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From sanjay Radia <sanjayo...@gmail.com>
Subject Re: [DISCUSSION] Merging HDFS-7240 Object Store (Ozone) to trunk
Date Wed, 14 Feb 2018 01:03:12 GMT
Dear 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 
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 layer
We were asked to provide a security design
There were questions around stability given ozone brings in a large body of code.
Why can’t they be separate projects forever or merged in when production ready?

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:

The new code base benefits to HDFS scaling and a roadmap has been provided. Summary:
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.
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)
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.
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).
Security design, including a threat model and and the solution has been posted.
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 merge. 
The issues raised today will be raised all the same then.


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