RFC: detect and manage power cut on MLC NAND

[nick]

On 2015-03-19 08:25 PM, Iwo Mergler 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.
> 
> 
> 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.
> 
> 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.
> 
> Would this work?
> 
> 
> Best regards,
> 
> Iwo
> 
> 
> 
Iwo,
Not a bad idea but what happens if the power gets cut after the first 2 pages then we lose
30 pages worth of data.  Unless the hardware is very fast and can move the pages over in
a millisecond or less then this can't really worth. However if we make these transactions
atomic in nature this may work better as CPU instruction respond is in the nanosecond range
even on low cost embedded boards using UBI for raw flash embedded on to themselves. 
Nick
________________________________________
> From: linux-mtd [linux-mtd-bounces at lists.infradead.org] On Behalf Of Boris Brezillon [boris.brezillon at free-electrons.com]
> Sent: Thursday, 19 March 2015 8:12 PM
> To: Andrea Marson
> Cc: Andrea Scian; Jeff Lauruhn (jlauruhn); linux-mtd at lists.infradead.org; dedekind1 at gmail.com; Richard Weinberger
> Subject: Re: RFC: detect and manage power cut on MLC NAND
> 
> On Thu, 19 Mar 2015 09:47:21 +0100
> Andrea Marson <andrea.marson at dave.eu> wrote:
> 
>>> Disturb is a block level affect, as long as partition A and B are in different blocks there will be no disturb between them.   Disturbs, does not damage cells; ERASE returns cells to undisturbed levels.
>> I think there are two options here: MTD partitioning and UBI
>> partitioning. AFAIK one should prefer UBI partitioning to preserve
>> device-wide wear leveling. Boris, am I right?
> 
> Both of them act at block level, meaning that your the partition size
> must be a multiple of the block size (logical block size in case of UBI
> volume and physical block size in case of MTD partition).
> IOW, you shouldn't bother whether you're using UBI on top of MTD or
> directly using MTD partitions, both are immune to cross partition/volume
> read/write disturbance.
> 
> 
>>
>>> Officially I would say don't use SLC emulation, but technically I know what your doing.   The reason I say no is because we have very precise recipes designed to create very tight distibutions, and although the first pass distributions might look like an SLC, they are really designed with the expectation of the upper page being programmed.  Not a true SLC.
>>> With MLC lithography of 25 nm and less  the difference between each level (L0, L1, L2 and L3) is just a few 10's of electrons.  The distribution have to be very tight, in order to meet retention requirements.
>> This is quite interesting, however I'm afraid I have not fully
>> understood it.
> 
> Me neither :-/.
> 
>> Let me try to rephrase it. Please correct me if I'm wrong.
>>
>> 1) Technically speaking, it is possible to use an MLC memory in SLC
>> mode, even if this is not recommended because MLC is not designed for
>> this usage.
> 
> That's what I understood, but I'm not sure to understand the
> constraints brought by SLC mode (only programming one of the paired
> pages).
> Jeff, Are you trying to explain what's described here [1] in slide 8
> (BTW I'm not sure to understand this diagram).
> If that's the case, could you explain us, how the NAND chip knows which
> threshold should be used (does it somehow store the information of
> which page has already been programmed)
> 
> [1]http://www.bswd.com/FMS09/FMS09-T2A-Grunzke.pdf
> 
> --
> Boris Brezillon, Free Electrons
> Embedded Linux and Kernel engineering
> http://free-electrons.com
> 
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