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The Shared Memory Device /dev/shm, shm = shared memory is disabled since version 2.3 on IPFire. Some Addons such as Qemu uses /dev/shm. If multiple VMs run at the same time you should adapt the size of /dev/shm accordingly.
Loggon to IPFire directly or via SSH, and change the following line in
filename = /etc/fstab
shm /dev/shm tmpfs default 0 0
shm /dev/shm tmpfs size=512m 0 0
After restarting the IPFire or remount the device the size of shared memory is raised up to 512 MB.
To remount the device type:
mount -o remount /dev/shm
Ensures that at least that much memory is available!