Re: Why is O_DSYNC on linux so slow / what's wrong with my SSD?

[Ric Wheeler]

On 11/23/2013 07:22 PM, Pavel Machek wrote:
> On Sat 2013-11-23 18:01:32, Ric Wheeler wrote:
> > On 11/23/2013 03:36 PM, Pavel Machek wrote:
> > > On Wed 2013-11-20 08:02:33, Howard Chu wrote:
> > > > Theodore Ts'o wrote:
> > > > > Historically, Intel has been really good about avoiding this, but
> > > > > since they've moved to using 3rd party flash controllers, I now advise
> > > > > everyone who plans to use any flash storage, regardless of the
> > > > > manufacturer, to do their own explicit power fail testing (hitting the
> > > > > reset button is not good enough, you need to kick the power plug out
> > > > > of the wall, or better yet, use a network controlled power switch you
> > > > > so you can repeat the power fail test dozens or hundreds of times for
> > > > > your qualification run) before being using flash storage in a mission
> > > > > critical situation where you care about data integrity after a power
> > > > > fail event.
> > > > Speaking of which, what would you use to automate this sort of test?
> > > > I'm thinking an SSD connected by eSATA, with an external power
> > > > supply, and the host running inside a VM. Drop power to the drive at
> > > > the same time as doing a kill -9 on the VM, then you can resume the
> > > > VM pretty quickly instead of waiting for a full reboot sequence.
> > > I was just pulling power on sata drive.
> > >
> > > It uncovered "interesting" stuff. I plugged power back, and kernel
> > > re-estabilished communication with that drive, but any settings with
> > > hdparm were forgotten. I'd say there's some room for improvement
> > > there...
> > Hi Pavel,
> >
> > When you drop power, your drive normally loses temporary settings
> > (like a change to write cache, etc).
> >
> > Depending on the class of the device, there are ways to make that
> > permanent (look at hdparm or sdparm for details).
> >
> > This is a feature of the drive and its firmware, not something we
> > reset in the device each time it re-appears.
> Yes, and I'm arguing that is a bug (as in, < 0.01% people are using
> hdparm correctly).

Almost no end users use hdparm. Those who do should read the man page and add 
the -K flag :)

Or system scripts that tweak should invoke it with the right flags.....

Ric

> So you used hparm to disable write cache so that ext3 can be safely
> used on your hdd. Now you have glitch on power. Then, system continues
> to operate in dangerous mode until reboot.
>
> I guess it would be safer not to reattach drives after power
> fail... (also I wonder what this does to data integrity. Drive lost
> content of its writeback cache, but kernel continues... Journal will
> not prevent data corruption in this case).
>
> 									Pavel

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