1. A method for data storage, comprising: in a system that comprises multiple servers and multiple storage devices, holding in a server a definition of a stripe that comprises 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; and modifying, by the server, one or more of the data elements in the stripe 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.