[Borgbackup] What do sizes mean when pruning?

[Wojtek Swiatek]

Hello everyone,

I started to use borg as my backup platform some time ago, running a backup
every 4 hours. Each backup name is time stamped with the time the backup
started.
I therefore ended up with ~60 backups and decided it is time to prune them
(and then prune on a regular basis).

I read the documentation about the command and went for a dry run first

root at srv ~# borg prune -v --list --dry-run --keep-daily=7 --keep-weekly=4
--stats  /services/backup/borg/
Keeping archive: srv-2018-08-28T12:00:01+02:00        Tue, 2018-08-28
12:00:02 [753f5c42bb554f1a3a7614a860f31d9b6e80ef3151635d6842dbd30eecdf58e0]
Would prune:     srv-2018-08-28T08:00:01+02:00        Tue, 2018-08-28
08:00:02 [42399cbafe9ad4a8f752392bde648459529bc9811b91a97e4953525574e16ac7]
(...)
Would prune:     srv-2018-08-27T00:00:01+02:00        Mon, 2018-08-27
00:00:03 [961e092dff8754806c7b9bbb7d5327c65ca25cd1d1a22f1b09bc78ea24c87310]
Keeping archive: srv-2018-08-26T20:00:01+02:00        Sun, 2018-08-26
20:00:02 [d96c3b2590afdd088c051a20a604a52355ecd4db28abad6badf13e43666fea87]
Would prune:     srv-2018-08-26T16:00:01+02:00        Sun, 2018-08-26
16:00:03 [30920cb17a5109f395145f6b3555111bdd9734a9501ab40661dcf974e75fa4c8]
Would prune:     srv-2018-08-26T12:00:00+02:00        Sun, 2018-08-26
12:00:01 [abf9a1781c01a6a35a214f68e67331a44379cea5d7e8f136e90ad8033b432f7a]
(...)
Would prune:     srv-2018-08-17T20:22:26+02:00        Fri, 2018-08-17
20:22:27 [911446a4de9adff4b6b50a8340f6408e2ad2277d677f8f88f994b1b4b08ef8d9]
Would prune:     srv                                  Fri, 2018-08-17
16:00:04 [e59c0bb486ba06677e20775783e9efd266f99cb1cb29d820d5b95d7225468077]
------------------------------------------------------------------------------
                       Original size      Compressed size    Deduplicated
size
Deleted data:                    0 B                  0 B
0 B
All archives:               24.01 TB             21.32 TB            340.27
GB

                       Unique chunks         Total chunks
Chunk index:                  568754             73359328
------------------------------------------------------------------------------

The numbers above are weird as the whole backed up drive is 500 GB but the
docs mention something about sizes so my understanding is that:
- there is 340 GB of files backed up
- but since there are several backups, this corresponds (backup after
backup) to a total of "virtual" 21 TB of files
- and it does not matter because deduplication.

In other words, a 1 GB file is backuped 10 times so it looks like the
backup takes 100 GB, but since there is deduplication, it is only 1 GB.

I then ran the real prunning:

root at srv ~# borg prune -v --list --keep-daily=7 --keep-weekly=4 --stats
/services/backup/borg/
Keeping archive: srv-2018-08-28T12:00:01+02:00        Tue, 2018-08-28
12:00:02 [753f5c42bb554f1a3a7614a860f31d9b6e80ef3151635d6842dbd30eecdf58e0]
Pruning archive: srv-2018-08-28T08:00:01+02:00        Tue, 2018-08-28
08:00:02 [42399cbafe9ad4a8f752392bde648459529bc9811b91a97e4953525574e16ac7]
(1/58)
(...)
Pruning archive: srv                                  Fri, 2018-08-17
16:00:04 [e59c0bb486ba06677e20775783e9efd266f99cb1cb29d820d5b95d7225468077]
(58/58)
------------------------------------------------------------------------------
                       Original size      Compressed size    Deduplicated
size
Deleted data:              -21.04 TB            -18.70 TB            -16.35
GB
All archives:                2.97 TB              2.62 TB            323.92
GB

                       Unique chunks         Total chunks
Chunk index:                  505291              9340546
------------------------------------------------------------------------------

I now see that there is a virtual 21 TB of data gone, the actual size is
more or less the same as before.


*My question is: what is the value of the "original size" information? It
is for me an indication of how much space I would have needed if there was
no deduplication but beside that I do not really see the usage I can make
of it. is there anything more behind it?*

Cheers,
Wojtek
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