Ariel Weisberg created CASSANDRA-20692:
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Summary: Direct IO commit log does not flush data safely
Key: CASSANDRA-20692
URL: https://issues.apache.org/jira/browse/CASSANDRA-20692
Project: Apache Cassandra
Issue Type: Bug
Components: Local/Commit Log
Reporter: Ariel Weisberg
[~maxwellguo] spotted this a few days ago.
The commit log is not safe as currently written with Direct IO. It writes to
the file, but doesn't sync the metadata on flush. That means the commit log may
claim it has flushed (and made durable) the data, but the filesystem journal
has not been flushed so the file length could be wrong and could truncate the
file on restart.
Additionally Direct IO doesn't actually make data durable on disk (emit write
barriers) it just flushes it to the cache of the disk. If the disk cache is
volatile then it can lose metadata and data.
It can probably be fixed pretty trivially by opening the file with {{D_SYNC}}
because the commit log writes up to the entire segment when it flushes so there
is no issue with needing to add buffering to avoid too many small writes.
[~amit_pawar] [~jlewandowski] [~blambov]
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