Thursday, March 14, 2019

Oracle RAC Startup sequence with ASM file system

OHASD is the root for bringing up Oracle Clusterware.

OHASD  ------------------------------------->   OLR (Provided Needed data to complete OHASD initialization)
                   has  access

2.        OHASD     --------------------------------------------->    GPNPD & CSSD (Cluster Sync Services)
                       Brings UP
                                                                                                                                            CSSD Has access of Grid Plug and Play
GPNP-à Profile Stored on the Local File system

è  ASM Diskgroup Discovery String
è  ASM SPFILE location (Diskgroup Name)
è  Name of the ASM Diskgroup containing the voting files.

3.       The Voting File location on ASM disks are accessed by CSSD with well-known pointers in the ASM disk header and CSSD is able to complete initialization and start or join an existing cluster.

4.       OHASD start as ASM instance and ASM can now operate with CSSD initialization and operating. The ASM instance uses special code to locate the contents of the ASM SPFILE assuming it is stored in diskgroup.

5.       With an ASM instance operating and its Diskgroup mounted, access to Clusterware’s OCR is available to CRSD.

6.       OHASD starts CRSD with access to the OCR in an ASM Diskgroup.

7.       Clusterware complete initialization and brings up other services under its control.

When Clusterware start, three files are involved.

OLR- First file to read and opened. This file contains information regarding where the voting disk is stored. And information to Startup the ASM (e.g. ASM Diskgroup string)

VOTING DISK- This is the 2nd file to be opened & read, this is depend on only OLR being accessible (ASM starts after CSSD or ASM does not start if CSSD is offline (i.e voting file missing).
To check voting file use- v$ASM_DISK column Voting_file

OCR- Finally the ASM instance start and mount all Diskgroup, then Clusterware daemon (CRSD) opens and reads the OCR which is stored on Diskgroup.
So, if ASM already started, ASM does not depends on OCR or OLR to be online. ASM depends on CSSD (vote disk) to be online.

- Piyush Mishra

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