Backing up into multiple USB drives

[rhatto]

Summary

• As workaround to backup into multiple locations (#340), it seems possible to backup one phone to two or more USB drives using SeedVault.
• This was tested, and the trick is to use the same disk label and UUID in every thumb drive.
• It seems possible to backup more than one phone using the same set of two or more USB drives (not yet tested).
• This use-case allows to keep air gapped backups in different physical locations, without relying on any online service.
• Is this use case fragile? Does it goes beyond SeedVault's capacity?

Requirements

• Each USB drive has the same partition scheme (perhaps the same size?).
• Each partition has the same label.
• Each partition has the same UUID.

Preparing the drives

Due to the way SeedVault currently works, all the external drives need to have the same UUID (and perhaps the same label).

Chose a UUID, like F0F0F0F0.

Then do something like this for each new USB stick (assuming a working shell script in a GNU/Linux compatible system):

export LABEL="BackupStick"
export UUID="F0F0F0F0"
export PARTITION="/dev/sda1" # careful not to choose the wrong device

sudo mkfs.vfat -i 28EA95AA $PARTITION
sudo fatlabel $PARTITION $LABEL

The order of the commands matters!

Checking the result:

lsblk -e7 -o type,name,path,size,pttype,fstype,label,partlabel

Doing the backup

• Attach the first drive into your phone.

• Configure SeedVault to use the drive as the backup destination.

• Do the first backup.

• Test the backup integrity.

• Eject and remove the drive from the phone, as usual.

• Reboot your phone. If you do not reboot the phone and try to backup into the next USB drive, the result will be corrupted backups.

• After the reboot, proceed doing the same with the second thumb drive.

Findings

• Each backup procedure should happen after rebooting the phone, otherwise backups will be corrupted.

• If you want to save time by copying previous backup data from one drive to another, it won't work: backup procedure succeeds, but the integrity checks (both apps and files) will fail!

Questions

• Is this use case fragile? Does it goes beyond SeedVault's intended usage?

• Any ideas why a reboot is needed at every backup? Is this expected or buggy behavior?

• Would restores work in this scenario? While this seems to work, I did not test backup restoration from any of the drives, so I wonder whether this is supposed to work.

• Could multiple phones being backed up into the same set of USB sticks?
  That was not tested.

[grote]

I'd be really careful with such a scheme especially file backup which may be less likely to tolerate that as it uses an internal cache of which chunks it thinks you have on the storage medium. These randomly changing may cause it to throw old chunks away and start from scratch or in the worst case cause data loss.

[grote]

No ideas why backups would be corrupted if the phone was rebooted before. Ideally, find minimal steps to reproduce this behavior and export logs that show the corruption in a dedicated ticket.

[rhatto]

Thanks for the reply.

No ideas why backups would be corrupted if the phone was rebooted before.

Actually the backups are reported as corrupted if I switch the USB sticks without rebooting the phone. I imagine it has something to do with the internal cache as you mention (maybe this cache is refreshed after a reboot?).

I'll probably give up this approach for now, since it's not recommended and currently I can't do much more beyond sharing my findings.

[grote]

Can you screenshot and post the part where it says the backups are corrupted? Did you safely eject the drives?

I think the safest option for having several backup locations, is to backup to one location and then replicate that to other places.