Output of restic version

restic 0.12.0 compiled with go1.15.8 on linux/arm

How did you run restic exactly?

export RESTIC_REPOSITORY=.
export RESTIC_PASSWORD=<SECRET_PW>
cd /home/pi/extern/restic
/home/pi/restic check --read-data --no-cache

Output:

~/extern/restic $ /home/pi/restic check --read-data --no-cache
repository xxxxxxx opened successfully, password is correct
create exclusive lock for repository
load indexes
check all packs
check snapshots, trees and blobs
read all data%  16 / 16 snapshots
[1:19] 100.00%  16 / 16 snapshots
[3:39:52] 5.90%  20405 / 346096 packs

What backend/server/service did you use to store the repository?

a 3.5" external USB HDD with EXT4 filesystem mounted to /home/pi/extern

Expected behavior

restic reads all data from local repository and checking them without writing a copy to temp folder

Actual behavior

Restic copies all data stored on directly attached harddrive to temp directory.

lsof | grep restic lists a lot of these entrys

restic    2810                              pi    6u      REG      179,2   4386561       5494 /tmp/restic-temp-421096629 (deleted)
restic    2810                              pi    8u      REG      179,2   6581784      11442 /tmp/restic-temp-657619758 (deleted)
restic    2810                              pi    9u      REG      179,2   4294885       1945 /tmp/restic-temp-362046705 (deleted)
restic    2810                              pi   10u      REG      179,2   5544330      10340 /tmp/restic-temp-922992203 (deleted)

This is just an excerpt. List is really long.

Steps to reproduce the behavior

Check a local restic repostory with restic check command (on a Raspberry PI3).
On an idle system you can check the caused writeload of restic via cat /sys/fs/ext4/mmcblk0p2/lifetime_write_kbytes replace mmcblk0p2 with where your root partition "/" is mounted.

After checking 1000 packs I had about 5 GB more lifetime writes

restic/data/00 $ ls -lh | head
total 6,8G

restic/data/00 $ ls -lh | wc -l
1381

So average size of one pack is 4,92 MB. Reading 1000 of this giving about 5 GB more written data.

Do you have any idea what may have caused this?

Restic treats it as remote repository where it has to download the data from before it can do useful stuff with it.

I think this behaviour probably will also occur when restoring data.

Probably it is because used system is too slow so that temp files get written do disk before restic deletes them again.

Do you have an idea how to solve the issue?

Current workaround is

sudo mkdir /run/restic
sudo chown pi:pi /run/restic
export TMPDIR=/run/restic

I know that restic needs a temp directory when backing up before uploading. It was just very unexpected that restic copies whole local repository when doing a check. So far I've done this several times, meaning a few TB were written on the SD card, so I should now replace it and buy a new one.

Due to the kind restic works it may be a lot of work to change behaviour for local repositories not to copy the file data. Perhaps this only occurs because I do this on a raspberry PI 3B which is so slow that pack files got written to disk before they were deleted again.
So just a simple warning when executing check that this operation may cause write of the whole repository to local disk and one should consider using export TMPDIR with tempdir on a ramdisk.

Did restic help you today? Did it make you happy in any way?

Restic is a really great program. I really like it very much and I'm looking very forward to when it is possible to increase filesize of packfiles via commandline, see #2750

I backup now all my machines with restic, even raspberry PIs complete OS and made a full restore procedure which I can share I someone is interested.
I really like restic. It is the best backup program of all those I've tried (Duplicati, Hardlinkbackup, Windows integrated Backup, using Winrar, robocopy, synchthing used as backup software).