A method for data storage includes, in a system that includes multiple servers and multiple storage devices, holding in a server a definition of a stripe that includes multiple memory locations on the storage devices, to be used by the servers for storing multiple data elements and at least a redundancy element calculated over the data elements. One or more of the data elements in the stripe are modified by the server, by executing in the storage devices an atomic command, which updates the redundancy element to reflect the modified data elements only if a current redundancy element stored in the storage devices reflects the multiple data elements prior to modification of the data elements, and storing the modified data elements in the storage devices only in response to successful completion of the atomic command.