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https://issues.apache.org/jira/browse/HADOOP-14965?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17512529#comment-17512529
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Steve Loughran commented on HADOOP-14965:
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[~yzhangal] what is the application reading the data doing?
* is it doing random IO, sequential, or forward-with
* which APIs is it using (seek and read, readFully, etc)
"normal" is sequentia; except on a backwards seek, when it switches to random.
so the only way this would be worse is if you read something at the end of a
file, then went back and did a large read to the end of the file. Is this what
is happening?
also, what is the value of "fs.s3a.readahead.range"? try a larger number (say
512k) and see what it does
> s3a input stream "normal" fadvise mode to be adaptive
> -----------------------------------------------------
>
> Key: HADOOP-14965
> URL: https://issues.apache.org/jira/browse/HADOOP-14965
> Project: Hadoop Common
> Issue Type: Sub-task
> Components: fs/s3
> Affects Versions: 2.8.1
> Reporter: Steve Loughran
> Assignee: Steve Loughran
> Priority: Major
> Fix For: 3.1.0, 3.0.1
>
> Attachments: HADOOP-14965-001.patch, HADOOP-14965-002.patch,
> HADOOP-14965-003.patch, HADOOP-14965-004.patch
>
>
> HADOOP-14535 added seek optimisation to wasb, but rather than require the
> caller to declare sequential vs random, it works out for itself.
> # defaults to sequential, lazy seek
> # if the caller ever seeks backwards, switches to random IO.
> This means that on the use pattern of columnar stores: of go to end of file,
> read summary, then go to columns and work forwards, will switch to random IO
> after that first seek back (cost: one aborted HTTP connection)/.
> Where this should benefit the most is in downstream apps where you are
> working with different data sources in the same object store/running of the
> same app config, but have different read patterns. I'm seeing exactly this in
> some of my spark tests, where it's near impossible to set things up so that
> .gz files are read sequentially, but ORC data is read in random IO
> I propose the "normal" fadvise => adaptive, sequential==sequential always,
> random => random from the outset.
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