SCAN Concepts
- Single client access name (SCAN) is the virtual hostname to provide for all clients connecting to the cluster (as opposed to the vip hostnames in 10g and 11gR1).
- SCAN is a domain name registered to at least one and up to three IP addresses, either in the domain name service (DNS) or the Grid Naming Service (GNS).
- By default, the name used as the SCAN is also the name of the cluster and must be globally unique throughout your enterprise. The default value for the SCAN is based on the local node name. SCAN name must be at least one character long and no more than 15 characters in length, must be alphanumeric - cannot begin with a numeral and may contain hyphens (-). If you require a SCAN that is longer than 15 characters, then select an Advanced installation.
- For installation to succeed, the SCAN must resolve to at least one address.
- SCAN VIP addresses must be on the same subnet as virtual IP addresses and public IP addresses.
- Oracle strongly recommends that you do not configure SCAN VIP addresses in the hosts file. But if you use the hosts file to resolve SCAN name, you can have only one SCAN IP address.
- If hosts file is used to resolve SCAN hostname, you will receive Cluster Verification Utility failure at end of installation (see Note: 887471.1 for more details)
- For high availability and scalability, Oracle recommends that you configure the SCAN to use DNS Round Robin resolution to three addresses.
- Because the SCAN is associated with the cluster as a whole, rather than to a particular node, the SCAN makes it possible to add or remove nodes from the cluster without needing to reconfigure clients. It also adds location independence for the databases, so that client configuration does not have to depend on which nodes are running a particular database.
- Clients can continue to access the cluster in the same way as with previous releases, but Oracle recommends that clients accessing the cluster use the SCAN. Clients using the SCAN can also access the cluster using EZCONNECT.
- Grid Infrastructure will start local listener LISTENER on all nodes to listen on local VIP, and SCAN listener LISTENER_SCAN1 (up to three cluster wide) to listen on SCAN VIP(s); 11gR2 database by default will set local_listener to local LISTENER, and remote_listener to SCAN listener.
The Basics of 11gR2:
- The clusterware is now known as Grid Infrastructure.
- CRS and ASM binaries reside under a single Oracle Home known as GRID Home.
- GRID Home and the Oracle Home must be installed in different locations.
- Oracle Grid Infrastructure cannot be placed under Oracle Base since permissions will be changed to root.
- OCR and Voting File can no longer be placed on RAW or block devices. They need to be placed either in ASM or on Shared NFS Storage.
- Voting file does not need to be backed up using dd command and is not supported anymore.The contents of Voting file are backed into the OCR.
Oracle 11g bug.
BUGs:
- Hugepages not being used. First check if /etc/security/limits.conf is set correctly for 'memlock'. You can also check 'ulimit -l'. However there is a bug with 11.2.0.1 where if database is started by srvctl and not sqlplus then hugepages are not used. The workaround is to edit either $GRID_HOME/bin/ohasd or /etc/init.d/ohasd and put 'ulimit -l unlimited' explicitly. This bug is supposedly fixed in 11.2.0.2.
- If loopback database link is used, the SQL would hang forever waiting on 'DFS Lock Wait'. Till date the issue persists and there doesn't seem to be fix from Oracle. Unofficially though I know one of the underscore parameter fixes the issue but would essentially disable majority of cluster features.
