A computer system, comprising: a plurality of host computers each having one or more virtual machines, VMs, running therein and system software that supports the VMs; a first shared storage device connected to each of the host computers; and a second shared storage device (141, 142) that has a larger capacity and higher input/output latency than the first shared storage device, wherein the system software is configured to cache data that are to be written in the second shared storage device (141, 142) in the first shared storage device in a write-back mode; and wherein the data cached in the first shared storage device are copied into the second shared storage device (141, 142) asynchronously by the system software with respect to when the data are cached in the first shared storage device; wherein the system software in each of the host computers is configured to cache data into the first shared storage device at a first rate; and wherein the system software in each of the host computers is configured to copy the data cached in the first shared storage device into the second shared storage device (141, 142) at a second rate; characterised in that: a coordinating host computer is provided amongst the plurality of host computers; each of the host computers (100-1, 100-N) is configured to report the first rate to the coordinating host computer; the system software of the coordinating host computer is configured to compute a target rate for all the host computers (100-1, 100-N) based on an average of the first rates reported thereto by the system software of all the host computers (100-1, 100-N) and to communicate the calculated target rate to each of the plurality of host computers (100-1, 100-N); and each of the plurality of host computers (100-1, 100-N) is configured to set the second rate to copy the data cached in the first shared storage device into the second shared storage device (141, 142) based on the target rate. a plurality of host computers each having one or more virtual machines, VMs, running therein and system software that supports the VMs; a first shared storage device connected to each of the host computers; and a second shared storage device (141, 142) that has a larger capacity and higher input/output latency than the first shared storage device, wherein the system software is configured to cache data that are to be written in the second shared storage device (141, 142) in the first shared storage device in a write-back mode; and wherein the data cached in the first shared storage device are copied into the second shared storage device (141, 142) asynchronously by the system software with respect to when the data are cached in the first shared storage device; wherein the system software in each of the host computers is configured to cache data into the first shared storage device at a first rate; and wherein the system software in each of the host computers is configured to copy the data cached in the first shared storage device into the second shared storage device (141, 142) at a second rate; characterised in that: a coordinating host computer is provided amongst the plurality of host computers; each of the host computers (100-1, 100-N) is configured to report the first rate to the coordinating host computer; the system software of the coordinating host computer is configured to compute a target rate for all the host computers (100-1, 100-N) based on an average of the first rates reported thereto by the system software of all the host computers (100-1, 100-N) and to communicate the calculated target rate to each of the plurality of host computers (100-1, 100-N); and each of the plurality of host computers (100-1, 100-N) is configured to set the second rate to copy the data cached in the first shared storage device into the second shared storage device (141, 142) based on the target rate. a coordinating host computer is provided amongst the plurality of host computers; each of the host computers (100-1, 100-N) is configured to report the first rate to the coordinating host computer; the system software of the coordinating host computer is configured to compute a target rate for all the host computers (100-1, 100-N) based on an average of the first rates reported thereto by the system software of all the host computers (100-1, 100-N) and to communicate the calculated target rate to each of the plurality of host computers (100-1, 100-N); and each of the plurality of host computers (100-1, 100-N) is configured to set the second rate to copy the data cached in the first shared storage device into the second shared storage device (141, 142) based on the target rate.