J-D thanks for your reply. I have some doubts which I posted inline . Kindly help me
On Tue, Mar 30, 2010 at 2:23 AM, Jean-Daniel Cryans <jdcry...@apache.org>wrote: > Inline. > > J-D > > On Mon, Mar 29, 2010 at 11:45 AM, john smith <js1987.sm...@gmail.com> > wrote: > > Hi all, > > > > I read the issue HBase-57 ( > https://issues.apache.org/jira/browse/HBASE-57 ) > > . I don't really understand the use of assigning regions keeping DFS in > > mind. Can anyone give an example usecase showing its advantages > > A region is composed of files, files are composed of blocks. To read > data, you need to fetch those blocks. In HDFS you normally have access > to 3 replicas and you fetch one of them over the network. If one of > the replica is on the local datanode, you don't need to go through the > network. This means less network traffic and better response time. > Is this the scenario that occurs for catering the read requests? In the thread "Data distribution in HBase" , one of the people mentioned that the data hosted by the Region Server may not actually reside on the same machine . So when asked for data , it fetches from the system containing the data. Am I right? Why is the data hosted by a "Region Server" doesn't lie on the same machine . Doesn't the name name "Region Server" imply that it holds all the regions it contains? Is it due to splits or restarting the HBase ? > > > Can > > map-reduce exploit it's advantage in any way (if data is distributed in > the > > above manner) or is it just the read-write performance that gets > improved . > > MapReduce works in the exact same way, it always tries to put the > computation next to where the data is. I recommend reading the > MapReduce tutorial > http://hadoop.apache.org/common/docs/r0.20.0/mapred_tutorial.html#Overview > Also the same case Applies here I guess . When a map is run on a Region Server, It's data may not actually lie on the same machine . So it fetches from the machine containing it. This reduces the data locality ! > > > Can some one please help me in understanding this. > > > > Regards > > JS > > >
