US Patent 7328226 Coordinated distributed log-based snapshots in a multi-host environment

A method for coordinating access to a storage volume between several hosts, each of which has an associated log, may involve: one of the hosts requesting permission from a coordinator to perform a write to a region of the storage volume; the host storing write data for the write to an associated log; and the coordinator sending a response granting permission to perform the write to the host, where the response indicates whether an old version of data in the region of the storage volume should be relocated to a COW (Copy On Write) snapshot volume. The host may signal completion of the write to an application that initiated the write prior to performing the write to the region of the storage volume and subsequent to both storing the write data to the associated log and receiving the response from the coordinator.