The recovery catalog obtains crucial RMAN metadata from the target database control file. This metadata must be kept up-to-date;
hence, RMAN must synchronize with the control file of the target data frequently. RMAN generates a snapshot control file, which is a temporary backup control file, each time it resynchronizes. This snapshot control file ensures that RMAN has a consistent view of the control file either when refreshing the recovery catalog or when querying the control file.
Oracle Shared Server does not guarantee that only one RMAN session accesses a snapshot control file at any point in time.
The Oracle Shared Server is a process that manages multiple database sessions on a single server process.
It does this by sharing the resources of the server process among the multiple sessions. This can improve performance and scalability for database applications.
Warning: The Shared Server does not provide any special handling for snapshot control files.
This means that multiple RMAN sessions can access the same snapshot control file at the same time. This can lead to problems if the RMAN sessions are trying to update the snapshot control file at the same time.
For example, if one RMAN session is trying to back up the control file and another RMAN session is trying to restore the control file, this could cause
data corruption.
To avoid these problems, it is important to coordinate access to the snapshot control file when using RMAN with the Shared Server.
You can do this by using a locking mechanism, such as a
semaphore[1].
Here is an example of how to use a semaphore to coordinate access to the snapshot control file:
-- Create a semaphore to lock the snapshot control file.
CREATE SEMAPHORE snapshot_control_file_lock;
-- Acquire the semaphore before accessing the snapshot control file.
ACQUIRE snapshot_control_file_lock;
-- Access the snapshot control file here.
-- Release the semaphore after accessing the snapshot control file.
RELEASE snapshot_control_file_lock;
By using a semaphore to coordinate access to the snapshot control file, you can ensure that only one RMAN session accesses the snapshot control file at any point in time. This will help to prevent data corruption. Oracle recommends that you use a locking mechanism to coordinate access to the snapshot control file when using RMAN with the Shared Server.
If the recovery catalog is destroyed and its backup is not available, you can partially reconstruct the catalog from the current control file or control file backups. But, you should always aim to have a valid, recent backup of your recovery catalog.
