Problem:
========
From 10.6.13(86767bcc0f) version,
InnoDB purge thread does free the TRX_UNDO_CACHED undo segment.
Pre-10.6.13 version data directory can contain TRX_UNDO_CACHED
undo segment in system tablespace even though it
has external undo tablespace.
During slow shutdown, InnoDB collects the segment from tables
that exist in system tablespace and cached undo segment in
the system tablespace as used segment exist in system tablespace.
While shrinking the system tablespace, last used extent can be
used by undo cached segment. This extent blocks the shrinking of
system tablespace in a effective way.
Solution:
========
While freeing the unused segment, InnoDB should free the cached
undo segment header page exists in system tablespace and
reset the TRX_RSEG_UNDO_SLOTS to 0xff for the rollback segment
header page exists in system tablespace. This could improve
the shrinking of system tablespace further.
