Oracle Scratchpad

March 13, 2014

Shrink Space

Filed under: Bugs,deadlocks,fragmentation,Index Rebuilds,Indexing,Locks,Oracle — Jonathan Lewis @ 7:08 am BST Mar 13,2014

Here’s a lovely effect looking at v$lock (on 11.2.0.4)

select
        sid, type, id1, id2, lmode, request, ctime, block
from
        V$lock
where   sid in (
                select  sid
                from    V$session
                where   username = 'TEST_USER'
        )
order by
        sid, type desc
;

       SID TY        ID1        ID2      LMODE    REQUEST      CTIME      BLOCK
---------- -- ---------- ---------- ---------- ---------- ---------- ----------
       145 TX     589833       7518          6          0        489          0
           TM      92275          0          2          6        489          1
           SK          7   25165955          6          0        489          0
           AE        100          0          4          0        582          0

       148 TX     524308       7383          6          0        490          0
           TM      92275          0          2          6        490          1
           SK          7   25173379          6          0        490          0
           AE        100          0          4          0        998          0

You’ll notice I’ve got two sessions holding a TM lock on the same table (object 92275) in mode 2 (sub-share) and waiting for an exclusive lock on the same table. Both sessions are recording the fact that they are blocking something else. You’ll have trust me when I say there are no other user sessions on the system at this point, and none of the background sessions is doing anything with that table.

The clue to what’s happening is the SK lock – it’s the “segment shrink” lock. I had two sessions start an “alter index I_n shrink space” (two different indexes on the same table) at the same time. The problem is that “shrink space” without the “compact” tries to drop the highwater mark on the index’ allocated space after completing the shrink phase – and there’s a defect in the internal code that tries to get the required exclusive lock on the underlying table: it doesn’t seem to allow for all the possible ways you can fail to get the lock. If you look at v$session_wait_history for either of these sessions, you’ll see something like the following:


  SID  SEQ#     EVENT# EVENT                            P1         P2         P3  WAIT_TIME WAIT_TIME_MICRO TIME_SINCE_LAST_WAIT_MICRO
----- ----- ---------- ------------------------ ---------- ---------- ---------- ---------- --------------- --------------------------
  145     1        798 Wait for shrink lock              0          0          0         10           99954                      70137
          2        235 enq: TM - contention     1414332422      92275          0        600         6002022                        101
          3        798 Wait for shrink lock              0          0          0         10          100723                      69335
          4        235 enq: TM - contention     1414332422      92275          0        600         6001589                         81
          5        798 Wait for shrink lock              0          0          0         10          100500                      69542
          6        235 enq: TM - contention     1414332422      92275          0        600         6002352                         86
          7        798 Wait for shrink lock              0          0          0         10          100618                      69145
          8        235 enq: TM - contention     1414332422      92275          0        600         6001545                        144
          9        798 Wait for shrink lock              0          0          0         10          100996                      69445
         10        235 enq: TM - contention     1414332422      92275          0        600         6002744                        310

The attempt to acquire the TM enqueue (lock) times out every three seconds – and I think the session then releases and re-acquires the SK lock before trying to re-acquire the TM lock – and it’s never going to get it.

The alert log, by the way, looked like this:


Wed Mar 12 12:53:27 2014
ORA-00060: Deadlock detected. More info in file /u01/app/oracle/diag/rdbms/test/test/trace/test_ora_2242.trc.
Wed Mar 12 12:53:30 2014
ORA-00060: Deadlock detected. More info in file /u01/app/oracle/diag/rdbms/test/test/trace/test_ora_1832.trc.
ORA-00060: Deadlock detected. More info in file /u01/app/oracle/diag/rdbms/test/test/trace/test_ora_2242.trc.
ORA-00060: Deadlock detected. More info in file /u01/app/oracle/diag/rdbms/test/test/trace/test_ora_1832.trc.
ORA-00060: Deadlock detected. More info in file /u01/app/oracle/diag/rdbms/test/test/trace/test_ora_2242.trc.
ORA-00060: Deadlock detected. More info in file /u01/app/oracle/diag/rdbms/test/test/trace/test_ora_1832.trc.
Wed Mar 12 12:53:40 2014
ORA-00060: Deadlock detected. More info in file /u01/app/oracle/diag/rdbms/test/test/trace/test_ora_2242.trc.
ORA-00060: Deadlock detected. More info in file /u01/app/oracle/diag/rdbms/test/test/trace/test_ora_1832.trc.
ORA-00060: Deadlock detected. More info in file /u01/app/oracle/diag/rdbms/test/test/trace/test_ora_2242.trc.
Wed Mar 12 12:53:43 2014
ORA-00060: Deadlock detected. More info in file /u01/app/oracle/diag/rdbms/test/test/trace/test_ora_1832.trc.
ORA-00060: Deadlock detected. More info in file /u01/app/oracle/diag/rdbms/test/test/trace/test_ora_2242.trc.

I’ve said it before, and I keep repeating it when people say “Oracle resolves deadlocks automatically”: Oracle does NOT resolve deadlocks automatically – one of the sessions will rollback its last DML statement to clear the deadlock, but the other session will (almost invariably) still be waiting. It’s up to the application to do something sensible to resolve the deadlock after it receives the ORA-00060 error.

Don’t ask about the trace files – but they had both reached 400MB by the time I finished this note.

Strategy Note

If you are going to shrink objects, it’s probably best to do it in two steps: “shrink space compact, followed by “shrink space”.
If you’re going to try to use multiple sessions to shrink several indexes as quickly as possible, make sure there’s no way that two sessions can try to “shrink space” on the indexes on the same table at the same time.

Footnote:

This article was prompted by the recent appearance of bug 18388128 on MoS.

3 Comments »

  1. Good find, JL!

    And (agreeing) regarding “Oracle resolves deadlocks automatically,” it would be far better for folks trying to convey what Oracle does about deadlocks to start such claims with “Oracle breaks deadlocks automatically.”

    Then perhaps it would be easier to get people to take seriously exactly the state of the youngest lock holder’s session, resolution of which has become the application’s province after Oracle has intervened to break the deadlock.

    Here’s hoping the bit of this application (Oracle RDBMS) that needs to avoid or handle that deadlock is well contained enough that it can be fixed and back ported quickly and reliably!

    Comment by rsiz — March 13, 2014 @ 10:59 am BST Mar 13,2014 | Reply

  2. > If you’re going to try to use multiple sessions to shrink several indexes as quickly as possible, make sure there’s no way that two sessions can try to “shrink space” on a single table concurrently.

    Hi, JL. How to comprehend it?

    Comment by wei — March 21, 2014 @ 8:04 am BST Mar 21,2014 | Reply


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