We have a service which writes one copy of a logfile directly into HDFS (writes go to namenode). As Dennis mentions, since HDFS does not support atomic appends, if a failure occurs before closing a file, it never appears in the file system. Thus we have to rotate logfiles at a greater frequency that we'd like to "checkpoint" the data into HDFS. The system certainly isn't perfect but bulk-loading the data into HDFS was proving rather slow. I'd be curious to hear actual performance numbers and methodologies for bulk loads. I'll try to dig some up myself on Monday.
On 8/2/07, Dennis Kubes <[EMAIL PROTECTED]> wrote: > > You can copy data from any node, so if you can do it from multiple nodes > your performance would be better (although be sure not to overlap > files). The master node is updated once a the block is copied it > replication number of times. So if default replication is 3 then the 3 > replicates must be active before the master is updated and the data > "appears" int the dfs. > > How long the updates take to happen is a function of your server load > and network speed and file size. Generally it is fast. > > So the process is the data is loaded into the dfs, replicates are > created, and the master node is updated. In terms of consistency, if > the data node crashes before the data is loaded then the data won't > appear in the dfs. If the name node crashes before it is updated but > all replicates are active, the data would appear once the name node has > been fixed and updated through block reports. If a single node crashes > that has a replicate once the namenode has been updated then the data > will be replicated from one of the other 2 replicates to another 3 > system if available. > > Dennis Kubes > > Venkates .P.B. wrote: > > Am I missing something very fundamental ? Can someone comment on these > > queries ? > > > > Thanks, > > Venkates P B > > > > On 8/1/07, Venkates .P.B. <[EMAIL PROTECTED]> wrote: > >> > >> Few queries regarding the way data is loaded into HDFS. > >> > >> -Is it a common practice to load the data into HDFS only through the > >> master node ? We are able to copy only around 35 logs (64K each) per > minute > >> in a 2 slave configuration. > >> > >> -We are concerned about time it would take to update filenames and > block > >> maps in the master node when data is loaded from few/all the slave > nodes. > >> Can anyone let me know how long generally it takes for this update to > >> happen. > >> > >> And one more question, what if the node crashes soon after the data is > >> copied into one it. How is data consistency maintained here ? > >> > >> Thanks in advance, > >> Venkates P B > >> > > >
