RFC: detect and manage power cut on MLC NAND

[Nick Krause]

On Fri, Mar 20, 2015 at 4:26 AM, Boris Brezillon
<boris.brezillon at free-electrons.com> wrote:
> Hi Iwo,
>
> On Fri, 20 Mar 2015 11:25:18 +1100
> Iwo Mergler <Iwo.Mergler at netcommwireless.com> wrote:
>
>>
>> Hi all,
>>
>>
>> I've been thinking a little about the paired pages problem.
>>
>> The patented mitigation methods I've seen so far are
>>
>> 1)  SanDisk: Use a separate log mechanism to back up LSB page
>> while writing MSB page.
>>
>> 2) M-Systems: Within the block, interleave backup copies of LSB
>> pages when writing MSB pages.
>>
>> 3) Micron: Use as a half-capacity SLC device by writing LSB and MSB
>> pages such that the levels are reinforced. Jeff mentioned this one.
>
> All these solutions involves storing a backup of the first paired page,
> before writing the 2nd one (I guess that's even worse with TLC NANDs:
> you'll have to save the 2nd page when writing the 3rd one).
> If that's possible, I think we should find a way to avoid this
> duplication.
>
>>
>>
>> Here is another idea. I assume it's patented already, in which case
>> I'd like to hear about it. If not, consider this published as of now :-)
>>
>>
>> Get UBI to map the paired pages into a single write unit. Jumbo page
>> if you will.  So, instead of a block with 64 pages, we get 32 pages,
>> twice as large.
>>
>> Thus, the paired pages will be written in quick succession. A power
>> cut during this is reduced to the unstable bits issue we already have
>> with SLC.
>
> That's actually a good idea, but I'd like to have feedback from Jeff
> and/or Qi regarding the feasibility of this proposal.
> I've read many times that MLC pages should be programmed in ascending
> order (0, 1, 2, 3, ..., N), and if you take a look at MLC datasheet
> you'll see that paired pages are not contiguous (here is an example
> [1], page 55-56 describe how pages are paired together).
>
> My question is: is there a reason for interleaving paired pages with
> other pages (write disturbance mitigation ?) ?
>
> If there is no specific reason but to annoy software developers :-),
> then I think we should consider this Jumbo page approach, because it
> would contain the paired pages handling in the NAND layer (instead of
> exposing complexity to the UBI/UBIFS layers) while allowing us to use
> almost all the NAND chip capacity.
>
>>
>> UBI could take the risk and split the first page pair between EC and
>> VID headers, replacing the EC info with average in case of unlikely
>> failure.
>
> You want to do that in order to avoid using 2 Jumbo pages to only store
> a few bytes ?
> Artem suggested another solution to deal with that: duplicate the EC
> header in the VID header so that EC information can be recovered when
> writing to the page paired with the EC header.
> You'd still have to prevent any write on the page paired with the EC
> header, but you're still saving one page with this approach and avoid
> any corruption of UBI metadata caused by paired pages.
>
> Maybe there are better solution, and I've been thinking about a
> different approach to avoid writing the EC and VID headers in 2
> different pages: store EC header information in a specific UBI volume
> (containing one or two LEBs).
> This volume would store a log of EC information update, so that when
> you erase a page you don't have to write the EC header right away, but
> should just add an entry in the UBI EC log.
> This way you write EC and VID information in one go when someone maps a
> LEB (attach it to a specific volume).
>
> These are just thoughts ;-).
>
> [1]http://www.szyuda88.com/uploadfile/cfile/2013419135343223.pdf
> --
> Boris Brezillon, Free Electrons
> Embedded Linux and Kernel engineering
> http://free-electrons.com
>
> ______________________________________________________
> Linux MTD discussion mailing list
> http://lists.infradead.org/mailman/listinfo/linux-mtd/
Iwo,
I was assuming the pages were still in ram and volatile for my example.
Nick