Note that for SDK v12.3 : if application size * 2 > available space a single bank update is performed.
While I am using v15.3 and there seems no more infor about the single bank and dual bank settings, I want to free more space for data storage and disable dual bank dfu, my question is how to use single bank dfu directly?
solution1: final I find the NRF_DFU_SINGLE_BANK_APP_UPDATES in https://infocenter.nordicsemi.com/index.jsp?topic=%2Fcom.nordic.infocenter.sdk5.v15.3.0%2Flib_bootloader_dfu_banks.html&cp=5_1_3_5_1_2
solution2: increase the NRF_DFU_APP_DATA_AREA_SIZE to make the space less than dual bank.
Thanks for answering your question. Just want to add that option two may be better as it will prevent the bootloader from overwriting the app data in case an image is also too big for a single bank update.