Commits on Jun 21, 2022

1. Scrub mirror children without BPs …

   When scrubbing a raidz/draid pool, which contains a replacing or
   sparing mirror with multiple online children, only one child will
   be read.  This is not normally a serious concern because the DTL
   records are used to determine where a good copy of the data is.
   As long as the data can be read from one child the mirror vdev
   will use it to repair gaps in any of its children.  Furthermore,
   even if the data which was read is corrupt the raidz code will
   detect this and issue its own repair I/O to correct the damage
   in the mirror vdev.
   
   However, in the scenario where the DTL is wrong due to silent
   data corruption (say due to overwritting one child) and the scrub
   happens to read from a child with good data, then the other damaged
   mirror child will not be detected nor repaired.
   
   While this is possible for both raidz and draid vdevs, it's most
   pronouced when using draid.  This is because by default the zed
   will sequentially rebuild a draid pool to a distributed spare,
   and the distributed spare half of the mirror is always preferred
   since it delievers better performance.  This means the damaged
   half of the mirror will go undetected even after scrubbing.
   
   For system administrations this behavior is non-intuitive and in
   a worst case scenario could result in the only good copy of the
   data being unknowingly detached from the mirror.
   
   This change resolves the issue by reading all replacing/sparing
   mirror children when scrubbing.  When the BP isn't available for
   verification, then compare the data buffers from each child.  They
   must all be indentical, if not there's silent damage and an error
   is returned to prompt the top-level vdev to issue a repair I/O to
   rewrite the data on all of the mirror children.  Since we can't
   tell which child was wrong a checksum error is logged against the
   replacing or sparing mirror vdev.
   
   Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>

   

   Brian D. Behlendorf authored and behlendorf committed Jun 21, 2022
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