Discussion: littlefs on SPI NAND

[hchaudhary1]

Hi I would like to use LittleFS directly on a NAND chip, such as Micron MT29F.

The minimum erasable unit on the MT29F is 128KiB
The smallest writable unit is 2KiB (random write)
The smallest writable unit is 1 byte (sequential write)

Thanks

[hchaudhary1]

Forgot to ask my question... will LittleFS work with such a memory?
Thanks

[geky]

Hi @hchaudhary1, at the moment you may run into problems.

LittleFS has only been used on NOR flash, eMMC, SD cards; devices with <4KB erase units, so I'm not sure how it will perform on NAND flash.

Concerns:

1. You must provide ECC

   LittleFS will hand write errors, which is the main effect of flash wear. But with NAND memory you also have to worry about read errors. NAND memory is so dense that the stored electrons start misbehaving pretty easily.

   Scanning the MT29F's datasheet, it looks like that part has built in ECC you can use.

2. On disk, each file uses 1 erase unit at minimum.

   For 128KiB erase units, this may waste a large amount of space.

   But NAND devices are also very large, so if you have few files this may not be a problem? I'm not sure. It probably depends a lot on how you use the filesystem.

   I have plans to improve this issue by storing multiple files in a block, but I don't know when I'll be able to implement the idea. So unfortunately at the moment 1 file per erase unit is the best LittleFS can do.

3. You may need 2KiB (writable unit) of RAM for the program cache?

   I'm not sure if you can take advantage of the 1 byte sequential writes.

   For each block, LittleFS does write the data out sequentially. But, it may partially write to multiple erase units multiple times (for example, partially write erase unit A, partially write erase unit B, finish writing erase unit A). Can the NAND chip handle that sort of pattern?

---

At the moment, I think LittleFS might need a flash translation layer (FTL) to be efficient. An FTL would provide a smaller erase unit and prevent all of the issues, though you may still want to use LittleFS for power resilience.

At the very least, LittleFS is probably better than putting FAT directly on top of raw NAND.

If you do try to put LittleFS on a NAND chip, let me know how it goes! I'd be interested to know what sort of problems you run into.

[hchaudhary1]

geky, thanks for elaborating on this

My hope is to avoid using a fully featured FTL layer such as dhara (https://github.com/dlbeer/dhara), since fail-safety, wear-leveling, and bad-block-management are already built into littlefs...

1. ECC correction upon physical read is handled by the chip itself. Up to 8-bits can be corrupted in the physical page, but when the read happens, the data is corrected and fixed before transferring over SPI.

2. The smallest capacity NAND (1Gbit) contains 1024 erasable units (each unit being 128KiB). This means I can have over 1000 files. This is more than enough for me. Capacity is also not an issue; I just need fast write speeds, which is why I am not using NOR.

3. This is something I will have to test; especially in a multi-threaded environment where I will have multiple files open simultaneously.

I guess my main concern is what will happen to stale data that over several years develops more than 8-bits of errors. I supposed only static wear leveling can reduce this issue...

Thanks. I will provide some feedback

[geky]

Ah interesting, that is a nice side effect of static wear leveling.

Though I've heard that errors from stale data are less concerning on SLC NAND parts. Looking at the MT29F's datasheet, it only mentions concerns about write errors:

Over time, some memory locations may fail to program or erase properly.

The same datasheet also lists a data retention of 10 years. I suspect that is in the case when the chip has no power, and may be the same for stale data. If you need longer data retention, you could probably increase the size of the ECC, though that may require software ECC.

Although it may be a good idea to ask someone who's a better expert on NAND memory.

[hchaudhary1]

implementation was a success!

[geky]

Ah! That is exciting! Thanks for the update.

[rahmanih]

Hi,

just to understand, does littleFS support NAND flash without any change?

regards
Haithem.

[hchaudhary1]

Yes, it supports NAND without modifications.
You have to write your own read, write, erase functions ofcoarse

EDIT:
Tested with Micron MT29F 1GiB model, that has hardware ECC.

[tomyqg]

is there any example,how to use littlefs on spi nand？

[hassanmj]

What is the status of LittleFS with NAND flash support? Any improvement?

[rojer]

there isn't anyhting special about nand and lfs, same as NOR pretty much. blocks are larger, but that's it.

[hchaudhary1]

NAND working well