Re: [RFC PATCH v1 0/7] Block/XFS: Support alternative mirror device retry

Darrick J. Wong
2018-12-10T04:30:46Z

On Sat, Dec 08, 2018 at 10:49:44PM +0800, Bob Liu wrote:
> On 11/28/18 3:45 PM, Christoph Hellwig wrote:
> > On Wed, Nov 28, 2018 at 04:33:03PM +1100, Dave Chinner wrote:
> >> - how does propagation through stacked layers work?
> >
> > The only way it works is by each layering driving it. Thus my
> > recommendation above bilding on your earlier one to use an index
> > that is filled by the driver at I/O completion time.
> >
> > E.g.
> >
> > bio_init: bi_leg = -1
> >
> > raid1: submit bio to lower driver
> > raid 1 completion: set bi_leg to 0 or 1
> >
> > Now if we want to allow stacking we need to save/restore bi_leg
> > before submitting to the underlying device. Which is possible,
> > but quite a bit of work in the drivers.
> >
>
> I found it's still very challenge while writing the code.
> save/restore bi_leg may not enough because the drivers don't know how to do fs-metadata verify.
>
> E.g two layer raid1 stacking
>
> fs: md0(copies:2)
> / \
> layer1/raid1 md1(copies:2) md2(copies:2)
> / \ / \
> layer2/raid1 dev0 dev1 dev2 dev3
>
> Assume dev2 is corrupted
> => md2: don't know how to do fs-metadata verify.
> => md0: fs verify fail, retry md1(preserve md2).
> Then md2 will never be retried even dev3 may also has the right copy.
> Unless the upper layer device(md0) can know the amount of copy is 4 instead of 2?
> And need a way to handle the mapping.
> Did I miss something? Thanks!

<shrug> It seems reasonable to me that the raid1 layer should set the
number of retries to (number of raid1 mirrors) * min(retry count of all
mirrors) so that the upper layer device (md0) would advertise 4 retry
possibilities instead of 2.

--D

> -Bob
>
> >> - is it generic/abstract enough to be able to work with
> >> RAID5/6 to trigger verification/recovery from the parity
> >> information in the stripe?
> >
> > If we get the non -1 bi_leg for paritity raid this is an inidicator
> > that parity rebuild needs to happen. For multi-parity setups we could
> > also use different levels there.
> >
>