Federation is orthogonal with Pluggable Namespaces. That is, one can use Federation if needed, even while a distributed K-V store is used on the backend.
Limitations of Federated namenode for scaling namespace are well-documented in several places, including the Giraffa presentation. HBase is only one of the several namespace implementations possible. Thus, if HBase-based namespace implementation does not fit your performance needs, you have a choice of using something else. - milind -----Original Message----- From: Azuryy Yu [mailto:azury...@gmail.com] Sent: Saturday, October 05, 2013 6:41 PM To: hdfs-dev@hadoop.apache.org Subject: Re: [Proposal] Pluggable Namespace Hi Milind, HDFS federation can solve the NN bottle neck and memory limit problem. AbstractNameSystem design sounds good. but distributed meta storage using HBase should bring performance degration. On Oct 4, 2013 3:18 AM, "Milind Bhandarkar" <mbhandar...@gopivotal.com> wrote: > Hi All, > > Exec Summary: For the last couple of months, we, at Pivotal, along > with a couple of folks in the community have been working on making > Namespace implementation in the namenode pluggable. We have > demonstrated that it can be done without major surgery on the > namenode, and does not have noticeable performance impact. We would > like to contribute it back to Apache if there is sufficient interest. > Please let us know if you are interested, and we will create a Jira and update the patch for in-progress work. > > > Rationale: > > In a Hadoop cluster, Namenode roughly has following main responsibilities. > . Catering to RPC calls from clients. > . Managing the HDFS namespace tree. > . Managing block report, heartbeat and other communication from data nodes. > > For Hadoop clusters having large number of files and large number of > nodes, name node gets bottlenecked. Mainly for two reasons . All the > information is kept in name node's main memory. > . Namenode has to cater to all the request from clients / data nodes. > . And also perform some operations for backup and check pointing node. > > A possible solution is to add more main memory but there are certain > issues with this approach . Namnenode being Java application, garbage > collection cycles execute periodically to reclaim unreferenced heap > space. When the heap space grows very large, despite of GC policy > chosen, application stalls during the GC activity. This creates a > bunch of issues since DNs and clients may perceive this stall as NN > crash. > . There will always be a practical limit on how much physical memory a > single machine can accommodate. > > Proposed Solution: > > Out of the three responsibilities listed above, we can refactor > namespace management from the namenode codebase in such a way that > there is provision to implement and plug other name systems other than > existing in-process memory-based name system. Particularly a name > system backed by a distributed key-value store will significantly > reduce namenode memory requirement.To achieve this, a new generic > interface will be introduced [Let's call it AbstractNameSystem] which > defines set of operations using which we perform the namespace > management. Namenode code that used to manipulate some java objects > maintained in namenode's heap will now operate on this interface. > There will be provision for others to extend this interface and plug their own NameSystem implementation. > > To get started, we have implemented the same memory-based namespace > implementation in a remote process, outside of the namenode JVM. In > addition, work is undergoing to implement the namesystem using HBase. > > Details of Changes: > > Created new class called AbstractNamesystem, existing FSNamesystem is > a subclass of this class. Some code from FSNamesystem has been moved > to its parent. Created a Factory class to create object of NS > management class.Factory refers to newly added config properties to > support pluggable name space management class. Added unit tests for > Factory. Replaced constructors with factory calls, this is because > the namesystem instances should now be created based on configuration. > Added new config properties to support pluggable name space management > class. This property will decide which Namesystem class will be > instantiated by the factory. This change is also reflected in some DFS > related webapps [JSP files] where namesystem instance is used to obtain DFS health and other stats. > > These changes aim to make the namesystem pluggable without changing > high level interfaces, this is particularly tricky since memory-based > name system functionality is currently baked into these interfaces, > and ultimate goal is to make the high level interface free from > memory-based name system. > > Consideration for Upgrade and Rollback: > > Current memory based implementation already has code to read from and > write to fsimage , we will have to make them publicly accessible which > will enable us to upgrade an existing cluster from FSNamespace to > newly added name system in future version. > > a. Upgrades: By making use of existing Loader class for reading > fsimage we can write some code load this image into the future name > system implementation. > > b. Rollback: Are even simpler, we can preserve the old fsimage and > start the cluster with that image by configuring the cluster to use > current file system based name system. > > Future work > > Current HDFS design is such that FSNameSystem is baked into even high > level interfaces, this is a major hurdle in cleanly implementing > pluggable name systems. We aim to propose a change in such interfaces > into which FSNameSystem is tightly coupled. > > - Milind > > > --- > Milind Bhandarkar > Chief Scientist > Pivotal >
