Date: 30 Mar 2006 06:23:49 -0500
Message-ID: <20060330112349.11324.qmail@science.horizon.com>
From: linux@horizon.com
To: linux@horizon.com, osv@javad.com
Subject: Re: Lifetime of flash memory
Cc: linux-kernel@vger.kernel.org
In-Reply-To: <87acb85tnb.fsf@javad.com>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2612
Lines: 51

>>>> Due to the multiplexing scheme used in high-density NAND flash devices,
>>>> even the non-programmed cells are exposed to a fraction of the programming
>>>> voltage and there are very low limits on the number of write cycles to
>>>> a page before it has to be erased again.  Exceeding that can cause some
>>>> unwanted bits to change from 1 to 0.  Typically, however, it is enough
>>>> to write each 512-byte portion of a page independently.
>>>
>>> Well, I'm not sure. The Toshiba and Samsung NANDs I've read manuals for
>>> seem to limit number of writes to a single page before block erase, --
>>> is 512-byte portion some implementation detail I'm not aware of?
>>
>> No.  I just meant that I generally see "you may program each 2K page a
>> maximum of 4 times before performing an erase cycle", and I assume the
>> spec came from 2048/512 = 4, so you can program each 512-byte sector
>> separately.
>
> I've a file system implementation that writes up to 3 times to the first
> 3 bytes of the first page of a block (clearing more and more bits every
> time), and it seems to work in practice, so maybe this number (4) came
> from another source? Alternatively, it works by accident and then I need
> to reconsider the design.

No, I'm sorry, I was still unclear.  The spec is 4 writes per page.
I believe that the REASON for this spec was so that people could write
512+16 bytes at a time just like they did with small-block devices and
it would work.

But I do not believe there is any limitation on the pattern you may use,
so your system should work fine.

What confuses me is that I thought I said (quoted above; paraphrasing
here) "there is a very low limit on the number of times you may write
to a page.  That limit is large enough that you can do pagesize/512 =
2048/512 = 4 separate 512-byte writes."  I didn't intend to imply that
that was the ONLY legal pattern.

But from your comments, I'm getting the impression that you think I did
say that was the only legal pattern.  If that impression is correct,
I'm not sure how you read that into my statements.


(I wonder if the actual limit is the number of writes per BLOCK, and
they just expressed it as writes per page.  I don't know enough about
the programming circuitry to know what's exposed to what voltages.
If the physics implied it, it would be useful flexibility for file system
design.)
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