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https://issues.apache.org/jira/browse/CASSANDRA-3116?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13095416#comment-13095416
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Tom Wilkie commented on CASSANDRA-3116:
---------------------------------------

Its worse than this too; a migration tried to get the write lock, which gets 
blocked on a big compaction (holding the read lock).  This migration waiting on 
the write lock then blocks all other compactions waiting on the read lock.  So 
you only get one compaction going on and thousands backing up.

A really hacky temporary fix would be to use a tryLock(timeout) and short sleep 
in a loop in the migration.  This would at least not starve the merges, but 
would starve the migrations quite badly.

> Compactions can (seriously )delay schema migrations
> ---------------------------------------------------
>
>                 Key: CASSANDRA-3116
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-3116
>             Project: Cassandra
>          Issue Type: Bug
>          Components: Core
>    Affects Versions: 0.8.4
>            Reporter: Eric Evans
>             Fix For: 1.1
>
>
> A compaction lock is acquired when dropping keyspaces or column families 
> which will cause the schema migration to block if a compaction is in progress.

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