Tuning the Storage Subsystem - Expert Oracle RAC Performance Diagnostics and Tuning - page 372

5 5 Used 29722
5 6 Free 73311
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How Many Diskgroups?
Quite often these question are asked: How many disks are required for an RAC database? What is the best practice for
creating diskgroups? While the best practice is to isolate the clusterware data and real data from recoverable data and
backup files, there are no fixed rules for how many diskgroups are ideal.
The number of diskgroups should depend on the type of implementation, the criticality of the data, and the
storage performance characteristics. For a small database with less I/O activity, the configuration could consist of
three diskgroups:

• GRID_DATA — to store OCR and voting disk

• SSKY_DATA — to store database files

• SSKY_FRA — to have the multiplex copies of redolog files and fast recovery area files

For a more critical database with high I/O, and when the decision to keep redolog files on RAID1 compared to
the other files, it would be good practice to have five diskgroups:

• GRID_DATA — to store OCR and voting files

• SSKY_DATA — to store database data files

• SSKY_REDO1 — to have the redolog files

• SSKY_REDO2 — to multiplex the redolog files

• SSKY_FRA — to store backup sets, fast recovery area files, and so on

In the above configuration, SSKY_REDO1, SSKY_REDO2, and SSKY_FRA will be on RAID1. GRID_DATA and
SSKY_DATA could be RAID10/RAID01 or RAID5 disk arrays.

Monitoring ASM
In an earlier section we discussed how an ASM instance and an RDBMS instance would communicate to complete
specific steps during instance startup. During this process of communication, and during the various administrative
functions performed by ASM on the diskgroups, ASM would require resources. As in a RDBMS instance, in spite of
ASM being a lightweight instance, it also contains an SGA. Similar to an RAC environment, ASM instances also have
cache fusion activity over the interconnect. For example, the default SGA is:

SQL> show sga
Total System Global Area 283930624 bytes
Fixed Size 2212656 bytes
Variable Size 256552144 bytes
ASM Cache 25165824 bytes
SQL>