Database Reference
In-Depth Information
The global cache coordination mechanism, also known as the RAC locking scheme, is centered on the following
premises:
1.
A block can be resident in a modifiable mode in one instance only. Before a block is
modified, a global cache lock is acquired on a resource in exclusive mode, effectively
protecting the block from concurrent changes.
2.
A block can be resident in Protected Read (PR) mode in multiple instances. Buffers
protected with PR mode locks are considered to be in read-only mode.
3.
A session must acquire the lock on a block in exclusive mode before modifying the block.
Therefore, other instances must downgrade the locks to null mode before the lock is
acquired in exclusive mode.
4.
Buffers reconstructed for Consistent Read (CR) mode do not require any BL locks to
be held.
Cache Fusion Processing
In a single instance, a database block can reside only in the local buffer cache. Therefore, if the block is not in the
local buffer cache, it is read from the disk. In RAC, the block can be resident in any buffer cache. So, a special type of
processing takes place that deviates sharply from single-instance processing.
The following section provides a rough outline of the cache fusion-processing scheme. This discussion should be
considered as an outline of cache fusion processing, not necessarily a documentation of the complete algorithm.
Figure
10-1
shows the mechanics of a cache fusion block transfer.
FG
4
1
5
LMS
LMS
LMS
2
3
block
block
BL
1
3
Instance 1
instance 2
instance 3
Figure 10-1.
Cache fusion processing 3-way
