Re: [patch] document flash/RAID dangers

[Ric Wheeler]

On 08/25/2009 08:20 PM, Pavel Machek wrote:
> > > > > ---
> > > > > There are storage devices that high highly undesirable properties
> > > > > when they are disconnected or suffer power failures while writes are
> > > > > in progress; such devices include flash devices and MD RAID 4/5/6
> > > > > arrays.  These devices have the property of potentially
> > > > > corrupting blocks being written at the time of the power failure, and
> > > > > worse yet, amplifying the region where blocks are corrupted such that
> > > > > additional sectors are also damaged during the power failure.
> > > >
> > > > I would strike the entire mention of MD devices since it is your
> > > > assertion, not a proven fact. You will cause more data loss from common
> > >
> > > That actually is a fact. That's how MD RAID 5 is designed. And btw
> > > those are originaly Ted's words.
> >
> > Ted did not design MD RAID5.
>
> So what? He clearly knows how it works.
>
> Instead of arguing he's wrong, will you simply label everything as
> unproven?
>
> > > > events (single sector errors, complete drive failure) by steering people
> > > > away from more reliable storage configurations because of a really rare
> > > > edge case (power failure during split write to two raid members while
> > > > doing a RAID rebuild).
> > >
> > > I'm not sure what's rare about power failures. Unlike single sector
> > > errors, my machine actually has a button that produces exactly that
> > > event. Running degraded raid5 arrays for extended periods may be
> > > slightly unusual configuration, but I suspect people should just do
> > > that for testing. (And from the discussion, people seem to think that
> > > degraded raid5 is equivalent to raid0).
> >
> > Power failures after a full drive failure with a split write during a rebuild?
>
> Look, I don't need full drive failure for this to happen. I can just
> remove one disk from array. I don't need power failure, I can just
> press the power button. I don't even need to rebuild anything, I can
> just write to degraded array.
>
> Given that all events are under my control, statistics make little
> sense here.
> 								Pavel

You are deliberately causing a double failure - pressing the power button after 
pulling a drive is exactly that scenario.

Pull your single (non-MD5) disk out while writing (hot unplug from the S-ATA 
side, leaving power on) and run some tests to verify your assertions...

ric

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/