Re: Race to power off harming SATA SSDs

David Woodhouse
2017-05-08T08:14:18Z

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On Mon, 2017-05-08 at 09:38 +0200, Ricard Wanderlof wrote:
> On Mon, 8 May 2017, David Woodhouse wrote:
>
> >
> > >
> > > [Issue is, if you powerdown during erase, you get "weakly erased"
> > > page, which will contain expected 0xff's, but you'll get bitflips
> > > there quickly. Similar issue exists for writes. It is solveable in
> > > software, just hard and slow... and we don't do it.]
> > It's not that hard. We certainly do it in JFFS2. I was fairly sure that
> > it was also part of the design considerations for UBI ? it really ought
> > to be right there too. I'm less sure about UBIFS but I would have
> > expected it to be OK.
> I've got a problem with the underlying mechanism. How long does it take to
> erase a NAND block? A couple of milliseconds. That means that for an erase
> to be "weak" du to a power fail, the host CPU must issue an erase command,
> and then the power to the NAND must drop within those milliseconds.
> However, in most systems there will be a power monitor which will
> essentially reset the CPU as soon as the power starts dropping. So in
> practice, by the time the voltage is too low to successfully supply the
> NAND chip, the CPU has already been reset, hence, no reset command will
> have been given by the time NAND runs out of steam.
>
> Sure, with switchmode power supplies, we don't have those large capacitors
> in the power supply which can keep the power going for a second or more,
> but still, I would think that the power wouldn't die fast enough for this
> to be an issue.
>
> But I could very well be wrong and I haven't had experience with that many
> NAND flash systems. But then please tell me where the above reasoning is
> flawed.

Our empirical testing trumps your "can never happen" theory :)

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