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https://issues.apache.org/jira/browse/HDFS-4089?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13481063#comment-13481063
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Todd Lipcon commented on HDFS-4089:
-----------------------------------
Hi Jan. The docs for this config say:
{quote}
If this configuration is enabled, the datanode will instruct the
operating system to enqueue all written data to the disk immediately
after it is written. This differs from the usual OS policy which
may wait for up to 30 seconds before triggering writeback.
This may improve performance for some workloads by smoothing the
IO profile for data written to disk.
If the Hadoop native libraries are not available, this configuration
has no effect.
{quote}
Do you think we could improve it? In DFSConfigKeys.java, it's in a lower
section than the HA-related config, though I agree we could add some more
comment lines to clearly delineate the different config keys.
> SyncBehindWrites uses wrong flags on sync_file_range
> ----------------------------------------------------
>
> Key: HDFS-4089
> URL: https://issues.apache.org/jira/browse/HDFS-4089
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: data-node, ha
> Reporter: Jan Kunigk
> Priority: Minor
> Attachments: syncBehindWrites.patch
>
>
> Hi, I stumbled upon this while trying to understand the append design
> recently. I am assuming when SyncBehindWrites is enabled we do indeed want to
> do a complete sync after each write. In that case the implementation seems
> wrong to me.
> Here's a comment from the manpage of sync_file_range on the usage of the
> SYNC_FILE_RANGE_WRITE flag in solitude: "This is an asynchronous
> flush-to-disk operation. This is not suitable for data integrity operations."
> I don't know why this syscall is invoked here instead of just fsync
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