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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