Re: Data corruption after resizing partition, when using bitmaps

[Jim Paris <jim@jtan.com>]

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  And the resync was thus guided by an out-of-date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?

Jim

> 
> 
> > 
> > I wrote a script (attached) to recreate what happened, using some loop
> > devices.  It works fine if BITMAP=none, and fails with BITMAP=internal.
> > 
> > Jim
>