it's fine two place 2 replica on the local rack's nodes first and then the third replica on the different rack node if the replica count is 3. now consider the scenario for the replication factor 2. if it place these two replica on the same rack, then you can loose all of your replica when the rack goes down. hope it will clear your doubt.
:::::::::::::::::::::::::::::::::::::::: Raj K Singh http://in.linkedin.com/in/rajkrrsingh http://www.rajkrrsingh.blogspot.com Mobile Tel: +91 (0)9899821370 On Sun, May 11, 2014 at 8:25 AM, jianan hu <hujia...@gmail.com> wrote: > Hi everyone, > > See HDFS documents, It says "For the common case, when the replication > factor is three, HDFS’s placement policy is to put one replica on one node > in the local rack, another on a node in a different (remote) rack, and the > last on a different node in the same remote rack." > > Assume there are two racks A and B. According to rack awareness, the first > block is put in rack A, and the the other two replicated blocks will be > pushed into rack B. > > However, why not store the first and second replicas in the local rack > (A), and the last in a different remote rack (B)? Both two scenarios have > same network traffic. What's the disadvantage of it? > > Thanks. > > Best Regards, > Jianan >
