FDS wear-leveling not working

[Adrian Eggenberger]

Hi

I've just done a few tests with the FDS system of the SDK12.2. My observation is, that the FDS doesn't properly use all assigned flash pages to reach a maximal lifetime.

I have a configuration with the space for the FDS is set to 4 pages. The FDS will use one page as SWAP and the other three pages as DATA pages. My code is writting approximatly 30 records a ~30Bytes to the FDS and randomly updates these records. Once one page is nearly full the garbage collector copies the valid data to the SWAP page and converts the previous SWAP page into a DATA page and the erased page becomes the new SWAP page. Once the new data page is nearly full the valid records are copied back to the SWAP page and so on. In fact only two of the four pages are used.

In my opinion a good flash manager should use all assigned flash pages to reach the maximal possible lifetime. This will allow the developer to add more space to the FDS to increase lifetime.

The problem is based in the fds.c in the function "write_space_reserve" this function always starts at the same position in the flash page list.

Any thoughts how this could be improved?

Adrian

[Hung Bui]

Hi Adrian,

As I understand, the behaviour you described only happens if you call fds_gc() every time the first page is full. Actually I don't know how you can get an event when one page is full. We only have FDS_ERR_NO_SPACE_IN_FLASH event when all pages are full.

In the case when all pages are full, and fds_gc() is called by the main application, each page in the fds file system that got erased will be used as swap page.

So I think it won't be just 2 pages out of 4 will be used as swap page, but rotating around ?

[Adrian Eggenberger]

Hi Hung In our solution the GC is called as soon as in the FDS the number of freeable_words exceeds a given threshold. This triggers a GC as soon as the system is idle. We are not really sure what is the best strategy for the GC call. Waiting until the error FDS_ERR_NO_SPACE_IN_FLASH is called seams also a bit odd in my opinion. What is your recommended way for calling GC?

[emdi]

Hi,

if you run garbage collection when the flash is full then the swap page will be moved around as you would expect and wear leveling will be maximized.

[Adrian Eggenberger]

It could be correct that the wear leveling is ok if the GC is only called if the FDS is full. In my case I modified the FDS to follow a different strategy. I've added a function to the FDS that can be called if the system is idle. This function will check if it makes sense to cleanup one of the pages based on various thresholds. If the function returns true the classic fds_gc() may be called to cleanup the page.

[Hung Bui]

Hi Adrian,

Could you explain a little bit more on what you want to achieve with your strategy ? I guess you want to take advantage of the CPU idle time so you can do gb collection instead of waiting till the fds system is full and flash storing will have to wait for gb collection?