DFS for WebFarm Usage - Content Replication and Failover
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by Web Team at ORCS Web
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Content Replication

With R2, DFS replication uses what is called Remote Differential Compression (RDC) which will only update changes to files and will not send the entire file across the wire.  This is especially handy when replicating across a wide area network, but it is also good for this situation.

If you set up two or more folder targets using DFS Management, the wizard should have asked you if you want to set up replication, but if you did things in a different order, you can set it up manually after the fact.  This can be done using the DFS Management tool as well.

Changes to the servers are not immediate so DFS does not work well for transactional type data where both servers need to be 100% in sync within a couple seconds of each other.  However, for a website related situation that is mostly read intensive, DFS works great.

You have a few options, but in our situation we will use the Full mesh which means that any server will write to any other server.  This means that in a failure situation, the content changes made on the backup server will push back to the primary server when it is online again.

How Good Is It?

DFS failovers are impressive.  If the primary content server becomes unavailable, DFS will fail over to the backup content server in a small number of seconds.  In this webfarm situation almost every time that the primary server fails, the HTTP protocol will retry for a few seconds until IIS is able to serve up a successful page.

This means that there is zero downtime if the primary content server fails.  The only issue I ran into when testing is if the page load was 1/2 done when the primary server failed using master pages or web controls.  It could potentially process 1/2 of an ASP.NET page and fail processing the rest.  This is pretty rare and I would say that the failover is as close to perfect as can be.

A failure of the namespace server is even smoother, resulting in no noticeable downtime or slowness.


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