Re: Thoughts on using fs/jbd from drivers/md

Mike Fedyk
2002-05-18T01:35:55Z

On Fri, May 17, 2002 at 07:34:10PM +0100, Stephen C. Tweedie wrote:
> Hi,
>
> On Fri, May 17, 2002 at 11:29:42AM -0700, Mike Fedyk wrote:
> > On Thu, May 16, 2002 at 04:17:49PM +0100, Stephen C. Tweedie wrote:
>
> > > Right. The ability of soft raid5 to lose data in degraded mode over a
> > > reboot (including data that was not being modified at the time of the
> > > crash) is something that is not nearly as widely understood as it
> > > should be, and I'd love for us to do something about it.
> >
> > Are there workarounds to avoid this problem?
>
> No.
>
> > What does it take to trigger the corruption?
>
> It just takes degraded mode, an unexpected power cycle, and concurrent
> write activity.
>
> Degraded mode relies on the parity disk being in sync at all times ---

Doesn't degraded mode imply that there are not any parity
disk(raid4)/stripe(raid5) updates?

> you can't recover data from the missing spindle unless that is true.
> However, writes to a stripe are not atomic, and you can get a reboot
> when, say, a write to one of the surviving data chunks has succeeded,
> but the corresponding write to the parity disk has not. If this
> happens, the parity is no longer in sync, and the data belonging to
> the missing spindle in that stripe will be lost forever.
>
> > I ask this because I have used a degraded raid5 because the source drive
> > would become a member, but I needed to copy the data first. While doing so,
> > I had to reboot a couple times to reconfigure the boot loader. All seems to
> > be working fine on the system today though.
>
> If it was a clean shutdown and reboot, you're fine.
>

OK, that's good.