[Yaffs] Access data in YAFFS

Good Day!

I am going to manipulate data in YAFFS just before data is going to write to disk. I looked for it a lot but finally I could not understand where I can do that.
I mean, is there anywhere that I can get right access to data?

I think the data is store in the iovec structure in kernel(I am not sure. am I right?)
I could not find where it passes to yaffs?

with many thanks.