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https://issues.apache.org/jira/browse/CASSANDRA-14210?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16369382#comment-16369382
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Marcus Eriksson commented on CASSANDRA-14210:
---------------------------------------------
pushed a few improvments here:
https://github.com/krummas/cassandra/commits/kgreav/14210
> 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
> Assignee: Kurt Greaves
> Priority: Major
> Fix For: 4.x
>
>
> 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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