1. A method of prioritising an input/output (I/O) operation in a computer storage system, the computer storage system including a host computing device having a host storage software stack at a kernel level of a host operating system, the host storage software stack including a plurality of software layers, the method comprising: receiving a command indicating one or more I/O operations to which priority is to be assigned; and interfacing with each of the software layers of the host storage software stack at the kernel level of the host operating system so as to prioritise allocation of resources of the software layers to the one or more I/O operations, based on the received command, wherein the prioritization is performed synchronously across the software layers.