In later version of the DSS V7 the Write Back function was taken off for iSCSI Volumes that where in Block IO mode, only iSCSI Volumes created in File IO mode will allow you to use the Write Back mode settings. Also you cannot use the File IO Volumes with Failover.
Thank you. May be you are rigth....
But in my DSS 6 i see checked check box WB on all Block iSCSI targets. Very interesting... is the cache working for block-iscsi targets in dss6 ?