On Tue, Dec 05, 2017 at 04:07:35PM +0800, Qu Wenruo wrote:
>
>
> On 2017年12月05日 06:40, Liu Bo wrote:
> > There is a scenario that can end up with rebuild process failing to
> > return good content, i.e.
> > suppose that all disks can be read without problems and if the content
> > that was read out doesn't match its checksum, currently for raid6
> > btrfs at most retries twice,
> >
> > - the 1st retry is to rebuild with all other stripes, it'll eventually
> > be a raid5 xor rebuild,
> > - if the 1st fails, the 2nd retry will deliberately fail parity p so
> > that it will do raid6 style rebuild,
> >
> > however, the chances are that another non-parity stripe content also
> > has something corrupted, so that the above retries are not able to
> > return correct content, and users will think of this as data loss.
> > More seriouly, if the loss happens on some important internal btree
> > roots, it could refuse to mount.
> >
> > This extends btrfs to do more retries and each retry fails only one
> > stripe. Since raid6 can tolerate 2 disk failures, if there is one
> > more failure besides the failure on which we're recovering, this can
> > always work.
>
> This should be the correct behavior for RAID6, try all possible
> combination until all combination is exhausted or correct data can be
> recovered.
>
> >
> > The worst case is to retry as many times as the number of raid6 disks,
> > but given the fact that such a scenario is really rare in practice,
> > it's still acceptable.
>
> And even we tried that much times, I don't think it will be a big problem.
> Since most of the that happens purely in memory, it should be so fast
> that no obvious impact can be observed.
>
It's basically a while loop, so it may cause some delay/hang, anyway,
it's rare though.
> While with some small nitpick inlined below, the idea looks pretty good
> to me.
>
> Reviewed-by: Qu Wenruo <wqu@xxxxxxxx>
>
> >
> > Signed-off-by: Liu Bo <bo.li.liu@xxxxxxxxxx>
> > ---
> > fs/btrfs/raid56.c | 18 ++++++++++++++----
> > fs/btrfs/volumes.c | 9 ++++++++-
> > 2 files changed, 22 insertions(+), 5 deletions(-)
> >
> > diff --git a/fs/btrfs/raid56.c b/fs/btrfs/raid56.c
> > index 8d09535..064d5bc 100644
> > --- a/fs/btrfs/raid56.c
> > +++ b/fs/btrfs/raid56.c
> > @@ -2166,11 +2166,21 @@ int raid56_parity_recover(struct btrfs_fs_info *fs_info, struct bio *bio,
> > }
> >
> > /*
> > - * reconstruct from the q stripe if they are
> > - * asking for mirror 3
> > + * Loop retry:
> > + * for 'mirror == 2', reconstruct from all other stripes.
>
> What about using macro to makes the reassemble method more human readable?
>
> And for mirror == 2 case, "rebuild from all" do you mean rebuild using
> all remaining data stripe + P? The word "all" here is a little confusing.
>
Thank you for the comments.
It depends, if all other stripes are good to read, then it'd do
'data+p' which is raid5 xor rebuild, if some disks also fail, then
it'd may do 'data+p+q' or 'data+q'.
Is it better to say "for mirror == 2, reconstruct from other available
stripes"?
Thanks,
-liubo
> Thanks,
> Qu
>
> > + * for 'mirror_num > 2', select a stripe to fail on every retry.
> > */> - if (mirror_num == 3)
> > - rbio->failb = rbio->real_stripes - 2;
> > + if (mirror_num > 2) {
> > + /*
> > + * 'mirror == 3' is to fail the p stripe and
> > + * reconstruct from the q stripe. 'mirror > 3' is to
> > + * fail a data stripe and reconstruct from p+q stripe.
> > + */
> > + rbio->failb = rbio->real_stripes - (mirror_num - 1);
> > + ASSERT(rbio->failb > 0);
> > + if (rbio->failb <= rbio->faila)
> > + rbio->failb--;
> > + }
> >
> > ret = lock_stripe_add(rbio);
> >
> > diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
> > index b397375..95371f8 100644
> > --- a/fs/btrfs/volumes.c
> > +++ b/fs/btrfs/volumes.c
> > @@ -5094,7 +5094,14 @@ int btrfs_num_copies(struct btrfs_fs_info *fs_info, u64 logical, u64 len)
> > else if (map->type & BTRFS_BLOCK_GROUP_RAID5)
> > ret = 2;
> > else if (map->type & BTRFS_BLOCK_GROUP_RAID6)
> > - ret = 3;
> > + /*
> > + * There could be two corrupted data stripes, we need
> > + * to loop retry in order to rebuild the correct data.
> > + *
> > + * Fail a stripe at a time on every retry except the
> > + * stripe under reconstruction.
> > + */
> > + ret = map->num_stripes;
> > else
> > ret = 1;
> > free_extent_map(em);
> >
>
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