IT sounds like you could benefit from reading the basic papers on map-reduce in general. Hadoop is a reasonable facsimile of the original Google systems.
Try looking at this: http://research.google.com/archive/mapreduce.html On Mon, Nov 19, 2012 at 7:14 AM, Kartashov, Andy <andy.kartas...@mpac.ca>wrote: > Thank you Kai.. One more question please. > > > > Does MapReduce run tasks of redundant blocks ? > > > > Say you have only 1 block of data replicated 3 times, one block over each > of three DNodes, block 1 – DN1 / block 1(replica #1) – DN2 / block1 > (replica #2) – DN3 > > > > Will MR attempt: > > > > a. to start 3 Map tasks (one per replicated block) end execute them > all > > b. to start 3 Map tasks (one per replicated block) end drop the > other two as soon as one of the three executed successfully > > c. will start only 1 Map task (for just one block avoiding all > replicated ones) and will attempt to start (another one of the replicated > blocks) when and only when the initially task running (say on DN1)failed > > > > Thanks, > > > > *From:* Kai Voigt [mailto:k...@123.org] > *Sent:* Monday, November 19, 2012 10:01 AM > > *To:* user@hadoop.apache.org > *Subject:* Re: a question on NameNode > > > > > > Am 19.11.2012 um 15:43 schrieb "Kartashov, Andy" <andy.kartas...@mpac.ca>: > > > > So, what if DN2 is down, i.e. it is not sending any blocks’ report. > Then NN (I guess) will figure out that it has 2 blocks (3,4) that has no > home and that (without replication) it has no way of reconstructing the > file A.txt. It must spit the error then. > > > > One major feature of HDFS is its redundancy. Blocks are stored more than > once (three times by default), so chances are good that another DataNode > will have that block and report it during the safe mode phase. So the file > will be accessible. > > > > Kai > > > > -- > > Kai Voigt > > k...@123.org > > > > > > > NOTICE: This e-mail message and any attachments are confidential, subject > to copyright and may be privileged. Any unauthorized use, copying or > disclosure is prohibited. If you are not the intended recipient, please > delete and contact the sender immediately. Please consider the environment > before printing this e-mail. AVIS : le présent courriel et toute pièce > jointe qui l'accompagne sont confidentiels, protégés par le droit d'auteur > et peuvent être couverts par le secret professionnel. Toute utilisation, > copie ou divulgation non autorisée est interdite. Si vous n'êtes pas le > destinataire prévu de ce courriel, supprimez-le et contactez immédiatement > l'expéditeur. Veuillez penser à l'environnement avant d'imprimer le présent > courriel >
