When setting up a bucket, I can see an option of "memcached" and "couchDB"
What are the difference between these two options?
The memcached bucket is just plain memcached with some nice clustering features. The other bucket is the Couchabse bucket, not couchDB bucket. This bucket provides persistence as well as caching and in 2.0 it will have support for views. I recommend using the Couchbase bucket unless you want Couchbase Server to mimic memcached behavior.
I can see that after a while , the data goes to the disk. I ain't sure about this...
if so, what is the time interval ? and what is the circumstance to make this happen?
All data is written to disk as soon as possible. Once an item is set in Couchbase it is immediately put into a replication and persistence queue. Their is no specific time interval for data to be persisted to disk since and the only factor that changes the persistence latency is how heavy your workload is.
So using Couchbase option means that it automatically back-ups data on disk as soon as possible?
Is this to prevent failover?
Couchbase is a database. We provide two ways to run. The first is as a cache. If you want to do this you use the memcached bucket. The second way is as a database which is through the use of the Couchbase bucket. Please take a look at our white paper and let me know if you have any other questions.
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