ed for LVM labels. This affects the ability of mirrors to
be used as physical volumes. If this setting is enabled, it is
impossible to create VGs on top of mirror LVs, i.e. to stack VGs on
mirror LVs. If this setting is disabled, allowing mirror LVs to be
scanned, it may cause LVM processes and I/O to the mirror to become
blocked. This is due to the way that the mirror segment type handles
failures. In order for the hang to occur, an LVM command must be run
just after a failure and before the automatic LVM repair process
takes place, or there must be failures in multiple mirrors in the
same VG at the same time with write failures occurring moments before
a scan of the mirror's labels. The 'mirror' scanning problems do not
apply to LVM RAID types like 'raid1' which handle failures in a
different way, making them a better choice for VG stacking.