Some host is down, need to reset the slot allocation

Hi,

Can someone help for this error how to fix it.

2018-05-24T14:54:42.409Z: Couldn't get a slot

2018-05-24T14:54:42.409Z: <DS> Error in opening stat file for device: naa.60002ac0000000000000006c000191e1.Ignoring this device.

2018-05-24T14:54:46.416Z: <DS> Some host is down, need to reset the slot allocation

2018-05-24T14:54:46.422Z: <DS> Some host is down, need to reset the slot allocation