Snapshot size much larger than data in the set

TrueNAS General
1

Very new to Truenas, I setup a SMB share for PC Backups, wondering why my available space is getting used up. I noticed that my data set is at 13Tb but the data is only 4.13Tb. Snapshots are taking up 9.17Tb.

Snapshot for this dataset is set to once a day and it was set to 2 weeks retention.

I have ran zfs list -o space

tank/pcbackup 9.48T 13.3T 9.17T 4.13T 0B 0B

I also ran zfs list -r -t snapshot -o name,creation,used,refer tank/pcbackup

tank/pcbackup@auto-2025-08-18_10-00 Mon Aug 18 10:00 2025 682K 5.37T
tank/pcbackup@auto-2025-08-19_10-00 Tue Aug 19 10:00 2025 781K 5.38T
tank/pcbackup@auto-2025-08-20_10-00 Wed Aug 20 10:00 2025 760K 5.38T
tank/pcbackup@auto-2025-08-21_10-00 Thu Aug 21 10:00 2025 838K 5.40T
tank/pcbackup@auto-2025-08-23_10-00 Sat Aug 23 10:00 2025 881K 7.42T
tank/pcbackup@auto-2025-08-24_10-00 Sun Aug 24 10:00 2025 689K 4.13T
tank/pcbackup@auto-2025-08-25_10-00 Mon Aug 25 10:00 2025 724K 4.14T
tank/pcbackup@auto-2025-08-26_10-00 Tue Aug 26 10:00 2025 739K 4.15T
tank/pcbackup@auto-2025-08-27_10-00 Wed Aug 27 10:00 2025 717K 4.17T
tank/pcbackup@auto-2025-08-28_10-00 Thu Aug 28 10:00 2025 753K 4.18T
tank/pcbackup@auto-2025-08-29_10-00 Fri Aug 29 10:00 2025 824K 4.19T
tank/pcbackup@auto-2025-08-30_10-00 Sat Aug 30 10:00 2025 646K 4.10T
tank/pcbackup@auto-2025-08-31_10-00 Sun Aug 31 10:00 2025 14.2K 4.11T
tank/pcbackup@auto-2025-08-31_12-00 Sun Aug 31 12:00 2025 14.2K 4.11T
tank/pcbackup@auto-2025-09-01_12-00 Mon Sep 1 12:00 2025 781K 4.11T
tank/pcbackup@auto-2025-09-02_12-00 Tue Sep 2 12:00 2025 717K 4.14T
tank/pcbackup@auto-2025-09-03_12-00 Wed Sep 3 12:00 2025 795K 4.15T
tank/pcbackup@auto-2025-09-04_12-00 Thu Sep 4 12:00 2025 866K 4.16T
tank/pcbackup@auto-2025-09-05_12-00 Fri Sep 5 12:00 2025 902K 4.17T
tank/pcbackup@auto-2025-09-06_12-00 Sat Sep 6 12:00 2025 1.69T 5.86T
tank/pcbackup@auto-2025-09-07_12-00 Sun Sep 7 12:00 2025 646K 4.12T
tank/pcbackup@auto-2025-09-08_12-00 Mon Sep 8 12:00 2025 142K 4.13T

Wondering why it is so high what I can do to save a bit of space.

2

This would be a case to look at reducing retention period or reducing data churn in the dataset. The former is fairly straightforward, and the latter involves taking a close look at how you’re using the dataset and tweaking application and client behavior.

2 Likes
3

What tool do you use for backups? Can it by any chance be windows’ built-in backup&restore?

Does your tool take an image of the entire system?

4

I am using Veeam Agent, It backs up changes daily a full backup every week. It has a 7 day retention.

5

I reduced the retention from 2 weeks to a week to see if that helps. Will monitor it for September.

6

You removed almost 3TB of data these days, but as long as it referenced in a snapshot, the space is needed nevertheless.

I’m not familiar with Veeam Agent, but if it tracks the changes on a daily basis – no need to snapshot it daily. This “double-tracking” can eat up much space. Realistically speaking, you only need these snapshots if your backup solution somehow screws up its storage.

I, for one, have a separate top-level parent dataset for backups, that is only snapshotted weekly.

7

What type of files are you backing up?

If they are large files that are “modified”, but not “in-place”, then even a small change of only a few KB will duplicate the entire file’s size when copied over to the server. The snapshot will reference all data blocks of the older file, which ZFS views as completely different blocks than the “modified” file of the same name.

Even if the files were modified in-place, I don’t believe Veeam supports this. Very few backup tools actually update files in-place. The only one I can think of is Rsync with the --inplace flag.

1 Like
8

Just wondering if over time that space will show available again? I will try weekly snapshots, see if that makes a difference. Thanks for the help.

9

Yeah, when the snapshots are gone.

10

If you bulk delete snapshots, it may take a little time to see the space get added back to available. zpool get freeing dozer. See man zpoolprops and man zfsconcepts.

     freeing           After a file system or snapshot is destroyed, the space it was using is returned to the pool asynchronously.
                       freeing is the amount of space remaining to be reclaimed.  Over time freeing will decrease while free in‐
                       creases.
1 Like