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https://issues.apache.org/jira/browse/HDFS-4960?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13703745#comment-13703745
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Colin Patrick McCabe commented on HDFS-4960:
--------------------------------------------
Two proposals:
* check the version on the DataNode side rather than the client side. Then, we
don't have to re-check the header of checksum files that are already in
FileInputStreamCache.
* Add an optional boolean to OpRequestShortCircuitAccessProto that can be used
to request *only* the block file, not the checksum file. This will avoid the
overhead of duplicating the other file descriptor. In our testing, this
overhead was much higher than just doing a read.
Also some reminders:
* if you're optimizing for SSDs (as it says in the description), seeks don't
matter!
* the .meta file will likely be in the cache after a few reads, so seeks won't
happen anyway
* let's not commit anything without at least one test that shows it's better.
> Unnecessary .meta seeks even when skip checksum is true
> -------------------------------------------------------
>
> Key: HDFS-4960
> URL: https://issues.apache.org/jira/browse/HDFS-4960
> Project: Hadoop HDFS
> Issue Type: Bug
> Affects Versions: 3.0.0, 2.1.0-beta
> Reporter: Varun Sharma
> Assignee: Varun Sharma
> Attachments: 4960-branch2.patch, 4960-trunk.patch
>
>
> While attempting to benchmark an HBase + Hadoop 2.0 setup on SSDs, we found
> unnecessary seeks into .meta files, each seek was a 7 byte read at the head
> of the file - this attempts to validate the version #. Since the client is
> requesting no-checksum, we should not be needing to touch the .meta file at
> all.
> Since the purpose of skip checksum is to also avoid the performance penalty
> of the extra seek, we should not be seeking into .meta if skip checksum is
> true
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