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https://issues.apache.org/jira/browse/HDFS-1780?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Todd Lipcon resolved HDFS-1780.
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Resolution: Fixed
Hadoop Flags: [Reviewed]
> reduce need to rewrite fsimage on statrtup
> ------------------------------------------
>
> Key: HDFS-1780
> URL: https://issues.apache.org/jira/browse/HDFS-1780
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Affects Versions: Edit log branch (HDFS-1073)
> Reporter: Daryn Sharp
> Assignee: Todd Lipcon
> Fix For: Edit log branch (HDFS-1073)
>
> Attachments: hdfs-1780.txt, hdfs-1780.txt
>
>
> On startup, the namenode will read the fs image, apply edits, then rewrite
> the fs image. This requires a non-trivial amount of time for very large
> directory structures. Perhaps the namenode should employ some logic to
> decide that the edits are simple enough that it doesn't warrant rewriting the
> image back out to disk.
> A few ideas:
> Use the size of the edit logs, if the size is below a threshold, assume it's
> cheaper to reprocess the edit log instead of writing the image back out.
> Time the processing of the edits and if the time is below a defined
> threshold, the image isn't rewritten.
> Timing the reading of the image, and the processing of the edits. Base the
> decision on the time it would take to write the image (a multiplier is
> applied to the read time?) versus the time it would take to reprocess the
> edits. If a certain threshold (perhaps percentage or expected time to
> rewrite) is exceeded, rewrite the image.
> Somethingalong the lines of the last suggestion may allow for defaults that
> adapt for any size cluster, thus eliminating the need to keep tweaking a
> cluster's settings based on its size.
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