Does Couchbase store documents in-memory first before moving the data to filestore? Is there any configuration available to specify how long the data has to be store in-memory before it can be flushed to file store?
Couchbase stores data both on disk and in RAM. The default behavior is to write the document to disk at some arbitrary time (usually quickly) after storing in RAM. This leaves a short window where node failure can result in loss of data. I can't find anything in the documentation for the current version of Couchbase, but it used to be that you could request the "set" method to only complete once the data has been persisted to disk (default is to RAM only).
In any case, after writing to RAM, the document will eventually be written to disk. Couchbase keeps a disk write queue which you can check on the metrics report page in the management console. Now, CB does synchronize writes across the cluster, and I believe a write will be synchronized across a cluster before Couchbase will acknowledge that the write happened (e.g. before the write method returns to the caller). Again, the documentation is hard to determine on this, as prior versions the documentation was much more detailed.
If you have more documents than available RAM, only the most-frequently accessed documents will be stored in RAM for quick retrieval, with all others being "evicted" to disk.