Tuning Global Cache - Expert Oracle RAC Performance Diagnostics and Tuning - page 395

a

b

c

SSKY1 SSKY2 SSKY4

SSKY3

R7

R5

R3

R1

R8

500:9996

1

R4

3

R2

2

SCN 9996

Block 450

Block 459 Block 550

Block 500

SSKYDB

Block 600 Block 490

Figure 13-5. Cache Fusion 3 way

i) Read the block from the disk.

A user session or process attached to instance SSKY3 makes a request for a block at DBA
file# 100, block# 500 (100/500). SSKY3 determines the master for this specific block that
contains the row is SSKY4 . The request is directed to instance SSKY4 where the GRD for
the object is maintained. Oracle allocates a node to be the resource master based on the
demand for the resource on a specific instance. If the object access increases on another node,
Oracle performs a remastering operation to move the resource master for the object to the node .

1.

Instance SSKY4 after checking against the GRD determines that neither instance SSKY4 nor
any other instance in the cluster has a copy of the block (100/500), so the data must be on
disk. Hence, a grant message is sent to the requesting instance to read the block from disk.
The grant gives the reader on instance SSKY3 the permission to read the block from disk.

2.

Instance SSKY3 reads the block from disk. The foreground process in instance SSKY3
initiates the I/O request to read the block 100/500 from disk. The row is contained in block
500 and has SCN 9996. Because Oracle reads a block of data at a time, other rows are also
retrieved as part of this read operation. The block is read into the buffer of instance SSKY3 .
Instance SSKY3 holds the block with SCN 9996 using a shared local (SL) mode.

3.