What exactly does "Last upload" represent on a backup plan?

[james-franchino]

Hi!

First, thank you for Backrest. I'm really enjoying it and just finished migrating my backups to it.

I'm trying to understand one of the dashboard statistics.

I have a backup plan that reports:

Protected: 95.31 GiB
Last upload: 50.29 GiB

This was the first successful backup for this particular backup plan, but not the first backup in the repository. The repository already contained several snapshots from other backup plans before this one ran.

My question is:

What exactly does "Last upload" measure?

Is it:

Bytes transmitted over the network?
New repository data after deduplication?
Size of newly written pack files?
Something else?

I'm mainly trying to understand how to interpret this number when comparing it to the "Protected" size, since the documentation doesn't seem to define it.

Thanks!

[LJAYi]

I traced these two values through the current Backrest and restic code.

• Protected comes from restic's total_bytes_processed: the total logical size processed by that backup.
• Last upload comes from restic's data_added: the amount of new, uncompressed data added to the repository after deduplication.

Therefore, Last upload is not:

• the total amount scanned or protected;
• the exact number of bytes transmitted over the network; or
• the final compressed size of newly written pack files.

It can include newly added data blobs and tree metadata. Restic exposes a separate field named data_added_packed for the size after compression, but the current Backrest dashboard uses data_added, not data_added_packed.

This also explains your numbers. Although it was the first backup for this plan, the repository already contained snapshots from other plans. Any chunks already present in that repository could be deduplicated:

Protected:   95.31 GiB  ← total logical data processed
Last upload: 50.29 GiB  ← new uncompressed repository data after deduplication

The relevant mappings are in Backrest's summary API, while restic documents data_added and data_added_packed separately in its JSON output fields.

So the short interpretation is: Protected tells you how much data the plan covered; Last upload tells you how much new uncompressed data that run contributed to the shared repository.

[james-franchino]

Thank you very much! That clears it up perfectly.

I had read through the documentation before posting, but I couldn't quite figure out what "Last upload" actually represented. Your explanation makes complete sense now, especially the distinction between data_added and data_added_packed.

I really appreciate you taking the time to trace it through both the Backrest and Restic code. Thanks again!