linux-kernel - Re: Race to power off harming SATA SSDs

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Message-ID: <alpine.DEB.2.11.1705081034420.6882@lnxricardw1>
Date:   Mon, 8 May 2017 10:36:54 +0200
From:   Ricard Wanderlof <ricard.wanderlof@...s.com>
To:     David Woodhouse <dwmw2@...radead.org>
CC:     Pavel Machek <pavel@....cz>, Tejun Heo <tj@...nel.org>,
        <boris.brezillon@...e-electrons.com>, <linux-scsi@...r.kernel.org>,
        Hans de Goede <hdegoede@...hat.com>,
        <linux-kernel@...r.kernel.org>, <linux-ide@...r.kernel.org>,
        <linux-mtd@...ts.infradead.org>,
        Henrique de Moraes Holschuh <hmh@....eng.br>
Subject: Re: Race to power off harming SATA SSDs


On Mon, 8 May 2017, David Woodhouse wrote:

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

I'm sure it does. But what is the explanation then? Has anyone analyzed 
what is going on using an oscilloscope to verify relationship between 
erase command and supply voltage drop?

/Ricard
-- 
Ricard Wolf Wanderlöf                           ricardw(at)axis.com
Axis Communications AB, Lund, Sweden            www.axis.com
Phone +46 46 272 2016                           Fax +46 46 13 61 30