Elek, Marton created HDDS-1284:
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Summary: Adjust default values of pipline recovery for more
resilient service restart
Key: HDDS-1284
URL: https://issues.apache.org/jira/browse/HDDS-1284
Project: Hadoop Distributed Data Store
Issue Type: Bug
Reporter: Elek, Marton
Assignee: Elek, Marton
As of now we have a following algorithm to handle node failures:
1. In case of a missing node the leader of the pipline or the scm can detected
the missing heartbeats.
2. SCM will start to close the pipeline (CLOSING state) and try to close the
containers with the remaining nodes in the pipeline
3. After 5 minutes the pipeline will be destroyed (CLOSED) and a new pipeline
can be created from the healthy nodes (one node can be part only one pipwline
in the same time).
While this algorithm can work well with a big cluster it doesn't provide very
good usability on small clusters:
Use case1:
Given 3 nodes, in case of a service restart, if the restart takes more than
90s, the pipline will be moved to the CLOSING state. For the next 5 minutes
(ozone.scm.pipeline.destroy.timeout) the container will remain in the CLOSING
state. As there are no more nodes and we can't assign the same node to two
different pipeline, the cluster will be unavailable for 5 minutes.
Use case2:
Given 90 nodes and 30 pipelines where all the pipelines are spread across 3
racks. Let's stop one rack. As all the pipelines are affected, all the
pipelines will be moved to the CLOSING state. We have no free nodes, therefore
we need to wait for 5 minutes to write any data to the cluster.
These problems can be solved in multiple ways:
1.) Instead of waiting 5 minutes, destroy the pipeline when all the containers
are reported to be closed. (Most of the time it's enough, but some container
report can be missing)
2.) Support multi-raft and open a pipeline as soon as we have enough nodes
(even if the nodes already have a CLOSING pipelines).
Both the options require more work on the pipeline management side. For 0.4.0
we can adjust the following parameters to get better user experience:
{code}
<property>
<name>ozone.scm.pipeline.destroy.timeout</name>
<value>60s</value>
<tag>OZONE, SCM, PIPELINE</tag>
<description>
Once a pipeline is closed, SCM should wait for the above configured time
before destroying a pipeline.
</description>
<property>
<name>ozone.scm.stale.node.interval</name>
<value>90s</value>
<tag>OZONE, MANAGEMENT</tag>
<description>
The interval for stale node flagging. Please
see ozone.scm.heartbeat.thread.interval before changing this value.
</description>
</property>
{code}
First of all, we can be more optimistic and mark node to stale only after 5
mins instead of 90s. 5 mins should be enough most of the time to recover the
nodes.
Second: we can decrease the time of ozone.scm.pipeline.destroy.timeout. Ideally
the close command is sent by the scm to the datanode with a HB. Between two HB
we have enough time to close all the containers via ratis. With the next HB,
datanode can report the successful datanode. (If the containers can be closed
the scm can manage the QUASI_CLOSED containers)
We need to wait 29 seconds (worst case) for the next HB, and 29+30 seconds for
the confirmation. --> 66 seconds seems to be a safe choice (assuming that 6
seconds is enough to process the report about the successful closing)
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