Sorry the formatting got messed by my email client. Here it is again
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
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 layer
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
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:
1) The new code base benefits 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.
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
posted.
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
merge.
** The issues raised today will be raised all the same then.
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