Wednesday, October 6, 2021

RAC : 11gR2 Clusterware Startup Sequence 

Here is the brief explanation that how the clusterware brings up step by step .

1. When a node of an Oracle Clusterware cluster start/restarts, OHASD is started by platform-specific means. OHASD is the root for bringing up Oracle Clusterware. OHASD has access to the OLR (Oracle Local Registry) stored on the local file system. OLR provides needed data to complete OHASD initialization.
2. OHASD brings up GPNPD and CSSDCSSD has access to the GPNP Profile stored on the local file system. This profile contains the following vital bootstrap data;
        a. ASM Diskgroup Discovery String
        b. ASM SPFILE location (Diskgroup name)
        c. Name of the ASM Diskgroup containing the Voting Files
3. The Voting Files locations on ASM Disks are accessed by CSSD with well-known pointers in the ASM Disk headers and CSSD is able to complete initialization and start or join an existing cluster.
4. OHASD starts an ASM instance and ASM can now operate with CSSD initialized and operating. The ASM instance uses special code to locate the contents of the ASM SPFILE, assuming it is stored in a Diskgroup.
5. With an ASM instance operating and its Diskgroups 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 completes initialization and brings up other services under its control.

When Clusterware starts three files are involved.

1. OLR – Is the first file to be read and opened. This file is local and this file contains information regarding where the voting disk is stored
and information to startup the ASM. (e.g ASM DiscoveryString)

2. VOTING DISK – This is the second file to be opened and read, this is dependent on only OLR being accessible.

ASM starts after CSSD or ASM does not start if CSSD is offline (i.e voting file missing)

How are Voting Disks stored in ASM?

Voting disks are placed directly on ASMDISK. Oracle Clusterware will store the votedisk on the disk within a disk group that holds the Voting Files.
Oracle Clusterware does not rely on ASM to access the Voting Files, which means Oracle Clusterware does not need of Diskgroup to read and write on ASMDISK. It is possible to check for existence of voting files on a ASMDISK using the V$ASM_DISK column VOTING_FILE.
So, voting files not depend of Diskgroup to be accessed, does not mean that the diskgroup is not needed, diskgroup and voting file are linked by their settings.

3. OCR – Finally the ASM Instance starts and mount all Diskgroups, then Clusterware Deamon (CRSD) opens and reads the OCR which is stored on Diskgroup.

So, if ASM already started, ASM does not depend on OCR or OLR to be online. ASM depends on CSSD (Votedisk) to be online.

There is a exclusive mode to start ASM without CSSD (but it’s to restore OCR or VOTE purposes)

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