[jira] [Commented] (KAFKA-615) Avoid fsync on log segment roll

["Neha Narkhede (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/KAFKA-615?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13728922#comment-13728922 ] 

Neha Narkhede commented on KAFKA-615:
-------------------------------------

Thanks for all the patches. Overall, it's a good patch. Few minor comments -

Log

1. Remove recoverFrom in the comments for Log
2. Remove count from comments for LogAppendInfo
3. In recoverLog(), it seems that it makes sense to break out of the while loop as soon as you find a segment that needs to be truncated. Right now, it goes back and tries to iterate through remaining unflushed segments which have been deleted
                
> Avoid fsync on log segment roll
> -------------------------------
>
>                 Key: KAFKA-615
>                 URL: https://issues.apache.org/jira/browse/KAFKA-615
>             Project: Kafka
>          Issue Type: Bug
>            Reporter: Jay Kreps
>            Assignee: Neha Narkhede
>         Attachments: KAFKA-615-v1.patch, KAFKA-615-v2.patch, KAFKA-615-v3.patch, KAFKA-615-v4.patch, KAFKA-615-v5.patch
>
>
> It still isn't feasible to run without an application level fsync policy. This is a problem as fsync locks the file and tuning such a policy so that the flushes aren't so frequent that seeks reduce throughput, yet not so infrequent that the fsync is writing so much data that there is a noticable jump in latency is very challenging.
> The remaining problem is the way that log recovery works. Our current policy is that if a clean shutdown occurs we do no recovery. If an unclean shutdown occurs we recovery the last segment of all logs. To make this correct we need to ensure that each segment is fsync'd before we create a new segment. Hence the fsync during roll.
> Obviously if the fsync during roll is the only time fsync occurs then it will potentially write out the entire segment which for a 1GB segment at 50mb/sec might take many seconds. The goal of this JIRA is to eliminate this and make it possible to run with no application-level fsyncs at all, depending entirely on replication and background writeback for durability.

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