Re: [patch] document flash/RAID dangers

[Ric Wheeler]

On 08/25/2009 08:06 PM, Pavel Machek wrote:
> On Tue 2009-08-25 19:48:09, Ric Wheeler 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.

> > 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?

> > > Otherwise, file systems placed on these devices can suffer silent data
> > > and file system corruption.  An forced use of fsck may detect metadata
> > > corruption resulting in file system corruption, but will not suffice
> > > to detect data corruption.
> >
> > This is very misleading. All storage "can" have silent data loss, you are
> > making a statement without specifics about frequency.
>
> substitute with "can (by design)"?

By Pavel's unproven casual observation?

> Now, if you can suggest useful version of that document meeting your
> criteria?
>
> 								Pavel

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