Oleksandr Shulgin created CASSANDRA-14210:
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Summary: Optimize SSTables upgrade task scheduling
Key: CASSANDRA-14210
URL: https://issues.apache.org/jira/browse/CASSANDRA-14210
Project: Cassandra
Issue Type: Improvement
Components: Compaction
Reporter: Oleksandr Shulgin
When starting the SSTable-rewrite process by running {{nodetool upgradesstables
--jobs N}}, with N > 1, not all of the provided N slots are used.
For example, we were testing with {{concurrent_compactors=5}} and {{N=4}}.
What we observed both for version 2.2 and 3.0, is that initially all 4 provided
slots are used for "Upgrade sstables" compactions, but later when some of the 4
tasks are finished, no new tasks are scheduled immediately. It takes the last
of the 4 tasks to finish before new 4 tasks would be scheduled. This happens
on every node we've observed.
This doesn't utilize available resources to the full extent allowed by the
--jobs N parameter. In the field, on a cluster of 12 nodes with 4-5 TiB data
each, we've seen that the whole process was taking more than 7 days, instead of
estimated 1.5-2 days (provided there would be close to full N slots
utilization).
Instead, new tasks should be scheduled as soon as there is a free compaction
slot.
Additionally, starting from the biggest SSTables could further reduce the total
time required for the whole process to finish on any given node.
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