Yaffs has a lower memory footprint than most other log-structured Flash file systems. Because it is log-structured, RAM is required to build up runtime data structures for acceptable performance. As a rule of thumb, Yaffs uses approximately 2 bytes per chunk of NAND flash, where a chunk is typically one page of NAND. For NAND with 512byte pages, a budget of approximately 4kbytes of RAM per 1Mbyte of NAND is usually sensible. And for 2kbyte page devices a budget of approximately 1kbyte per 1Mbyte of NAND usually suffices.
You can find out details of Yaffs memory use in the document http://www.yaffs.net/documents/how-yaffs-works