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From: Vinod Kumar Vavilapalli <vinodkv@hortonworks.com>
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Subject: Re: [Proposal] Pluggable Namespace
Date: Sun, 6 Oct 2013 12:20:35 -0700
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In order to make federation happen, the block pool management was already separated. Isn't that the same as this effortt?

Thanks,
+Vinod

On Oct 6, 2013, at 9:35 AM, Milind Bhandarkar wrote:

> 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:azuryyyu@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" <mbhandarkar@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
>> 


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