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 
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. 
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 
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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