Capacity Planning and Architecture - Expert Oracle RAC Performance Diagnostics and Tuning - page 52

a

c b

SSKY2 SSKY3 SSKY4

SSKY1

R7

R5

R1

R3

R8

500:9996

1

R4

R2

3

2

SCN 9996

Block 450
Block 490

Block 459 Block 550

Block 500

Block 600

SSKYDB

Figure 2-14. Three-way block transfer

As illustrated in Figure 2-14 , there are two possibilities when the block is not found on the instance that is the
master of the object (resource).

• Read the block from the disk.

1. Instance SSKY1 requests for the block 500 from the GCS located on instance SSKY4 .

2. 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 requested. Hence,
it sends a message to the requesting instance to read the block from disk.

3. Instance SSKY3 reads the block from disk.
Request another instance to transfer the block. •

a. Instance SSKY2 requests block 500 from the GCS located on instance SSKY4 .

b. Instance SSKY4 verifies against its GRD and determines that the block is currently

held by instance SSKY3 . It sends a message to instance SSKY3 requesting it to send a
copy of the block to instance SSKY2 .

c. Instance SSKY3 accepts the request and sends the block to instance SSKY2 .