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[FREEBSD] How to clean up swap from old rubbish

You want to clear your swap – all articles are saying – just do swapoff/swapon but you getting error:
swapoff: /dev/xbd25: Cannot allocate memory
What to do? Easy, make some file with size of your swap, add it as swap, turn off old one, turn on old one and remove the file! Easy:

dd if=/dev/zero of=/usr/jails/swap0 bs=1m count=10240
#10gb of swap
mdconfig -a -t vnode -f /usr/jails/swap0 -u 0
mdconfig -l -v md0    vnode       10G    /usr/jails/swap0
swapon /dev/md0
swapinfo 
    Device          1K-blocks     Used    Avail Capacity
    /dev/xbd25       15728640  7773260  7955380    49%
    /dev/md0         10485760        0 10485760     0%
    Total            26214400  7773260 18441140    30%
swapoff /dev/xbd25
swapon /dev/xbd2
swapoff /dev/md0

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Marek Knappe