Row-level locks are the locks that protect
selected rows. When a transaction updates or
modifies a row, its transaction identifier is
recorded in the entry as a part of the
transaction list. The list is located in the
header of the data block itself, and the row
header is modified to point to the transaction
list entry. The following statements create
row-level locks:
These row locks
or row-level locks are stored in the block, and
each lock refers to the global transaction lock.
As in the case of a single instance, Oracle RAC
controls concurrency down to the row level. The
finest lock granularity is at the row level.
However, to keep the cache coherent, access to
the data blocks is controlled by the GCS. This
has no effect on the row-level lock. GCS
resources and row locks
operate independently of the GCS. An instance can
request or ship the data block to another
instance in the cluster without affecting the
row-level locks that are held inside the data
block. The row-level lock is fully controlled by
the transaction that causes the row-level lock.
When the transaction commits or rolls back, the
row-level lock is released. In the meantime, if
another transaction intends to update the same
row, it has to wait until the initial
transaction commits or rolls back.
The row lock method has an important advantage
in maintaining data consistency, even if there
are multiple instances, as in the RAC system.
The behavior of the row lock and the release is
the same, whether it is a single standalone
database or a multi-instance RAC system. During
the row lock period, even if the data block gets
transferred to another instance, the row lock
remains intact until released.
Global Resource Directory
The GES and GCS together maintain a Global Resource
Directory (GRD) to
record information about resources and enqueues.
The GRD remains in memory and is stored on all
the instances. Each instance manages a portion
of the directory. The distributed nature of the
GRD is a key point for the fault tolerance of
RAC.
The GRD is an internal database that records and
stores the current status of the data blocks.
Whenever a block is transferred out of a local
cache to another instance’s cache, the GRD is
updated. The following resource information is
available in GRD:
-
Data Block Addresses
(DBA) - This is the address of the block being
modified
-
Location of most current version
of the data block. This exists only if
multiple nodes share the data block.
-
Modes of the data blocks -
(N)Null, (S)Shared, and (X)Exclusive
-
The roles of the data blocks
(local or global) - This indicates the role
in which the data block is being held by the
instance.
-
SCN – System Change Number
-
Image of the Data Block – It
could be past image or current image.
Current image represents the copy of the
block held by the current instance. Past
image represents the global dirty data block
image maintained in the cache.
