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From Jitendra Pandey <jiten...@hortonworks.com>
Subject [VOTE] Merging branch HDFS-7240 to trunk
Date Mon, 26 Feb 2018 21:18:56 GMT
    Dear folks,
           We would like to start a vote to merge HDFS-7240 branch into trunk. The context
can be reviewed in the DISCUSSION thread, and in the jiras (See references below).
    HDFS-7240 introduces Hadoop Distributed Storage Layer (HDSL), which is a distributed,
replicated block layer.
    The old HDFS namespace and NN can be connected to this new block layer as we have described
in HDFS-10419.
    We also introduce a key-value namespace called Ozone built on HDSL.
    The code is in a separate module and is turned off by default. In a secure setup, HDSL
and Ozone daemons cannot be started.
    The detailed documentation is available at 
    I will start with my vote.
            +1 (binding)
    Discussion Thread:
            On 2/13/18, 6:28 PM, "sanjay Radia" <sanjayosrc@gmail.com> wrote:
                Sorry the formatting got messed by my email client.  Here it is again
                 Hadoop Community Members,
                   We had multiple community discussions, a few meetings in smaller groups
and also jira discussions with respect to this thread. We express our gratitude for participation
and valuable comments. 
                The key questions raised were following
                1) How the new block storage layer and OzoneFS benefit HDFS and we were asked
to chalk out a roadmap towards the goal of a scalable namenode working with the new storage
                2) We were asked to provide a security design
                3)There were questions around stability given ozone brings in a large body
of code.
                4) Why can’t they be separate projects forever or merged in when production
                We have responded to all the above questions with detailed explanations and
answers on the jira as well as in the discussions. We believe that should sufficiently address
community’s concerns. 
                Please see the summary below:
                1) The new code base benefits HDFS scaling and a roadmap has been provided.

                  - New block storage layer addresses the scalability of the block layer.
We have shown how existing NN can be connected to the new block layer and its benefits. We
have shown 2 milestones, 1st milestone is much simpler than 2nd milestone while giving almost
the same scaling benefits. Originally we had proposed simply milestone 2 and the community
felt that removing the FSN/BM lock was was a fair amount of work and a simpler solution would
be useful
                  - We provide a new K-V namespace called Ozone FS with FileSystem/FileContext
plugins to allow the users to use the new system. BTW Hive and Spark work very well on KV-namespaces
on the cloud. This will facilitate stabilizing the new block layer. 
                  - The new block layer has a new netty based protocol engine in the Datanode
which, when stabilized, can be used by  the old hdfs block layer. See details below on sharing
of code.
                2) Stability impact on the existing HDFS code base and code separation. The
new block layer and the OzoneFS are in modules that are separate from old HDFS code - currently
there are no calls from HDFS into Ozone except for DN starting the new block  layer module
if configured to do so. It does not add instability (the instability argument has been raised
many times). Over time as we share code, we will ensure that the old HDFS continues to remains
stable. (for example we plan to stabilize the new netty based protocol engine in the new block
layer before sharing it with HDFS’s old block layer)
                3) In the short term and medium term, the new system and HDFS  will be used
side-by-side by users. Side by-side usage in the short term for testing and side-by-side in
the medium term for actual production use till the new system has feature parity with old
HDFS. During this time, sharing the DN daemon and admin functions between the two systems
is operationally important:  
                  - Sharing DN daemon to avoid additional operational daemon lifecycle management
                  - Common decommissioning of the daemon and DN: One place to decommission
for a node and its storage.
                  - Replacing failed disks and internal balancing capacity across disks -
this needs to be done for both the current HDFS blocks and the new block-layer blocks.
                  - Balancer: we would like use the same balancer and provide a common way
to balance and common management of the bandwidth used for balancing
                  - Security configuration setup - reuse existing set up for DNs rather then
a new one for an independent cluster.
                4) Need to easily share the block layer code between the two systems when
used side-by-side. Areas where sharing code is desired over time: 
                  - Sharing new block layer’s  new netty based protocol engine for old HDFS
DNs (a long time sore issue for HDFS block layer). 
                  - Shallow data copy from old system to new system is practical only if within
same project and daemon otherwise have to deal with security setting and coordinations across
daemons. Shallow copy is useful as customer migrate from old to new.
                  - Shared disk scheduling in the future and in the short term have a single
round robin rather than independent round robins.
                While sharing code across projects is technically possible (anything is possible
in software),  it is significantly harder typically requiring  cleaner public apis etc. Sharing
within a project though internal APIs is often simpler (such as the protocol engine that we
want to share).
                5) Security design, including a threat model and and the solution has been
                6) Temporary Separation and merge later: Several of the comments in the jira
have argued that we temporarily separate the two code bases for now and then later merge them
when the new code is stable:
                  - If there is agreement to merge later, why bother separating now - there
needs to be to be good reasons to separate now.  We have addressed the stability and separation
of the new code from existing above.
                  - Merge the new code back into HDFS later will be harder. 
                    **The code and goals will diverge further. 
                    ** We will be taking on extra work to split and then take extra work to
                    ** The issues raised today will be raised all the same then.
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