Rsync backup to a Synology NAS takes more space than expected

TrueNAS General
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1

Hi,

I’m trying to backup my TrueNAS data to a Synology NAS with one of their app that uses Rsync (Synology Active Backup for Business). But I face a problem : all files were resynced completely at one point

The Multi-version task I setup on the Synology NAS is suppose to backup only the edited files since the last backup, taking only the space of the modified files. Not the whole files as duplicates, taking twice the space

Source files on TN takes 20.91 TiB, while on the Synology NAS, destination files are taking 33.16 TB

While I contacted Syno support, they suggested to see here if something might happening on TrueNAS side

Is there something in ZFS that can result in an apparent modification of all files on the server? Something that can be setup to avoid that?

Here’s my TrueNAS System Informations :

  • Platform: Generic
  • Version: ElectricEel-24.10.0.2
  • Data VDEVs : 2 x RAIDZ1 | 3 wide | 14.55 TiB
  • Log VDEVs : 1 x MIRROR | 2 wide | 931.51 GiB
  • Auto TRIM: On
  • Compression Level: LZ4
  • ZFS Deduplication: OFF

Thanks in advance!

2

Why not check the space consumed by the “rsync backup”[1] files on the Synology side?

This will immediately let you know if the extra space is accounted by duplication of entire files.

From there, you can investigate further.

  1. This “folder” is usually set by an option called --backup-dir when using rsync in the command-line, in conjunction with its --backup parameter. There’s also the related --suffix option as well, usually used to timestamp “old versions” of files. ↩︎

3

Thanks for the quick reply
I did checked the consumed space by the rsync backup, the space being the 33.16 TB compared to the real 20.91 TiB from the TrueNAS server

The consumed space is from only 2 backup runs where close to none of the original files were edited, hence my curiosity of why this space is so much

Am I missing something in what you suggest?

4

What parameters are you using on your rsync? If you are using the delay option, it actually moves all the files to a temp directory ~tmp~ or something, using almost twice the space. Check that and post your rsync command. If you are using the data protection setup, try turning off delay updates and clear the remote of files and run it again, or find the ~tmp~ directories and delete them. That likely is the problem.

5

Since the rsync is used through a Synology app (Active Backup for Business), it’s hard to know exactly which rsync command is used
Will ask them about this process though, to be sure it’s not using this kind of feature