s still included in the previous snapshots.

It’s most likely the newest snapshot on the destination (since the last time you ran a replication), and you can check if it exists on the source.


Snapshots never “add” data. They can, however, “retain” space that would otherwise be freed.

So if the pool’s used space keeps climbing, then something is getting written to it. Sure, you can keep pruning snapshots, but it doesn’t address why you have a significant amount of data regularly being written.

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Hmmm…OK interesting. I clearly have not been understanding how snapshots work but that makes sense. So yeah, I do need to figure out what is getting written.

I will try to delete the oldest snapshots and hopefully free up enough space to access the data, and then hopefully figure out where the additions are coming from.

Thanks for all the info!

I try to explain how snapshots work, how to use them, and how to delete them in this video:

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Super helpful, thank you!