Find Oracle RAC Locks

Oracle Database Tips by Donald Burleson

Oracle RAC is extremely complex and special scripts are required to identify locks within any RAC cluster. The monitoring of the RAC global enqueue services (GES) process is performed using the gv$enqueue_stat view.

For full scripts, download the Oracle script collection. Also see these scripts for finding RAC blocking sessions.

The RAC resources managed by the GES include the following lock areas:

· Transaction locks - It is acquired in the exclusive mode when a transaction initiates its first row level change. The lock is held until the transaction is committed or rolled back.

· Library Cache locks - When a RAC database object (such as a table, view, procedure, function, package, package body, trigger, index, cluster, or synonym) is referenced during parsing or compiling of a SQL, DML or DDL, PL/SQL, or Java statement, the process parsing or compiling the statement acquires the library cache lock in the correct mode.

· Dictionary Cache Locks - Global enqueues are used in the cluster database mode. The data dictionary structure is the same for all Oracle instances in a cluster database, as it is for instances in a single-instance database.

The Oracle script collection contains a complete list of Oracle RAC locking scripts, but this script can be used to quickly identify and isolate RAC locking issues:

select o.name object_name, u.name owner, lid.*
from (select
               s.inst_id, s.sid, s.serial#, p.spid,nvl (s.sql_id, 0), s.sql_hash_value,
               decode (l.type,
                       'tm', l.id1,
                       'tx', decode (l.request,
                                     0, nvl (lo.object_id, -1),
                                     s.row_wait_obj#
                                    ),
                       -1
                      ) as object_id,
                 l.type lock_type,
               DECODE (l.lmode,
                       0, 'NONE',
                       1, 'NULL',
                       2, 'ROW SHARE',
                       3, 'ROW EXCLUSIVE',
                       4, 'SHARE',
                       5, 'SHARE ROW EXCLUSIVE',
                       6, 'EXCLUSIVE',
                       '?'
                      ) mode_held,
               DECODE (l.request,
                       0, 'NONE',
                       1, 'NULL',
                       2, 'ROW SHARE',
                       3, 'ROW EXCLUSIVE',
                       4, 'SHARE',
                       5, 'SHARE ROW EXCLUSIVE',
                       6, 'EXCLUSIVE',
                       '?'
                      ) mode_requested,
               l.id1, l.id2, l.ctime time_in_mode,s.row_wait_obj#, s.row_wait_block#,
               s.row_wait_row#, s.row_wait_file#
          from gv$lock l,
               gv$session s,
               gv$process p,
               (select object_id, session_id, xidsqn
                  from gv$locked_object
                 where xidsqn > 0) lo
         where l.inst_id = s.inst_id
           and s.inst_id = p.inst_id
           and s.sid = l.sid
           and p.addr = s.paddr
           and l.sid = lo.session_id(+)
           and l.id2 = lo.xidsqn(+)) lid,
       SYS.obj$ o,
       SYS.user$ u
WHERE o.obj#(+) = lid.object_id
AND o.owner# = u.user#(+)
AND object_id <> -1;

Oracle Grid and Real Application Clusters

See working examples of Oracle Grid and RAC in the book Oracle Grid and Real Application Clusters.

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