why usart6'dma must use the FIFO mode.  if I not use ,the data recived by usart6 is whole,then I write to the SD,the data in SD is loss

Thank you ,my problem is solved,I use the FIFO mode with the USART6'DMA ,the SD data was not loss.but,I don't understand why? 

yes,when I don't use the USART6'S DMA,it can work normally,and not loss data.I also use the DMA with the SDIO

just loss data in sd card

if I don't use USART6'DMA ,I can write to SD card normally. But,if I use the USART's DMA ,When I write to SD,it will loss many data