ubi: why ubi wear leveling always pick highly worn-out free physical eraseblock

[Zhihao Cheng]

在 2024/2/23 18:14, Ryder Wang 写道:
> Hi Zhihao,
> 
> Thanks for your reply.
> 
> You explained why e1 and e2 use picked to compare with CONFIG_MTD_UBI_WL_THRESHOLD to decide whether WL is needed. That's right as I also mentioned. However, the point is that why such high worn-out free physical eraseblock (in ubi->free tree) should be used to store the data of e1? High worn-out PEB should always be avoided to use (low worn-out PEB of ubi->free tree should be preferred), right?
> 

Since e1 has smaller ec counter. because it comes from ubi->used tree  
and likely holds cold data, so it won't be erased for a long time. If we  
choose low worn-out PEB from ubi->free tree as e2, the high worn-out PEB  
can still be used to store new data and be erased again, which will  
aggravate the worn-out of high worn-out PEB, but actually the high  
worn-out PEB should not be scheduled for using. So, the best choice is  
that make high worn-out PEB hold cold data, which can stop frequently  
erasing worn-out PEB.

> ________________________________________
> From: Zhihao Cheng <chengzhihao1 at huawei.com>
> Sent: Friday, February 23, 2024 15:11
> To: Ryder Wang; linux-mtd at lists.infradead.org; richard at nod.at
> Subject: Re: ubi: why ubi wear leveling always pick highly worn-out free physical eraseblock as the mov_to target
> 
> 在 2024/2/23 11:27, Ryder Wang 写道:
>> Refer to the ubi source code:
>> wear_leveling_worker
>>     e2 = get_peb_for_wl(ubi)
>>       e = find_wl_entry(ubi, &ubi->free, WL_FREE_MAX_DIFF);
>>
>> The function find_wl_entry() always find the  highly worn-out free physical eraseblock (e2):
>> 1. It's good to check such PEB (e2) to decide whether the following wear leveling procedure should be continued according to UBI_WL_THRESHOLD.
>> 2. But personally I can't understand why such high worn-out free physical eraseblock should also be used as target PEB(move_to) to store the move_from data for wear leveling purpose. Will it be much more reasonable to use low worn-out free physical eraseblock (from ubi->free tree) in this case for more perfect wear leveling?.
>>
> 
> Normally, e1 is the smaller ec counter picked from ubi->used, e2 is
> bigger ec counter picked from ubi->free, the wear-leveling worker
> follows that rule which is based on the realization of free PEBs fetching.
> First, let's talk about the simplest case without fastmap, assumpt that
> CONFIG_MTD_UBI_WL_THRESHOLD=2, there are total 3 PEBs.
> Only 1 EB(erase block) is used, then free it, and repeat the process
> forvever. According to the implementation in
> ubi_wl_get_peb->wl_get_wle->find_mean_wl_entry, the ubi->free tree will
> change like:
> (1,1,1) -> (1,1,2) -> (1,2,2) -> (1,2,3) -> (1,3,3) -> ... -> (1,4,5) ->
> (2,4,5), which means that wear-leveling worker is not needed, because
> find_mean_wl_entry has made sure that 'max_ec - min_ec <= 2 *
> CONFIG_MTD_UBI_WL_THRESHOLD'. Similar, when there are more EBs are used
> and freed, there is no need to trigger wear-leveling work.
> However, if one free PEB is taken and not be freed, other free PEBs are
> taken and freed, like:
> ubi->used: 1
> ubi->free: (1, 1) -> (1, 2) -> ... (x, y)
> After a while, min(x,y) - 1 will be greater than
> 2*CONFIG_MTD_UBI_WL_THRESHOLD. Then, wear-leveling should start working,
> cold data always takes certain PEB for a long time, which causes PEBs
> with smaller ec counter gather in ubi->used and PEBs with bigger ec
> counter gather in ubi->freed.
> I think that's the explainations of the rule how to pick e1/e2.
> .
>