Cleanmarker is a special JFFS2 node which is written to the beginning of a block just after the block has been erased. On NOR flashes it is a special small JFFS2 node at the beginning of the block. On NAND flashes it is placed to the spare area of the first page.
The main reason why cleanmarkers are used is the need to be sure that the block erase operation was correctly completed. All 0xFF bytes in the block are not necessarily mean the block is ready to be utilized. For example, if an unclean reboot happened just at the end of the block erase cycle, the block might have unstable bits, which are read as "1" one time and might be read as "0" next time.
When preparing a flash partition for JFFS2, it is recommended to put cleanmarkers to the erased blocks. This might be done my means of "-j" option of the "flash_eraseall" MTD utility. Otherwise, JFFS2 will re-
erase the blocks which contain all 0xFF and have no cleanmarker. This is an unneeded wasting of time.