Verification after backup - High download volume

I have multiple backup jobs from my NAS to JottaCloud.
For most of them I have set backup-test-percentage to 5 (in the general settings), for some of the backup jobs I have put it to 10 (in the individual backup job’s settings).
However, when looking at how much is downloaded, and comparing it to the total backup volume, it seems to me that the percentage that is downloaded is more or less double of what should be downloaded. I am checking this by comparing the value for “BytesDownloaded” in the full log (under “Testresults”,“BackendStatistics”) versus the backup size under the job in the main screen.
It seems consistent - for the jobs where I put backup-test-percentage to 5%, about 10% is downloaded and for those where I put it to 10% about 20% is downloaded. It is also consistent with the download volume shown in my ISP’s traffic statistics.
Backup-test-samples is not set so this could not be the cause.
I am running Duplicati - on a Synology DSM 216+ via Docker.

Is this expected behaviour? How much data is effectively being tested - the percentage that I put or the double?

Welcome to the forum @Bart80.2

The TEST command says what a sample is (it’s not one file – it’s one each of three types).
Backup Test block selection logic describes the detailed method by which files are chosen.

You can look at the size of your different backup files. The default size of a dblock is 50 MB.
There’s a smaller dindex for each dblock, and the dlist file size varies with backup file count.

One question is – what does it mean to test 10 percent of the backup? One view might say
that it means testing 10 percent of the dblock files, and because it’s a set, some other ones.
That would be the sample set count. To compute it roughly, take total file count * percent / 2.
The divide-by-two is because of dindex for every dblock. The question is – did code divide?

Add backup-test-percentage in addition to backup-test-samples [$25] #3296 asked for a “percentage of your backup”, but what does that mean, given the sample set idea? I’m thinking code should divide by 2.

How about if you look at backend statistics and post your KnownFileCount and FilesDownloadedCount
on a backup that doesn’t also do a Compact (which would add downloads) to see how the math lands?

Hi, thanks for your answer.
As a sample is 1 dblock, 1 dindex and 1 dlist file; all dblock files are ± same size and the sizes of dindex and dlist files are lot smaller (if not negligible if compared versus the dblock files), the percentage should reflect more or less the total size.
I did what you requested as a test. I enabled the option “no-auto-compact” and had a backup job running of a total size of 18,55 GB (backup data):
“KnownFileCount”: 773
“FilesDownloadedCount” does not appear in the log, instead I find “FilesDownloaded”: 83.
“BytesDownloaded”: 2046355815 => Amounts to about 2 GB which is a bit over 10% of the backup size (backend). However, percentage is set to 5.
So, still with “no-auto-compact” enabled, the downloaded size is still double.

Could there be another cause?