How to get real file offset in NAND by file name?

Question

Using linux, I can use raw access to NAND or access to files through filesystem. So, when I need to know, where my file is really located in NAND, what should I do? I cannot found any utilities providing this feature. Moreover, I cannot detect any possibility of this, besides hacking kernel with tons of "printk" (it's not nice way, I guess). Can anybody enlighten me on this? (I'm using YAFFS2 and JFFS2 filesystems)

Answer 1

You can make a copy of any partition with nanddump. Transfer that partition dump to a PC. The nandsim utility can be used to mount the partitions on a PC.

modprobe nandsim first_id_byte=0x2c second_id_byte=0xda \
   third_id_byte=0x90 fourth_id_byte=0x95 parts=2,64,64
flash_erase /dev/mtd3 0 0
ubiformat /dev/mtd3 -f rootfs.ubi

This command emulates a Micron 256MB NAND flash with four partitions. If you just capture the single partition and not the whole device, don't set parts. You can also do nanddump on each partition and then concatenate them all. This code targeted mtd3 with a UbiFs partition. For JFFS2 or YAFFS2, you can try nandwrite or some other appropriate flashing utility on the PC.

How to get real file offset in NAND by file name?

The files may span several NAND sectors and they are almost never contiguous. There is not much of an advantage to keep file data together as there is no disk head that takes physical time to seek. Some flash has marginally better efficiency for sequential reads; yet other flash will give better performance for reads from another erase block.

I would turn on debug at either the MTD layer or in the filesystem. In a live system, the position of the file may migrate over time on the flash even if it is not written. This is active wear leveling.