On 2015-07-11 02:46, Stefan Priebe wrote:
> Hi,
>
> while using a 40TB btrfs partition for VM backups. I see a massive
> slowdown after around one week.
>
> The backup task takes usally 2-3 hours. After one week it takes 20
> hours. If i umount and remount the btrfs volume it takes 2-3 hours again.
>
> Kernel 4.1.1
>
I've been seeing similar (although much less drastic) slowdowns over 
time myself pretty much since I started using BTRFS (IIRC, sometime 
around 3.16).  If you're not constantly writing to that backup volume, 
you might want to consider setting up automounting for it.