What’s the role of an Oracle VM refresh server?
By PoorFrodo on Aug 01, 2014
In order to understand how to effectively manage NFS file systems within the Oracle VM Manager, we must first understand the underlying assumption that each NFS server being accessed by Oracle VM servers are for the exclusive use of Oracle VM. It is assumed that no non-Oracle file systems are being shared/exported from a storage array that is part of an Oracle VM environment.
Prior to Oracle VM 3.2, it was assumed that NFS servers in the data center would export all NFS file systems with full read-write access to all Oracle VM servers across multiple server pools. In other words, all NFS file systems would be uniformly exported to all Oracle VM servers even if some file systems were for the exclusive use by Oracle VM servers in different server pools.
Using an Oracle VM Manager with three server pools as an example, NFS exports intended for Oracle VM servers belonging to pool1 would also be accessible by the Oracle VM servers belonging to pool2 and pool3. Oracle VM uses the term uniform exports to describe this model for exporting all NFS file systems to all Oracle VM servers. This is the only option in Oracle VM 3.0 and 3.1.
The concept of non-uniform exports was introduced with Oracle VM 3.2. Using the same example of three server pools above, the idea of non-uniform exports allows NFS file systems meant for pool1 to be exported only to the Oracle VM servers that belong to pool1; the same NFS export policy would be applied to the Oracle VM servers belonging to pool2 and pool3. So, Oracle VM servers belonging to one server pool may not have access to NFS file systems belonging to Oracle VM server in another server pool.
So, if you are discovering an NFS server using uniform exports, don’t assign any refresh servers even though the Discover File Server wizard in Oracle VM 3.2 and 3.3 present you with a screen asking you to assign refresh servers – ignore this last screen for refresh servers if you are using uniform exports. The next major release of Oracle VM 3 will not present an opportunity to assign refresh servers when using uniform exports.
However, if you are using non-uniform exports then you will assign refresh servers. The more refresh servers you assign, the longer your NFS file server discovery and subsequent refreshes will take. In addition, if an assigned refresh server becomes unavailable for any reason, then it must be manually removed as an active refresh server before any other assigned refresh servers will be used.
So, a best practice is to assign only one refresh server as an active refresh server to improve performance of discoveries and refreshes. Another best practice is to have at least one additional server that can be used as an “alternate” refresh server in case the active one becomes unavailable for any reason; in this case the “alternate” refresh server is simply an Oracle VM server that has the same access rights as the active refresh server, but it is not actually assigned as a refresh server until needed.
Finally, any Oracle VM server you want to use as refresh server must have full read-write access to all NFS file systems being exported from the NFS server. The illustration below shows the three fictitious server pools I mentioned above. Each server pool has two Oracle VM servers: myserver13 & 14 in pool1, myserver15 & 16 in pool2 and myserver17 & 18 in pool3. The Oracle VM servers belonging to each server pool only have read-write access to two out of the six NFS file systems being exported from the NFS server.
Let’s say I decide to use myserver13 as the one and only refresh server for all three server pools (remember, you should have at least one other refresh server in reserve, but I’m trying to keep a complex subject as simple as possible). I need to ensure that all six NFS file systems are exported as read-write to myserver13. The rest of the Oracle VM servers only have read-write access to the two NFS file systems meant for each server pool.
This is just a small sample of some of the concepts you need to understand in order to plan, implement and manage NFS file systems effectively in Oracle VM. Visit the new Oracle VM 3.3 Concepts Guide for more detailed information about discovering and managing NFS file servers in Oracle VM Manager.