Simulate fail disk in Linux

If you want to simulate fail disk when test any storage software such as CEPH, ScaleIO ...
Block any IO send to device:
echo offline > /sys/block/sda/device/state

Re-enable:

echo running > /sys/block/sda/device/state

Link:
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/5/html/Online_Storage_Reconfiguration_Guide/removing_path-to-storage-device.html