[jira] [Created] (HDDS-2447) Allow datanodes to operate with simulated containers

["Stephen O'Donnell (Jira)" <ji...@apache.org>]

Stephen O'Donnell created HDDS-2447:
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             Summary: Allow datanodes to operate with simulated containers
                 Key: HDDS-2447
                 URL: https://issues.apache.org/jira/browse/HDDS-2447
             Project: Hadoop Distributed Data Store
          Issue Type: Bug
          Components: Ozone Datanode
    Affects Versions: 0.5.0
            Reporter: Stephen O'Donnell


The Storage Container Manager (SCM) generally deals with datanodes and containers. Datanodes report their containers via container reports and the SCM keeps track of them, schedules new replicas to be created when needed etc. SCM does not care about individual blocks within the containers (aside from deleting them) or keys. Therefore it should be possible to scale test much of SCM without OM or worrying about writing keys.

In order to scale test SCM and some of its internal features like like decommission, maintenance mode and the replication manager, it would be helpful to quickly create clusters with many containers, without needing to go through a data loading exercise.

What I imagine happening is:

* We generate a list of container IDs and container sizes - this could be a fixed size or configured size for all containers. We could also fix the number of blocks / chunks inside a 'generated simulated container' so they are all the same.
* When the Datanode starts, if it has simulated containers enabled, it would optionally look for this list of containers and load the meta data into memory. Then it would report the containers to SCM as normal, and the SCM would believe the containers actually exist.
* If SCM creates a new container, then the datanode should create the meta-data in memory, but not write anything to disk.
* If SCM instructs a DN to replicate a container, then we should stream simulated data over the wire equivalent to the container size, but again throw away the data at the receiving side and store only the metadata in datanode memory.
* It would be acceptable for a DN restart to forget all containers and re-load them from the generated list. A nice-to-have feature would persist any changes to disk somehow so a DN restart would return to its pre-restart state.

At this stage, I am not too concerned about OM, or clients trying to read chunks out of these simulated containers (my focus is on SCM at the moment), but it would be great if that were possible too.

I believe this feature would let us do scale testing of SCM and benchmark some dead node / replication / decommission scenarios on clusters with much reduced hardware requirements.

It would also allow clusters with a large number of containers to be created quickly, rather than going through a dataload exercise.

This would open the door to a tool similar to https://github.com/linkedin/dynamometer which uses simulated storage on HDFS to perform scale tests against the namenode with reduced hardware requirements.

HDDS-1094 added the ability to have a level of simulated storage on a datanode. In that Jira, when a client writes data to a chunk the data is thrown away and nothing is written to disk. If a client later tries to read the data back, it just gets zeroed byte buffers. Hopefully this Jira could build on that feature to fully simulate the containers from the SCM point of view and later we can extend to allowing clients to create keys etc too.



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