Suresh, Sanjay, Thank you very much for addressing my questions.
Cheers, Doug On 04/26/2011 10:29 AM, suresh srinivas wrote: > Doug, > > >> 1. Can you please describe the significant advantages this approach has >> over a symlink-based approach? > > Federation is complementary with symlink approach. You could choose to > provide integrated namespace using symlinks. However, client side mount > tables seems a better approach for many reasons: > # Unlike symbolic links, client side mount tables can choose to go to right > namenode based on configuration. This avoids unnecessary RPCs to the > namenodes to discover the targer of symlink. > # The unavailability of a namenode where a symbolic link is configured does > not affect reaching the symlink target. > # Symbolic links need not be configured on every namenode in the cluster and > future changes to symlinks need not be propagated to multiple namenodes. In > client side mount tables, this information is in a central configuration. > > If a deployment still wants to use symbolic link, federation does not > preclude it. > >> It seems to me that one could run multiple namenodes on separate boxes > and run multile datanode processes per storage box > > There are several advantages to using a single datanode: > # When you have large number of namenodes (say 20), the cost of running > separate datanodes in terms of process resources such as memory is huge. > # The disk i/o management and storage utilization using a single datanode is > much better, as it has complete view the storage. > # In the approach you are proposing, you have several clusters to manage. > However with federation, all datanodes are in a single cluster; with single > configuration and operationally easier to manage. > >> The patch modifies much of the logic of Hadoop's central component, upon > which the performance and reliability of most other components of the > ecosystem depend. > That is not true. > > # Namenode is mostly unchanged in this feature. > # Read/write pipelines are unchanged. > # The changes are mainly in datanode: > #* the storage, FSDataset, Directory and Disk scanners now have another > level to incorporate block pool ID into the hierarchy. This is not a > significant change that should cause performance or stability concerns. > #* datanodes use a separate thread per NN, just like the existing thread > that communicates with NN. > >> Can you please tell me how this has been tested beyond unit tests? > As regards to testing, we have passed 600+ tests. In hadoop, these tests > are mostly integration tests and not pure unit tests. > > While these tests have been extensive, we have also been testing this branch > for last 4 months, with QA validation that reflects our production > environment. We have found the system to be stable, performing well and have > not found any blockers with the branch so far. > > HDFS-1052 has been open more than a year now. I had also sent an email about > this merge around 2 months ago. There are 90 subtasks that have been worked > on last couple of months under HDFS-1052. Given that there was enough time > to ask these questions, your email a day before I am planning to merge the > branch into trunk seems late! >
