A method for data storage includes preparing first data having a first size for storage in a memory device that stores data having a nominal size larger than the first size, by programming a group of memory cells to multiple predefined levels using a one-pass program-and-verify scheme. The first data is combined with dummy data to produce first combined data having the nominal size, and is sent to the memory device for storage in the group. The dummy data is chosen to limit the levels to which the memory cells in the group are programmed to a partial subset of the predefined levels. In response to identifying second data to be stored in the group, the second data is combined with the first data to obtain second combined data having the nominal size, and is sent to the memory device for storage, in place, in the group.