Backing up unionfs pool to external drives

1. You can't unmount the filesystem because it is used by some service. Possibly smb share?

2. Rsync is a sync tool. This means that it can very efficiently update a copy of a filesystem to match the original, as long as there are few differences. Perhaps a few new files added and some deleted. But most the same. Rsync will do the minimum possible to make the copy identical to the changed original.

This means that rsync is very inefficient if there are big changes between backups. And the first time you do a backup.

Rsync can ALSO be used to create versioned snapshots of the original. It can use hardlinks to avoid having to store a file more than once, but the hardlink to that file can appear in many snapshots and take up very little storage.

This means that you can have one snapshot that looks just like the original as it was yesterday. And one as it was last week or last month. And so on.

I use rsync in scripts to make versioned snapshots of folders like this. And also delete old snapshots. Typically I keep 7 daily snapshots, 4 weekly and 3-12 monthly. The rsnapshot plugin for OMV works in a similar way.

3. That depends on how you use the two backup HDDs. If you use them as a mergerfs filesystem during a backup, then it may be easiest to use them the same way to access the backup. It is not necessary, but then you don't have to search on two HDDs for a file.

You can also backup your original filesystem in several parts. Depending on how you are organized. For instance "cold" data that rarely changes only need to be backed up when it changes. For instance finished TV series/seasons and old movies. Folders that are frequently updated with new "hot" files often need to be backed up more frequently. For instance ongoing TV series and new movies. Or files that need renaming and metadata.

Perhaps a few times per year you migrate stuff from the "hot" side to the "cold". When metadata is perfect and/or the TV series season is over. And you update the "cold" manual backups.

But you really shouldn't need to access the external backups on some other system. Access the originals using the network. It is a NAS after all.

4. I would ignore snapraid and instead go for good backups. And split the data in "hot" (=changing often) and "cold" (=mostly static). Or have snapraid with just one parity drive.

I would use two drives for a pooled mergerfs filesystem shared by OMV. One or two drives, possibly also as a pooled mergerfs filesystem in the same NAS, but only for automatic frequent versioned rsync snapshots. Mainly for "hot" files, but also extra backup of valuable "cold" files. Family photos, for example. And two external drives for mainly "cold" manual versioned snapshots. Also valuable "hot" files. But updated only a few times per year, when "hot" files are moved to the "cold" side.

This is almost how I do it today, but between several separate NAS over the network, typically not using mergerfs but SBCs with big (12TB-16TB) single HDDs. Using scripts to make automatic versioned snapshots of folders.