Re: Data corruption after resizing partition, when using bitmaps

[NeilBrown <neilb@suse.de>]

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On Wed, 20 May 2015 02:31:50 -0400 Jim Paris <jim@jtan.com> wrote:

> NeilBrown wrote:
> > On Tue, 19 May 2015 10:12:40 -0400 Jim Paris <jim@jtan.com> wrote:
> > 
> > > I had a raid1 mirror consisting of big partitions on two disks.
> > > The first disk was 2TB, partitioned like this:
> > > 
> > >   [--sda1(128M)--][-------sda2(~2T)--------------]
> > > 
> > > The second disk was 3TB, partitioned like this:
> > > 
> > >   [--sdb1(128M)--][-------sdb2(~3T)------------------------------------]
> > > 
> > > sda2 and sdb2 were part of the array, which was only ~2TB in size due
> > > to the smaller disk.
> > > 
> > > I realized that I needed to add a BIOS boot partition to the 3TB disk,
> > > so I removed sdb2 from the array, and repartitioned sdb like this:
> > > 
> > >   [--sdb1(128M)--][--sdb2(1M)--][-------sdb3(~3T)----------------------]
> > > 
> > > Then I added sdb3 to the array.  And lost all my data. :(
> > > 
> > > What happened was that the last sector of the big partition did not
> > > change location.  So the metadata (0.90) at the end was still present.
> > 
> > This is one of the big reasons why 1.x was invented.
> > 
> > > Adding sdb3 to the array was considered a "re-add" because the UUID
> > > and array sizes still matched the array, even though the partition
> > > itself shrank.  And the resync was thus guided by an out-of-date
> > > bitmap, which caused very little data to actually be written to sdb3,
> > > so half the reads from the array started returning junk.  Once the
> > > filesystem got involved, the result was rapid corruption.
> > > 
> > > If I had not been using write-intent bitmaps, everything would have
> > > worked fine.  I only recently started using bitmaps, and never had any
> > > problems with adjusting partitions like this before that.
> > > 
> > > Perhaps mdadm can be more careful here -- for example, maybe checking
> > > the actual device size and not just the "used dev size" when
> > > determining whether to trust the bitmap.
> > 
> > It is perfectly acceptable to have the various devices in an array of
> > different sizes.  Unfortunately I don't think there is anything that mdadm
> > can usefully do here.
> > 
> > Thanks for the report anyway,
> > NeilBrown
> 
> Hi Neil,
> 
> Can we add u64 device_size to bitmap_super_t, and ensure that it
> matches the actual current device size before trusting the bitmap?

Well .... we could, but the bitmap_super is currently the same on all
devices.  This would  make it different.
And if we a going to change the metadata, why not just convert from 0.90 to
1.0?

 mdadm --stop /dev/mdXX
 mdadm --assemble /dev/mdXX --update=metadata /dev/...list-of-devices....

You might need to remove the bitmap first, and add it back afterwards.

NeilBrown

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