Latest results:

- it seems I can trigger the problem by doing the backup to my external
  usb disk

- the usb disk uses jfs

- jfs seems to take the major part in cache memory consumption

I've attached dmesg and slabtop outputs of various stages:

- before and during the backup

I've had to abort the backup, because memory pressure made the system
almost completly unusable.

Hope this helps & someone has a clue on what went wrong were...

Cheers,

Nico

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